Chapter 3 Immunopathology

All lymphocytes are derived from bone marrow prelymphoid stem cells, which give rise to two distinct cell lineages:

(1) T-lymphocytes named so because they mature in the thymus and (2) B-lymphocytes, named after the bone marrow in which they primarily reside.

The innate immune system protects the body principally by three mechanisms:

(1) by initiating inflammation, (2) by combating viral infections, and (3) by mounting a general response to damaged cell products.

The immune response has two different forms:

1) a relatively primitive, nonspecific set of innate immunity, and (2) a complex system of cellular and humoral reactions that evolve in response to repeated exposures to foreign substances, known as acquired immunity.

The first organ transplant was performed in

1954 in Boston, when the surgeons transplanted Ronald Herrick's kidney to his identical twin, Richard. The transplantation was uneventful and the recipient lived for 8 years following the procedure.

Transplantation

Solid tissues can be transplanted successfully from one individual to another, but the graft will be viable only if the donor and the recipient are immunologically similar enough to avoid immunologic rejection. Alternatively, the graft will "take" only if the immune system of the recipient is unable to react against the foreign antigens. This is the case in congenitally immunodeficient children born without a thymus or in nude athymic mice, which represent an animal model of this human immunodeficiency. The immune system can be partially inactivated with various immunosuppressive drugs, which are used in clinical medicine to facilitate the acceptance of transplants.

T- and B-lymphocytes have distinct functions, although morphologically they cannot be distinguished by light or electron microscopy.

Subtle differences between T- and B-lymphocytes can be recognized by immunochemical techniques designed to identify unique markers on the surface of these cells. In practice, this can be done by immunocytochemical staining of tissue sections or cell smears with specific, color-coded antibodies. Antibodies to so-called cluster differentiation (CD) antigens have been most useful in this regard. CD antigens are selectively expressed during specific stages of lymphocyte development. Some CD antigens are selectively expressed on T-cells, and others are selectively expressed on B-cells. Several subsets of T- and B-lymphocytes and their immature precursors can be recognized by using this approach. With the use of cell sorters, such as the fluorescence-activated cell sorter (FACS), it is possible to distinguish T- from B-lymphocytes and to determine their ratio in circulation or in various lymphoid organs. FACS also can be used for separation of subsets of T-lymphocytes, which is important for the diagnosis of immunodeficiency diseases, such as acquired immunodeficiency syndrome (AIDS).

Rh Factor Incompatibility

The Rh blood group system consists of a group of antigens typically expressed on the surface of human RBCs. The three antigens, known as cde/CDE, are strong antigens, and of these only the d/D antigen is of practical significance. Persons who have the dominant allele D are Rh positive (Rh+) and those who have two recessive d/d alleles are Rh negative (Rh-). Approximately 15% of whites and 5% of African Americans are Rh-, whereas most others, including Native Americans and Asians, are Rh+. In contrast to the ABO antigens, which are complemented with natural antibodies, Rh antigens do not have natural antibodies. Thus, antibodies to dominant Rh antigens will be formed in Rh− persons who are transfused with Rh+ blood. Furthermore, it is important to note that the antibodies to the ABO antigens are of the IgM class, whereas the newly generated anti-Rh antibodies are of the IgG class. The IgG antibodies can cross the placenta, which is an important consideration in maternal-fetal Rh factor incompatibility.

Allograft.

This transplant involves donation of organs or tissues between individuals of the same species who are not genetically identical. In clinical practice, allografted tissues are accepted only if the donor and the recipient are matched at several major histocompatibility loci (HLAs) and have the same blood group. Best results are obtained with transplantation between relatives or siblings. However, to avoid immune reaction against foreign antigens, the recipients routinely receive immunosuppressive therapy before transplantation.

Xenograft.

This transplant involves organs and tissue recovered from nonhuman species grafted into human recipients. Such transplants between different species (as in liver transplantation between monkeys and humans) are in general poorly tolerated, with a few exceptions. It has been shown, namely, that avascular tissues, such as the cornea or heart valves, can be used for xenografting. Porcine heart valves are thus commonly used to replace damaged human heart leaflets. Before allotransplantation, the most common clinically used form of organ grafting, the donor's tissue must be matched with that of the recipient. This is done by crossmatching the peripheral blood lymphocytes, because the lymphocytes carry the same major histocompatibility antigens as the cells of the solid organs. This test provides basic information on the similarities and disparities of the two individuals and makes it possible to determine to what extent they are histocompatible. The histocompatibility antigens form four major loci (HLA-A, HLA-B, HLA-C, and HLA-D). Together with a closely related antigen called HLA-DR, these antigens are inherited as a single unit of five loci known as the haplotype. Because haplotypes are inherited as single alleles, each of us has a 1 in 4 chance that a brother or sister has a haplotype that is identical to ours. Thus, siblings are ideal tissue donor-recipient pairs. If an ideal match cannot be arranged, however, tissues from the patient's closest relatives, or from unrelated donors whose tissues are histocompatible with the recipient, may be used.

Blood Transfusion

Transfusion of blood from one person to another is a form of transplantation. In contrast to solid organs, however, blood is a tissue that is composed of dissociated cells circulating inside the vessels. Because RBCs outnumber white blood cells by an order of magnitude, the success or failure of blood transfusions depends primarily on the compatibility of the donor and the recipient with regard to their RBC blood group antigens. If the blood of an A group donor is infused into a B group recipient, the natural antibodies to A in the recipient will react with the donor's RBCs and cause their hemolysis.

Anaphylactic Shock

a life-threatening, severe, systemic response to an allergen to which the body was previously sensitized. Anaphylactic shock following a bee sting in a person sensitized to bee venom is a typical example. Allergy to various nuts, especially peanuts, is yet another well-known cause. Drug induced anaphylactic reaction is most often related to administration of penicillin, but it can occur occasionally after intravenous injection of other drugs, anesthetics, or radiographic contrast media. Shock develops as a result of a massive release of histamine and other vasoactive substances into the circulation. Typical symptoms include stridor (high-pitched sound during breathing) caused by vocal cord spasm, choking secondary to laryngeal edema and narrowing, wheezing and shortness of breath resulting from bronchial spasm, and pulmonary edema and systemic circulatory collapse with fainting caused by hypotension, secondary to vasodilation and increased leakage of fluid from the hyperpermeable blood vessels.

ABO antigens have corresponding natural antibodies:

blood group A contains anti-B antibodies, group B anti-A antibodies, and the O group contains both anti-A and anti-B antibodi. Group AB blood does not contain natural antibodies to either A or B antigens. Thus, A blood can be given to group A and group AB recipients, because the blood of these recipients does not contain antibodies to the group A antigen. Group AB blood can be given only to AB group recipients, because all other group persons would have antibodies to A or B, or both. O group blood can be given to recipients of all blood groups. Accordingly, individuals with AB blood are called universal recipients and those with O blood are considered universal donors.

Asthma

considered a type I hypersensitivity reaction affecting the bronchi. There are several forms of asthma, not all of which are immunologically mediated. Asthma caused by hypersensitivity to inhaled antigens is primarily mediated by leukotrienes and prostaglandin D2, usually affecting children. Attacks of asthma are marked by coughing and wheezing related to the constriction of bronchi and overproduction of mucus by the bronchial glands.

Chronic Rejection

evolves slowly over a period of several months or years. It also involves both antibody- and cell-mediated responses. Vascular changes cause obliteration of the arterial lumen (endarteritis), which in turn leads to hypoperfusion of tissues and chronic ischemia. Interstitial tissue inflammation contributes to the destruction of parenchymal cells and the ultimate deterioration and loss of tubules. Glomeruli are also affected and their capillaries become obliterated. Arterial, glomerular, and tubular changes lead to chronic renal failure, which usually requires a new kidney transplant.

Lymphocytes are also present in other organs and are most prominent in the

gastrointestinal and bronchial mucosa, where they form the so-called mucosa-associated lymphoid tissue (MALT). In contrast to encapsulated lymph nodes and spleen, MALT has no capsule, and its cells are an integral part of the mucosa.

IgG

has the smallest molecular weight of the immunoglobulins but is nevertheless the most copious immunoglobulin. It is produced in small amounts upon initial immunization, but its production is boosted by reexposure to the antigen. Fc receptors for IgG exist on macrophages, PMNs, lymphocytes, eosinophils, and platelets and in the placenta. This allows the passage of IgG across the placenta to the fetus. IgG acts as an opsonin, that is, it coats bacteria and thus facilitates their phagocytosis.

IgD

is a cell membrane-bound immunoglobulin found exclusively on B-cells. It participates in the antigenic activation of B cells, but it is not released into the serum or body fluids.

Graves' Disease

is a form of hyperthyroidism that typically develops in women who have autoantibodies to the thyroid-stimulating hormone (TSH) receptor on the surface of their own follicular cells of the thyroid. The binding of the antibody to the receptor leads to the stimulation of cells, which is similar to the action of TSH. This results in hyperthyroidism resulting from an overproduction of thyroid hormones.

Myasthenia Gravis

is a muscle disease marked by severe muscle weakness. This autoimmune disease is mediated by antibodies to the receptors for acetylcholine on the surface of striated muscle cells. Acetylcholine is the neurotransmitter released from the nerves at the neuromuscular junction, and it mediates the transmission of signals for muscle contraction. Blockade of acetylcholine receptors prevents the binding of the neurotransmitter, causing progressive muscle weakness and even paralysis.

Poststreptococcal Glomerulonephritis

is an acute renal disease that typically follows an upper respiratory tract infection caused by certain nephritogenic (i.e., capable of inducing nephritis) strains of streptococci. Persons sensitized to streptococcal antigens during acute infection produce antibodies that react with soluble streptococcal antigens "planted" onto the glomerular basement membranes during filtration from the plasma. Antibodies may also react with the streptococcal antigens found in the circulation and thus form circulating immune complexes. Such antigen-antibody complexes are deposited in the glomerular basement membrane, evoking a complement-mediated inflammatory response.

Systemic Lupus Erythematosus

is an autoimmune disease of unknown origin, which is discussed in greater detail later in this chapter. Here, it should be mentioned that patients with SLE have circulating immune complexes formed between various autoantigens and equivalent antibodies. Although it is unclear what elicits the autoimmune reaction, the consequences of immune complex deposition in tissues are well known. These include kidney disease, arthritis, skin disease, and a variety of other diseases.

Acquired Immunity

is based on specific responses elicited by substances that act as antigens. An antigen is any chemical substance that can induce a specific immune response—that is, a reaction to production of specific antibodies or specifically sensitized immune cells. Acquired immunity is based on the ability of the body's immune system to distinguish self from nonself, to generate an immunologic memory, and to mount an integrated reaction of various cells. Acquired immunity is based on the reaction of the immune system and also involves other cells (auxiliary [helper] cells), such as macrophages, basophils, and eosinophils. The body's ability to mount an appropriate immune response is termed immunocompetence. Immunocompetence depends on adequate structural and functional development of the immune system and on the coordinated action of its components.

IgM

is composed of five basic units and is thus the largest immunoglobulin (macroglobulin) held together with a linker-J chain. Its function is to neutralize microorganisms. It is an avid complement activator, because it has five complement-binding sites. IgM is the first immunoglobulin to appear after immunization, and it is a natural antibody against blood group antigens ABO.

IgA

is found in blood but is more abundant in mucosal secretions (e.g., tears and nasal mucus), breast milk, and intestinal contents, where it has an important protective function.

Goodpasture's Syndrome

is marked by renal and pulmonary pathologic changes. These develop because of autoimmunity to a component of collagen type IV in the basement membranes of the glomeruli and alveoli. This antigenic epitope, which is normally hidden, becomes inappropriately exposed, allowing the circulating antibodies to attack the kidneys and lungs. "Membranotoxic" antibodies cause destruction of glomeruli and consequent renal failure, as well as massive pulmonary hemorrhage that may be lethal.

IgE

is present in trace amounts in serum. This immunoglobulin is secreted by sensitized plasma cells in tissues and is locally attached to mast cells. IgE mediates allergic type I hypersensitivity reactions, also known as atopic or anaphylactic reactions.

Hemolytic Anemia

is the prototype of a cytotoxic antibody-mediated reaction. The RBC antigens of these patients become antigenic and are recognized as foreign by the body's own immune system. Foreign chemicals, such as drugs, which are not antigenic per se, may attach to the surface of the RBCs as haptens and become antigenic in that context. Antibodies to these "incomplete antigens" also may cause hemolysis.

Atopic Dermatitis

is typically a disease of childhood, a chronic skin irritation known as eczema. Eczema affects approximately 10% of all children, 50% of whom have a family history of similar problems. This genetic predisposition is associated with hyperproduction of IgE in response to potential environmental allergens. However, a nonfamilial form of atopic dermatitis can occur as well. Bacterial superinfection of the irritated skin lesions is common and aggravates the course of the disease. Exposure to allergens usually occurs through direct skin contact. Other allergens may be inhaled or ingested in food. Atopic dermatitis improves with age, although affected children have a tendency to develop other type I hypersensitivity diseases in adulthood, such as asthma or hay fever.

Every RBC carries a set of surface antigens, which can be divided into three groups:

major blood group antigens, minor blood group antigens, and Rh blood antigens.

Acute Rejection

occurs most often within the first few weeks of transplantation but may also evolve later when the immunosuppressive treatment becomes ineffectual. It involves both antibody- and cell-mediated immune reactions. Antibodies tend to damage the blood vessels, such as the peritubular capillaries, which by immunofluorescence microscopy show deposits of immunoglobulin and activated fragments of complement, most often C4d. The T-cell mediated rejection presents in a form of tubulitis and arteritis involving the intima of renal artery branches. B-lymphocytes and plasma cells may also appear in the interstitial spaces between the tubules, indicating that this is a mixed T- and B-cell mediated transplant reaction.

Hay Fever

or allergic rhinitis, occurs typically as a seasonal allergy to pollens and other plant derived antigens. It may also be caused by other foreign substances, such as cat dander or house dust, and it is not always seasonal. Exposure to inhaled allergen causes nasal itching and sneezing. Swelling and inflammation of the nasal mucosa (rhinitis) is often associated with similar irritation and inflammation of the conjunctiva (conjunctivitis). All symptoms can be attributed to the effects of histamine and can be neutralized with antihistamines. Drugs that stabilize mast cells and prevent the discharge of their granules are also effective. Long-term relief can be achieved through desensitization to specific allergens. This treatment is based on repeated prophylactic injections of antigen, which induce a neutralizing IgG response. When a desensitized patient encounters the allergen again, the IgG that is bound to the antigen will prevent its contact with the IgE in the sensitized tissue. The adverse response of mast cells is thus prevented.

The bone marrow and thymus are called

primary lymphoid organs

From the primary lymphoid organs, the T- and B-lymphocytes enter the blood circulation and colonize various

secondary lymphoid organs. Among these, the most prominent are the lymph nodes and spleen, in which lymphocytes constitute a significant percentage of the total cell population. Lymphocytes are also present in other organs and are most prominent in the gastrointestinal and bronchial mucosa, where they form the so-called mucosa-associated lymphoid tissue (MALT). In contrast to encapsulated lymph nodes and spleen, MALT has no capsule, and its cells are an integral part of the mucosa.

Hyperacute Rejection

typically occurs because the recipient has preformed antibodies to the donor's antigens. Typically the reactions occur during the operation. When the surgeon connects the donor's and recipient's blood vessels and the recipient's blood enters the graft, the preformed circulating antibodies react with the endothelial cells. Damage to the endothelial cells leads to thrombosis, and the graft cannot perfuse normally. Such transplants must be removed immediately to prevent even more serious and inevitable complications.

Polyarteritis Nodosa

which is the clinical equivalent of the Arthus phenomenon, is an antigen-antibody-mediated inflammatory disease that typically involves small to medium-sized arteries. In the early stage the affected vessels show focal fibrinoid necrosis and acute inflammation. In the chronic stage the disease is marked by destruction of the vessel wall. Damaged vessels tend to thrombose and become occluded, causing tissue ischemia and infarcts.

The following diseases are some clinical examples of type II hypersensitivity reaction:

• Hemolytic anemia • Goodpasture's syndrome • Graves' disease • Myasthenia gravis

The reaction to homotransplanted kidneys, which can occur in three forms:

• Hyperacute reaction • Acute reaction • Chronic rejection

The most important clinical entities mediated by type III hypersensitivity reactions are as follows:

• Systemic lupus erythematosus • Poststreptococcal glomerulonephritis • Polyarteritis nodosa

Innate protective mechanisms are inherited and operational at the time of birth. They are relatively nonspecific, and in contrast to acquired immunity these mechanisms rely on antigenic stimulation—that is, they do not depend on previous exposure to foreign substances. These innate defense mechanisms include the following:

• Various mechanical barriers (e.g., the epidermis or the ciliated cells in the mucosa of the nose or bronchus) • Phagocytic cells, such as neutrophils, macrophages, and dendritic cells • Natural killer (NK) cells • Protective proteins found in tissues and plasma, such as properdin and lysozyme Properdin is a plasma protein that activates the alternative complement pathway. Lysozyme is a low molecular weight protein found in some body fluids, such as tears, nasal, and intestinal secretions. It nonspecifically kills many bacteria.

Transplant Rejection

All allografts invariably evoke some transplant rejection, which is mediated by antibodies and a delayed cellular immune reaction.

Cells of the Immune System

All cells of the immune system are descendants of primitive hematopoietic stem cells originally found in the bone marrow. The bone marrow stem cells give rise to two major cell lineages: lymphocytes, known as lymphoid cells, and all other nonlymphoid cells, including PMNs, eosinophils, basophils, macrophages, and megakaryocytes. Lymphoid cells are the primary cells of the immune system, whereas the other cells may or may not contribute to immune reactions and are in this context considered to be helper cells.

B-Lymphocytes

B-cells are lymphocytes that are essential for the production of antibodies. These cells are primed to differentiate into immunoglobulin-producing plasma cells. This differentiation occurs in a stepwise manner. Each of these intermediate stages is characterized by distinct cell surface and cytoplasmic changes that can be recognized in histologic slides under the microscope by modern immunohistochemistry methods. However, some features are shared by all B-cells and their mature descendants, the plasma cells. The most important shared feature is the activation of the immunoglobulin gene, which occurs only in the B-cell lineage. The immunoglobulin gene is similar to TCR, and its activation also occurs through a rearrangement of parts of the gene. The immunoglobulin gene rearrangement enables B-cells to produce immunoglobulins that are incorporated into the B-cell antigen receptor complex on the plasma membrane. Antigen-stimulated B-lymphocytes differentiate subsequently into plasma cells.

If the blood of an A group donor is infused into a B group recipient, the natural antibodies to A in the recipient will react with the donor's RBCs and cause their hemolysis. This transfusion reaction would produce the following clinical symptoms:

Chills, shivering, and even mild fever. In most instances transfusion reaction is readily recognized and the transfusion is discontinued. If the reaction is not diagnosed, massive hemolysis may cause shock with thrombi in small blood vessels and disseminated intravascular coagulation (DIC). Some patients may even die. Jaundice, from the bilirubin released from hemolyzed RBCs, develops in those who survive. Acute renal failure is a common complication. To avoid transfusion reaction, the donor's blood must be cross-matched with the blood of the recipient. This is done before the transfusion by mixing the serum of the donor with the RBCs of the recipient and vice versa. The RBCs are incubated at body temperature in a test tube. The RBCs that are compatible with the serum will remain suspended in the fluid. However, if there are antibodies in the serum they will attach to the RBCs and agglutinate them. Blood that agglutinates in the crossmatch is not suitable for transfusion. Cross-matching of donor and recipient blood is essential to avoid transfusion reactions caused by ABO incompatibility. In addition, this procedure can also detect significant incompatibilities of minor blood group antigens. In most instances, incompatibility at these loci has no consequence, but occasionally it may cause a significant hemolytic reaction.

Immunity

Derived from the Latin term denoting exemption from duty (munus, meaning "duty" or "service"), was originally defined as resistance to infections. Immune persons would be "exempt from suffering" inflicted by infectious diseases. Subsequently, it became apparent that immune reactions were not elicited only by various living pathogens but also by many other substances (e.g., snake poison), as long as they were perceived as foreign by the immune system. Furthermore, we have learned that these reactions occur in many forms and that immunity not only provides protection but also can cause disease. There are few human diseases that do not affect the immune system. Immunologic techniques are useful in research and are also used daily in clinical laboratories. This applied immunology forms the basis for immunodiagnostics. Finally, immunotherapy should be mentioned because it provides new modalities for the treatment of diseases. Immunopreventive techniques, such as vaccination and active and passive immunization, have contributed enormously to the fight against diseases in humans and animals. It is fair to say that immunization has probably saved more human lives than all other drugs combined.

Graft-Versus-Host Reaction

Develops as a result of the transfer of a donor's immunocompetent lymphocytes. In response to antigens on the recipient's tissues, the donor's lymphocytes initiate a cell-mediated type IV immune reaction. Because the host is usually immunosuppressed, the transplanted immunocompetent cells cannot be rejected; as a result, these cells proliferate and overwhelm the host's body. The donor's lymphocytes attack various tissues in the host, especially the epithelial cells of the skin or gastrointestinal tract, and the bile ductal cells inside the liver. Clinical signs of the GVH reaction usually involve dermatitis with scaling of the epidermis. Diarrhea and fever are typical signs of gastrointestinal GVH reaction. Jaundice is the most prominent sign of liver involvement. GVH reaction may be difficult to treat, and the patient usually dies as a result of overwhelming infections.

In the clinical setting, there are several forms of transplantation, which include the following:

Autograft, Allograft, Xenograft

Major Histocompatibility Complex

All processed antigens are presented to T cells in the context of the major histocompatibility complex (MHC) proteins expressed on the surface of APCs. These MHC proteins, first identified on leukocytes, are also known as human leukocyte antigens (HLAs), although they are expressed on other nucleated cells in the body. A unique set of MHC antigens determines the individuality of each person. Only identical twins have the same MHC antigens. In immune reactions, the MHC regulates the cell-to-cell contact during antigen presentation. Human MHC antigens belong to two groups. Type I MHC proteins, found on all nucleated cells of the body, serve as the receptors for CD8, thus linking macrophages to cytotoxic T-lymphocytes. By binding to type I MHC, cytotoxic T-lymphocytes kill virus infected cells and transplanted foreign cells, or tumor cells. Type II MHC molecules react with CD4, mediating the attachment of macrophages to helper T-lymphocytes. Type II molecules serve in the presentation of exogenous antigens (e.g., bacteria and soluble antigens) that are first internalized and processed before presentation to T-cells. The main function of the MHC is presentation of antigens to T-cells. T-cells can react only to membrane-bound antigens; thus without the APC there is no T-cell reaction to antigens. The MHC is also important for organ transplantation, and most transplant rejection reactions result from the HLA incompatibility of the host and the donor.

Hypersensitivity Reactions

An abnormal immune response to exogenous antigens or a reaction to endogenous autoantigens. Hypersensitivity reactions are the basis of hypersensitivity diseases, which are also known as allergic disorders.

Antibodies

Antibodies are proteins of the immunoglobulin class that are secreted by plasma cells. They can be defined operationally as proteins reacting with antigens. Chemically they can be classified into five classes: IgG, IgM, IgA, IgE, and IgD. These immunoglobulins share some common features: • Immunoglobulins are composed of light and heavy chains, and all the light chains are either kappa or lambda. A single molecule contains either two kappa or two lambda chains. Heavy chains are immunoglobulin class specific, i.e., unique to each of the five classes. • Each heavy and light immunoglobulin chain has a constant (CH, CL) and variable (VH, VL) part, these portions are important for the recognition of antigens. • Each antibody can be cleaved enzymatically into two fragments: the Fc portion, which contains the constant region, and the Fab fragment, which contains the variable region. Fc binds to specific Fc receptors that are expressed on macrophages, polymorphonuclear neutrophils (PMNs), and others. Fab serves as the antigen-binding site.

Antibody Production

Antibody production begins with contact between an antigen and the cells of the immune system. All substances identified by the body as foreign may serve as antigens and incite an immune response. This activation of B-cells culminates in the production of specific antibodies that can react with the antigens. To elicit antibody production, the antigen must bind to the B-lymphocyte antigen receptor complex. This complex includes IgM or IgD, which binds the antigen, and several membrane molecules that do not participate in antigen binding but are essential for signal transduction and initiation of antibody production. Antibody production requires the support of T helper cells. It is worth noting that B-cells can internalize the antigens and thereafter may function as antigen-presenting cells (APCs), providing the internalized antigens to T-cells.

Major blood group antigens (ABO) are encoded by three genes that produce the following six genotypes:

IAIA, IAi, IBIB, IBi, IAIB, and ii. The A and B genes are dominant over the O gene, so there are only four phenotypic blood groups with their respective genotypes: A (IAIA and IAi), B (IBIB and IBi), AB (IAIB), and O (ii).

Autograft.

In this form of transplantation the patient serves as both donor and recipient. Such grafts are typically used for skin, hair transplantation, and replacement of blood vessels of the heart with leg veins.

Plasma Cells

Plasma cells are fully differentiated descendants of B-lymphocytes. These cells have an oval shape and an eccentrically located round nucleus. The cytoplasm of plasma cells is basophilic because it contains an abundance of ribosomes. On electron microscopy, their cytoplasm contains numerous stacks of rough endoplasmic reticulum (RER). The RER is the site of synthesis of immunoglobulins, the primary secretory products of plasma cells.

Maternal-fetal RH incompatibility

Involving the Rh antigen D has, until recently, been the most important cause of neonatal hemolytic disease and several syndromes known as icterus gravis neonatorum, hydrops fetalis, and erythroblastosis fetalis. These conditions, which involve Rh− women with Rh+ mates, are all caused by the same mechanism; the different names for the syndromes merely reflect the extent of injury and its timing. If the child is Rh+, because he or she has inherited the paternal D allele, the child's Rh+ blood could sensitize the mother. During the first pregnancy, the Rh+ child will not be affected because the mother does not have natural antibodies to the Rh antigen D. However, the mixing of fetal and maternal blood at the time of delivery may expose the Rh− mother to the dominant D antigen on fetal RBCs. This will immunize the mother and cause her to produce IgG-type anti-Rh antibodies. If the fetus in the subsequent pregnancy is again Rh+ (i.e., d/D), the antibodies to D will cross the placenta and affect the fetal Rh+ RBCs. Hemolysis will ensue in the fetal circulation, and the fetus may die in utero showing signs of severe generalized swelling known as hydrops fetalis Essentially the hemolysis destroys fetal RBCs. The fetus becomes anemic and develops severe hypoxia and congestive heart failure. The term erythroblastosis fetalis, used as a synonym, indicates that the fetal bone marrow is maximally stimulated by the loss of RBCs and is trying to compensate for the loss. There is marked extramedullary hematopoiesis in the liver, spleen, and lymph nodes. The fetus is also jaundiced as a result of the bilirubin released from the hemolyzed RBCs. In milder cases, comparatively less hemolysis occurs and massive edema does not develop. The newborn child does not show signs of massive edema, and the only external evidence of hemolysis is marked jaundice. The major danger associated with this form of jaundice is kernicterus, or jaundice of the basal ganglia of the brain. The massive elevation of serum bilirubin breaches the blood-brain barrier and the bilirubin that would normally not cross into the brain is deposited preferentially in the basal ganglia. This bilirubin is toxic and may cause permanent neural damage. Clearly maternofetal Rh incompatibility has serious repercussions, and once the disease develops, it cannot be treated efficiently. Fortunately, this disease can be prevented by proper treatment of Rh− pregnant women at risk. At the time that an Rh− mother first delivers an Rh+ child, it is possible to prevent Rh immunization of the mother by injecting her with anti-D immunoglobulin (RhoGAM). This procedure, if performed during the first 72 hours postpartum, prevents maternal immunization and erythroblastosis fetalis in subsequent pregnancies. Unfortunately, if a pregnant Rh− woman becomes immunized to the D antigen during pregnancy or abortion, treatment with RhoGAM is of no avail. Immunoprophylaxis of maternal-fetal Rh incompatibility has almost completely eliminated this medical problem in North America and Europe. Hemolytic disease of the neonate still occurs, albeit rarely, as a result of the incompatibility of the fetal and maternal major blood groups or some minor blood group antigens. In practice, this is usually encountered in fetuses of the A1 subtype born to group O mothers who have acquired IgG antibodies to the A1 antigen. Fortunately, this incompatibility results only in mild hemolysis and the newborn child usually shows only anemia and jaundice. There is no prophylactic measure that could be applied to prevent the consequences of maternal-fetal incompatibility in ABO or minor blood group antigens.

Antigen-Antibody Reaction

Most antigens have more than one antigenic site, or epitope, and are thus able to bind more than one antibody to their surface. Antigens are thus multivalent. Antigens and antibodies are bound to each other by complex physical and chemical bonds, forming antigen-antibody complexes. If the antigen is soluble and circulating in the blood, the complexes will also circulate in the plasma, the fluid portion of blood. However, these complexes tend to enlarge as more and more antibodies and antigen molecules are included in the meshwork. The complexes reach the size of small particles, which are phagocytized in the spleen and the liver by fixed macrophages. The smaller complexes may remain in the circulation, depending on their overall solubility, size, and electrical charge. Contingent on these three properties, the immune complexes may remain suspended in circulation for a long time; alternatively, they may be attached to red blood cells (RBCs) or endothelial cells, or filtered through the capillary walls with other proteins. Antibodies to insoluble antigens, such as cell surface antigens, become fixed to the cell membrane. This is best illustrated by the antibodies against RBCs that typically coat the cell surface. Antibodies bind RBCs to each other, best seen by placing a drop of blood on a glass slide. The antibody-coated RBC clump together on the slide. This process called agglutination can be easily recognized because the clumped together RBCs will separate from the serum and form a dense red dot on the glass. Another way of recognizing antibodies in the laboratory is to add the antibody to the blood and induce hemolysis. By adding the antibody to the blood one allows the antibodies to form antigen-antibody complexes with surface molecules of the RBCs. These complexes activate the complement cascade in blood, leading to the formation of the membrane attack complex. MAC inserts into the membrane of RBCs, whereupon the cells rupture and hemoglobin spills out into the fluid. Complement activation in the blood flow of the body results from the circulating, soluble, or cell surface-fixed immune complexes inside the blood vessels. Antigen-antibody complexes formed outside of the vessels, or those that are pathologically deposited in tissues, also activate complement. Antigen binds to the Fab region of the antibody. The Fc region that protrudes on the opposite side of the antibody serves as a binding site for complement. At the same time, Fc can also attach to cells that have Fc receptors. The most important among these are the macrophages and PMNs, which act as scavengers of immunoglobulin-coated (opsonized) bacteria. RBC membrane fragments that are coated with antibodies and the large soluble immune complexes are taken in the same way by macrophages and removed from circulation.

Clinical Use of Transplantation

Transplants are used extensively in clinical practice. Kidney transplants have been performed with considerable success for more than 50 years. Skin transplants are used for the treatment of burns. Livers, hearts, lungs, or pancreases that have been terminally damaged by various diseases also can be replaced successfully with transplants. Bone marrow transplantation is used to treat aplastic anemia and bone marrow failure. This procedure is also used in the treatment of leukemia, a neoplastic disease of the bone marrow. In such cases the bone marrow of the leukemic patient is irradiated to kill all the tumor cells and the bone marrow is then replenished with the stem cells that have been removed from the bone marrow of a histocompatible donor. The best way to prevent transplant rejection is to match the donor and recipient carefully. Because an ideal match is not always possible, recipients must be prepared for transplantation with adequate immunosuppression. This is accomplished by administering drugs, such as cyclosporine, which inhibits IL-2 production and thus impairs the T-cell response, or cyclophosphamide, which inhibits proliferation of lymphocytes. Antibodies to T-cell antigens are also used to reduce the number of these cells. However, immunosuppression is not an innocuous procedure, and it predisposes the patient to infections. Some drugs, such as cyclosporine, may have significant side effects and are nephrotoxic. Immunosuppressed patients are at an increased risk for the development of infections with ubiquitous bacteria and fungi, which are then difficult to eradicate.

Type II Hypersensitivity

Type II hypersensitivity is mediated by cytotoxic antibodies that react with antigens in cells or tissue components, such as basement membranes. The antigen may be extrinsic or intrinsic, as is often the case in some autoimmune diseases. Intrinsic antigens include macromolecules, such as proteins, RNA, or DNA. The reason these components become antigenic are not known. Foreign antigens include drugs or simple chemicals that usually act as haptens. These substances bind to soluble plasma proteins, or proteins on the surface of RBCs and other cells that then immunize the body. Foreign substances released from bacteria and cells infected with viruses may provoke a similar response. Hypersensitivity reaction occurs upon reexposure to the pathogenic antigen. Persistent antigens, as in chronic infection and slow release of endogenous autoantigens, provoke deleterious hypersensitivity reactions. Type II hypersensitivity reaction is mediated by IgG or IgM, which form antigen-antibody complexes on cell membranes or the extracellular matrix, such as basement membranes. These complexes activate complement, the major effector mechanism accounting for the cell lysis that occurs, for instance, in the acute hemolytic reaction caused by transfusion of mismatched blood. Immunoglobulins attached to the antigen also may evoke an antibody-dependent, cellular, cytotoxic (ADCC) reaction. The antibody typically binds to the antigen at the Fab end, whereas the Fc portion is free and serves as the attachment site for various effector cells, such as NK cells, macrophages, and other leukocytes. Finally, some type II hypersensitivity reactions do not require either complement or an ADCC reaction. Such reactions are based on the binding of antibodies to the receptors on cell surfaces. The binding of antibodies to receptors may stimulate or inhibit the function of such cells.

Type III Hypersensitivity

Type III hypersensitivity is mediated by immune complexes that are formed between antigens and appropriate antibodies. In systemic reactions to soluble antigens, the immune complexes are in the circulation, whereas in localized reactions the immune complexes are formed in tissues. Serum sickness, which was common when horse serum was used extensively for passive immunization against tetanus, is the prototype of type III hypersensitivity A few days after the injection of the serum, foreign proteins appear in the circulation. As the titer of antibodies rises, the concentration of antigen decreases until all of it is completely complexed with antibodies and eliminated from circulation. Initially, the antigen-antibody complexes are small and sparse, but with time the antibody excess becomes overwhelming and the antigen is completely bound to large complexes. During the time of equilibrium, or mild antibody excess, antigen-antibody complexes form that are rather soluble and not large enough to be phagocytized by macrophages. Such soluble antigen-antibody complexes remain in circulation and are filtered through the basement membranes of glomeruli and other sites where the plasma is ultra-filtered to produce body fluids. Such anatomic sites include the anterior chamber of the eye, the choroid plexus of the brain, and the serosal surface covering the pleura, pericardium, and the peritoneal cavity. Immune complexes that are trapped in these semipermeable membranes activate complement, which attracts PMNs and results in acute inflammation. Localized immune complex formation occurs typically in various forms of vasculitis, such as polyarteritis nodosa. This disease can be reproduced experimentally in the form of the so-called Arthus phenomenon. The antigen is injected subcutaneously to produce sensitization and to stimulate antibody production. Upon rechallenge, with the same antigen injected into another site, the antibodies from the circulation diffuse toward the antigen in the tissue and react with it at the site of contact. Antigen-antibody complexes precipitate at the site of equilibrium, and this occurs typically in the vessel wall. Antigen-antibody complexes formed in the vessel wall activate complement, which attracts leukocytes. A localized acute inflammation develops, characterized by fibrinoid necrosis. Fibrinoid necrosis reflects the influx of plasma proteins that permeate the site of injury. Fibrinogen, which is also present, undergoes polymerization into fibrin, leading to localized clotting in the vessel walls.

Type IV Hypersensitivity

Type IV hypersensitivity is also known as cell-mediated or delayed-type immune reaction. It involves T-lymphocytes and macrophages, which typically aggregate at the site of injury to form granulomas. Type IV hypersensitivity reaction is initiated by complex antigens that are taken up by macrophages, or equivalent APCs, such as Langerhans' cells of the epidermis. The antigen is processed and presented to T-lymphocytes. Helper T-lymphocytes that are exposed to the antigen and the cytokines produced by the APCs become primed and activated. This leads to formation of immune memory, which is important for subsequent exposure and the recruitment of other cells, most notably macrophages, and additional helper T- and suppressor/cytotoxic T- lymphocytes. Under the influence of cytokines, the macrophages transform into epithelioid cells, which produce even more varied mediators of inflammation than their predecessors, further promoting the formation of granulomas. IFN-γ, considered the most important cytokine responsible for the formation of granulomas, acts on epithelioid cells by augmenting their phagocytic activity and their ability to kill antigen-bearing cells and bacteria (e.g., Mycobacterium tuberculosis or tumor cells). IFN-γ also promotes the fusion of epithelioid cells into multinucleated giant cells. Hence, fully formed granulomas consist of epithelioid cells, giant cells, and lymphocytes Type IV hypersensitivity reaction occurs in response to complex antigens of M. tuberculosis, Mycobacterium leprae, and various fungi. In addition to infectious granulomas, type IV hypersensitivity reaction accounts for granulomas that develop in response to tumors and for idiopathic granulomatous diseases, such as sarcoidosis. Granulomas can be induced by injecting humans or animals with the antigen. For example, tuberculin, prepared from M. tuberculosis, may induce delayed hypersensitivity reactions. Thus, it is possible to test whether somebody was exposed to tuberculosis by injecting purified tuberculin into the skin. If the tested person develops localized induration of the skin within 48 hours, the test is considered positive and the person is assumed to have been exposed to tuberculosis (or to have persistent disease). Contact dermatitis, the most common clinical form of type IV hypersensitivity, is not associated with granuloma formation. In this disease, which may be caused by allergy to a variety of allergens (e.g., latex gloves, gold rings, or poison ivy), the skin usually contains infiltrates of T-lymphocytes and macrophages but no granulomas. The inflammatory cells typically show perivascular cuffing, which correlates with altered vascular permeability, wheal formation, and edema of the affected skin. The best example of contact dermatitis is the so-called poison ivy reaction, a hypersensitive reaction to plant antigens. Surgeons may become allergic to latex gloves. Hypersensitivity reactions can occur to essentially any antigen in the environment, including antiperspirants, medical ointments, or laundry detergent

Hypersensitivity diseases are pathogenetically classified into four major groups, each of which is mediated by distinct mechanisms:

Type I—anaphylactic or atopic reaction Type II—cytotoxic antibody-mediated reaction Type III—immune complex-mediated reaction Type IV—cell-mediated, delayed-type reaction

Type I Hypersensitivity

also known as anaphylactic or atopic reaction, is primarily mediated by IgE and mast cells, or basophils. IgE is produced by plasma cells derived from B-lymphocytes controlled by TH2 helper cells, which have been sensitized to foreign antigens, such as pollen. Upon second exposure to the same antigen, the primed plasma cells secrete an antibody that diffuses locally toward mast cells and is fixed to the Fc receptors on their surface. Subsequent reexposure to the antigen leads to the formation of antigen-antibody complexes on the surface of mast cells. This triggers the release of vasoactive substances stored in mast cell granules. The most important among these is histamine, the well-known vasoactive biogenic amine. The release is instantaneous, as any sufferer of hay fever can testify. It is accompanied by increased vascular permeability, edema, and accumulation of inflammatory cells, most notably eosinophils. Eosinophilia—an increased number of eosinophils in the blood—is a common systemic feature of type I hypersensitivity reactions. Type I hypersensitivity reactions also produce a late-phase response that usually occurs 4 to 6 hours after exposure to allergens. Basophils and mast cells play an important role in this reaction, but other inflammatory cells also partake and are important for prolonging the tissue reaction to antigens. The late-phase response is mediated by arachidonic acid derivatives classified as leukotrienes and prostaglandin D2. This type of late-phase response is especially prominent in bronchial asthma, a chronic respiratory disease prone to frequent exacerbations in the form of asthmatic attacks, characterized by coughing and shortness of breath as a result of bronchospasm and excessive mucus production in the bronchi. Clinical examples of type I hypersensitivity are as follows: • Hay fever • Atopic dermatitis • Bronchial asthma • Anaphylactic shock

T-Lymphocytes

are lymphocytes that have matured in the thymus. They account for two thirds of all lymphocytes in the blood and also are found in the paracortical zone of lymph nodes and the periarteriolar sheath of the spleen. There are several subsets of T-cells, the most important of which are the T helper and T suppressor/cytotoxic cells. T helper cells actively participate in the immune response to antigens and help B-cells produce antibodies. T suppressor/cytotoxic cells suppress unwanted antibody production and mediate killing of virus-infected cells or tumor cells that are recognized by the body as foreign. Common to all T-cells is the surface T-cell receptor (TCR), which is linked to a membrane protein known as CD3. T-cells use the TCR for the recognition of antigens. The TCR-CD3 complex is thus essential for the activity of T-cells. Like all other genes inherited from our parents, the gene for TCR is in all the cells of the body. However, the gene is activated only in T-cells. TCR gene activation occurs through rearrangement of parts of the gene. Because TCR rearrangement occurs only in T-cells, it is a unique genetic marker of T-lymphocytes. This is important to note because 10% to 15% of peripheral lymphocytes have the same surface markers as T-lymphocytes but do not have TCR gene rearrangement. These cells are known as natural killer (NK) cells. NK cells mediate innate immune reactions and are not involved in T- and B-cell-mediated immune reactions. Their function is to react to virus-infected cells and to kill tumor cells and transplanted foreign cells without previous sensitization. T helper cells express CD4 on their surface, whereas the T suppressor/cytotoxic cells express the CD8 antigen. CD4 and CD8 are used as markers for these lymphocytes and for the counting of T helper and T suppressor/cytotoxic cells in the blood. In normal blood, CD4-positive cells predominate, and the cell ratio of CD4 to CD8 is approximately 2 : 1. In individuals with AIDS, CD4 cells are selectively lost and the cell ratio of CD4:CD8 is less than 1. CD4-positive helper cells can be activated to secrete regulatory proteins known as cytokines. T helper cells can be subdivided into two major groups on the basis of which cytokines they produce: T helper-1 (TH1) cells that synthesize interleukin-2 (IL-2) and interferon-gamma (IFN-g) and T helper-2 (TH2) cells that synthesize IL-4, IL-5, and IL-13. TH1 cells stimulate macrophages to become phagocytic and mediate the formation of granulomas, whereas the cytokines secreted by TH2 cells are important for the secretion of IgE and other immunoglobulins and for the activation of eosinophils.

Lymphocytes

are small cells, only slightly larger than erythrocytes. They have a round nucleus and very little cytoplasm.