IHO: Hypersensitivity Reactions (Dr. Nelson - 2 hrs)

Tissues of the Immune System:

*Generative lymphoid organs (primary or central)*: sites where T and B lymphocytes mature and become competent to respond to antigens (thymus for T lymphocytes, bone marrow for B lymphocytes). *Peripheral lymphoid organs (secondary)*: sites where the adaptive immune response is initiated (lymph nodes, spleen, mucosal and cutaneous lymphoid tissues (GI tract, respiratory tract, skin). T and B lymphocytes are segregated into different regions in the peripheral lymphoid organs. For example, in the lymph nodes, B lymphocytes are concentrated in the follicles, and T lymphocytes are concentrated in the paracortical regions, adjacent to the follicles. When B lymphocytes respond to an antigen, the follicle develops a germinal center and becomes hyperplastic (*reactive follicular hyperplasia*). Other slides in PP as well...

Systemic Anaphylaxis

Massive Mast Cell Activation! *Refers to a life threatening systemic allergic reaction* typically characterized by fall in blood pressure with vascular shock, bronchospasm and laryngeal edema with difficulty breathing (*massive mast cell activation*). Inciting agents include therapeutic agents (e.g. beta-lactam antiobiotics such as penicillin and cephalosporins, radiocontrast agents), exposure to food products (e.g. peanuts, seafood), insect toxin (e.g. bee or wasp sting), and latex allergy. Death can ensue within minutes if untreated. Death is usually due to *asphyxiation* from upper airway edema or acute respiratory failure due to bronchial constriction and obstruction, and/or shock with *cardiovascular collapse*. Emergent treatment includes intramuscular epinephrine. In some patients with honeybee allergy, venom immunotherapy is used to prevent future attacks. This form of *desensitization therapy* involves repeated injections of the allergen in increasingly greater amounts resulting in the production of IgG antibodies that can attach to allergens and prevent them from binding to mast cells.

B lymphocytes, Associated "Light Chain Restriction", and Recognition of Proliferative Neoplasm

Monoclonal (neoplastic) proliferations typically produce or express only one type of immunoglobulin, and thus the immunoglobulin light chain expressed by a neoplastic, monoclonal B lymphocyte population will be of either kappa or lambda type (this is called *light chain restriction*). Light chain restriction can be detected by a variety of techniques, including *flow cytometry, serum protein electrophoresis with immunofixation electrophoresis* (e.g. monoclonal spike in myeloma), kappa and lambda immunostaining, and *kappa and lambda in situ hybridization study* (the latter two can be performed on paraffin embedded tissue). In some cases, light chain restriction may be difficult to detect, and it can be problematic to distinguish a B lymphocyte reactive proliferation from a monoclonal (neoplastic) proliferation. *B lymphocyte immunoglobulin gene rearrangement analysis* can help, as the population of neoplastic B lymphocytes will have the same immunoglobulin gene rearrangement.

Innate (natural) Immunity

Refers to pre-existing defense mechanisms present prior to infection that have evolved to recognize microbial pathogens and protect the individual against infection. These defense mechanisms can also recognize nonmicrobial antigens that have been released during cell death or injury. Major components include: -Epithelial barriers -Phagocytic cells (neutrophils and monocytes/macrophages) -Eosinophils, basophils, mast cells -Dendritic cells -Natural killer cells (NK cells) -Innate lymphoid cells -Plasma proteins (complement system, mannose-binding lectin, C-reactive protein, lung surfactant)

Adaptive (acquired, specific) immunity

Refers to reactive immune mechanisms that are stimulated by (adapt to) microbes or other foreign antigens, and are capable of recognizing microbial and nonmicrobial substances. The term "immune response" refers to adaptive immunity. The adaptive immune system consists of lymphocytes and their products, including antibodies. There are two types of adaptive immunity: -Humoral immunity: protects against extracellular microbes and toxins. -Cell-mediated (cellular) immunity: protects against intracellular microbes, tumor cells.

IgG Kappa Myeloma Serum Protein Electrophoresis

Serum protein electrophoresis shows a M-spike (monoclonal spike) in the gamma region measuring 3.63 g/dl (arrows, circles). Immunofixation electrophoresis study shows monoclonal IgG kappa protein (arrows).

Waldenstrom's Macroglobulinemia Serum protein electrophoresis

Serum protein electrophoresis shows a M-spike in the gamma region measuring 2.27 g/dl (arrows). Immunofixation electrophoresis study shows monoclonal IgM kappa protein (arrows).

Waldenstrom's Macroglobulinemia (Histo Slide)

Some of the lymphocytes exhibit plasmacytoid features (arrows).

Immediate (Type 1) Hypersensitivity/Allergic Reactions, Patient Susceptibility

Susceptibility is genetically determined. *Atopy* refers to the predisposition to develop immediate hypersensitivity reactions. A *positive family history of allergy is found in 50% of atopic individuals*. Atopic individuals also tend to have higher serum IgE levels and more IL-4-producing TH2 cells. The antigens that elicit allergic reactions are typically called *allergens*. Incidence of allergic diseases is increasing. As this seems to be related to decreasing infections in early life, some have postulated that improved hygiene has diminished exposure to microbial antigens in early life which "educate" the immune system, setting the stage for subsequent pathologic allergic responses later in life (this is known as the *hygiene hypothesis*).

Tuberculin Skin Test

Test reaction should be read between 48 and 72 hours after administration. A patient who does not return within 72 hours will need to be rescheduled for another skin test. The reaction should be measured in millimeters of the induration (palpable, raised, hardened area or swelling). The reader should not measure erythema (redness). The diameter of the indurated area should be measured across the forearm (perpendicular to the long axis).

Kappa Light Chain Restriction in Chronic Lymphocytic Leukemia (CLL)

The Image shows a portion of a flow cytometry study on peripheral blood in a patient with chronic lymphocytic leukemia (CLL). In the left image, a population of B lymphocytes expressing CD20 predominates (arrow). In the right image, the B lymphocytes consist almost entirely of a subpopulation of B lymphocytes expressing kappa light chain (gray arrow). Virtually no B lymphocytes are present expressing lambda light chain (black arrow). This finding would be called "kappa light chain restriction" and indicates a "monoclonal kappa B cell lymphoproliferative disorder." If the additional flow cytometry markers were consistent with CLL, the diagnosis would be "monoclonal kappa B cell lymphoproliferative disorder exhibiting immunophenotypic features consistent with CLL." Please remember that flow cytometry is performed on fresh and unfixed tissue, blood, or body fluids.

Cirrhosis due to Alcoholic Liver Disease Serum Protein Electrophoresis

The elevated gamma fraction (arrows) appears polyclonal, and no monoclonal protein is seen

Immediate Hypersensitivity, Kinetics of the Immediate and Late-Phase Reactions

The immediate vascular and smooth muscle reaction to allergen develops within minutes after challenge (allergen exposure in a previously sensitized individual), and the late-phase reaction develops 2 to 24 hours later. B, C, Morphology: The *immediate reaction* (B) is characterized by vasodilation, congestion, and edema, and the *late-phase reaction* (C) is characterized by an inflammatory infiltrate rich in eosinophils, along with neutrophils and T cells. *Eosinophilic inflammation is a hallmark of allergic reaction, but can be seen in other situations (e.g. parasitic infection).*

Major Histocompatibility Complex (MHC)

The physiologic function of MHC molecules is to display peptide fragments of proteins for recognition by antigen specific T cells. In humans the MHC complex genes are found on chromosome 6 and are also known as the human leukocyte antigen (HLA) complex as they were initially detected on leukocytes. The MHC gene products are membrane bound glycoproteins which are found on all nucleated cells except mature red blood cells. The HLA system is highly polymorphic (many different alleles of each MHC gene). Key MHC gene products include: Class I MHC molecules: coded by HLA-A, HLA-B, and HLA-C genes; display antigens that are recognized by CD8+ T-lymphocytes and NK cells. Class II MHC molecules: coded by HLA-DP, HLA-DQ, and HLA-DR genes; display antigens that are recognized by CD4+ T lymphocytes.

What can go wrong with the Immune System?

Too little immunity: Disorders with decrease or loss of immune function. These disorders are called immunodeficiencies, and may be either acquired or congenital. Too much immunity: Disorders with pathologic (injurious) immune responses or reactions, called hypersensitivity reactions. Pathologic immune reactions can be seen in: -Allergic reactions -Autoimmune reactions (immune reactions against an individual's own tissue and cells)

Granulomatous inflammation

A section of a lymph node shows several granulomas, each made up of an aggregate of epithelioid cells (histiocytes) and surrounded by lymphocytes (star). The granuloma in the center shows several multinucleate giant cells (arrows). These granulomas would be classified as non-caseating granulomas as no necrosis is seen

Flow Cytometry

A technique that allows the quantification of cells in a stream of fluid by passing them by an electronic detection device. The cells can be incubated with fluorescent labeled antibodies which can bind to specific antigens on the cells. As the cells are detected, the fluorescent signal is also detected, allowing quantification of subpopulations expressing the antigen of interest. This technique is very useful in the identification and classification of leukemias and lymphomas. The above specimen is from a normal lymph node, and it demonstrates a population of B lymphocytes which express CD20 measuring approximately 15% of all cells (left image, arrow). In the right image, the normal B lymphocytes consist of subpopulations expressing kappa and lambda light chains (arrows) with a kappa to lambda ratio of approximately 1.5 to 1.0 (normal ratio ranges from 1.0 - 4.0 to 1.0). Flow cytometry is performed on fresh and unfixed tissue, blood, or body fluids.

HLA Alleles in the Determination of Diseases and its Relationship with Cytokines

A variety of diseases are associated with the inheritance of certain *HLA alleles, and HLA testing can be used to determine disease risk* (e.g. 90% of patients with ankylosing spondylitis are positive for HLA-B27). *HLA testing is also used in the transplantation workup*, as close matches of HLA-A, HLA-B, HLA-C, and HLA-D in both the donor and graft recipient increase the chance of graft survival. Also note that the induction of and regulation of immune responses involve multiple interactions amongst various cells and that many of these interactions are mediated by short acting secreted mediators called cytokines. Knowledge about the role of *cytokines* in immune responses has therapeutic applications (e.g., TNF antagonists for treatment of rheumatoid arthritis, use of recombinant cytokines to enhance immunity).

Mechanisms of antibody-mediated injury

A, Opsonization of cells by antibodies and complement components and ingestion by phagocytes. B, Inflammation induced by antibody binding to Fc receptors of leukocytes and by complement breakdown products. C, Anti-receptor antibodies disturb the normal function of receptors. In these examples, antibodies to the acetylcholine (ACh) receptor impair neuromuscular transmission in myasthenia gravis, and antibodies against the thyroid-stimulating hormone (TSH) receptor activate thyroid cells in Graves disease.

Delayed hypersensitivity reaction in the skin (Histo Slide)

A, Perivascular infiltration by T cells and mononuclear phagocytes, with mild dermal edema. B, Immunoperoxidase staining reveals a predominantly perivascular cellular infiltrate that marks positively with antibodies specific to CD4. An example of this type of delayed hypersensitivity reaction in the skin is the tuberculin reaction, which occurs following intracutaneous injection of purified protein derivative (PPD, a protein-containing antigen from the tubercle bacillus), in a individual previously sensitized to TB. This reaction is clinically seen as an area of induration with varying degrees of redness 12-72 hours following injection. Intracutaneous injection of PPD is also known as the Mantoux test or the tuberculin skin test (TST).

Mechanisms of T cell-mediated (type IV) hypersensitivity reactions

A. In cytokine mediated inflammation, upon repeat exposure to the antigen, CD4+ TH1 cells (and sometimes CD8+ T cells, not shown) respond to tissue antigens by secreting cytokines that stimulate inflammation and activate phagocytes, leading to tissue injury. CD4+ TH17 cells contribute to inflammation by recruiting neutrophils (and, to a lesser extent, monocytes). Examples of this mechanism include the tuberculin skin test (PPD test), forms of granulomatous inflammation, contact dermatitis, multiple sclerosis, and inflammatory bowel disease. B. In some diseases, CD8+ cytotoxic T lymphocytes (CTLs) directly kill tissue cells. Examples of this mechanism include type I diabetes and graft rejection. APC, antigen-presenting cell.

IgG Kappa Myeloma (Histo Slide)

Bone marrow clot section (Left) and bone marrow smear (Right) show numerous atypical, neoplastic plasma cells (arrows point to just a few of the numerous neoplastic plasma cells present).

Antibody-Mediated (Type II) Hypersensitivity

Caused by *antibodies that react with normal or altered cell surface antigens, or with antigens in the extracellular matrix*. This can cause disease by destroying cells, triggering inflammation, or by interfering with normal function. The antibodies may be specific for normal cell or tissue antigens (autoantibodies), or for exogenous antigens, such as chemical (e.g. drug) or microbial proteins, that bind to a cell surface or tissue.

Immune Complex-Mediated (Type III) Hypersensitivity

Caused by *deposition of antigen-antibody complexes* which elicit inflammation at the sites of deposition. The pathologic reaction begins when *antigen in the circulation combines with antibody in the circulation, and the circulating immune complexes are deposited in vessel walls*. In some instances, the complexes can form at extravascular sites where the antigen has been "planted" previously (called in situ immune complex deposition). The involved antigens can be exogenous, such as an injected foreign protein or an antigen produced by a microbe, or endogenous if the individual produces an antibody against self antigens (autoimmune disease). The sites of involvement can be systemic or localized. Blood vessels, as well as organs where blood is filtered to form other fluids, such as kidneys and synovium, are common sites of injury. Thus patients may develop symptoms related to vasculitis, glomerulonephritis, and arthritis. Key steps in immune complex-mediated hypersensitivity are: -Immune complex formation -Immune complex deposition -Immune complex-mediated inflammation and tissue injury.

T-Cell Mediated (Type IV) Hypersensitivity

Caused by *inflammation resulting from cytokines produced by CD4+ T lymphocytes and cell killing by CD8+ T lymphocytes* (no antibodies involved). CD4+ or CD8+ T lymphocytes are sensitized to exogenous or endogenous antigens. The resultant T cell immune response results in cell or tissue injury. There are two mechanisms of T cell mediated (type IV) hypersensitivity: CD4+ T cell-mediated inflammation CD8+ T cell-mediated cytotoxicity

Immediate (Type 1) Hypersensitivity/Allergic Reactions

Defined as a rapid immunologic reaction occurring within minutes after an antigen combines with antibody bound to mast cells in individuals previously sensitized to the antigen (allergic reaction). The reaction is typically *mediated by IgE antibody-dependant activation of mast cells, with degranulation and release of mast cell contents*. Initial exposure to the antigen (allergen) results in activation of B cells with the production of IgE, which attaches to mast cells (*this first step is called sensitization*). *Repeat exposure to the antigen (allergen) results in mast cell degranulation*, with the release of chemical mediators, causing vasodilation, vascular leakage, smooth muscle spasm, and recruitment of leukocytes, particularly eosinophils. *Eosinophils secrete major basic protein and eosinophil cationic protein, which are toxic to epithelial cells*. Activated eosinophils and neutrophils also activate mast cells to release mediators, amplifying and sustaining the inflammatory response without additional exposure to the triggering antigen (*late phase reaction*).

Cell-mediated Immunity

Dendritic cells (DCs) capture microbial antigens from epithelia and tissues and transport the antigens to lymph nodes. During this process, the DCs mature, and express high levels of MHC molecules and costimulators. Naive T cells recognize MHC-associated peptide antigens displayed on DCs. The T cells are activated to proliferate and to differentiate into effector and memory cells, which migrate to sites of infection and serve various functions in cell-mediated immunity. CD4+ effector T cells of the TH1 subset recognize the antigens of microbes ingested by phagocytes, and activate the phagocytes to kill the microbes; other subsets of effector cells enhance leukocyte recruitment and stimulate different types of immune responses. CD8+ cytotoxic T lymphocytes (CTLs) kill infected cells harboring microbes in the cytoplasm. Some activated T cells remain in the lymphoid organs and help B cells to produce antibodies, and some T cells differentiate into long-lived memory cells (not shown). APC, Antigen-presenting cell. "Follow the boxes in the Image"

Pemphigus vulgaris

Due to autoantibodies directed against desmogleins 1 and 3 which are found in desmosomes. Above right, direct immunofluorescence of pemphigus vulgaris: There is deposition of immunoglobulin along the plasma membranes of epidermal keratinocytes in a reticular or fishnet-like pattern. Also note the early suprabasal separation due to loss of cell-to-cell adhesion (acantholysis, bottom of image). Above left, suprabasal acantholysis results in an intraepidermal blister in which rounded (acantholytic) epidermal cells are identified; also note presence of inflammatory cells (inset).

T-Lymphocytes, Gene Rearrangement, and Neoplastic Proliferations

Each T lymphocyte recognizes a specific cell bound antigen by means of an antigen specific T-cell receptor (TCR). Monoclonal (neoplastic) proliferations of T lymphocytes will have the *same TCR gene rearrangement.* Neoplastic proliferations of T lymphocytes are often recognized on the basis of *flow cytometry studies, or by analysis of immunostains for T lymphocyte markers on paraffin embedded tissue*. In some situations neoplastic proliferations of T lymphocytes can be difficult to recognize, and use of *TCR gene rearrangement analysis* can be used to determine if a T lymphocyte proliferation is neoplastic, as the population of neoplastic T lymphocytes will have the same TCR gene rearrangement.

Table of Antibody-Mediated or Type II Hypersensitivity

Emphasize the importance of specific autoantibodies in both pathogenesis and diagnosis of organ specific autoimmune disease.

Immediate (Type 1) Hypersensitivity/Allergic Reaction, Localized and Systemic Reactions

*Allergic reactions can be localized or systemic. * A few examples of localized allergic reactions with pathologic manifestations include: -*Allergic rhinitis, sinusitis* (hay fever): Increased mucus secretion and inflammation -Some forms of *bronchial asthma* (atopic forms): Airway obstruction caused by bronchial smooth muscle hyperactivity; inflammation and tissue injury caused by the late-phase reaction. -*Food allergies* (allergic gastroenteritis): increased peristalsis due to contraction of intestinal mucosa. -*Urticaria (hives)*: often a manifestation of a systemic reaction, e.g. allergic drug reaction, may precede anaphylaxis) *Triggering allergens are numerous* and include such things as pollens, molds, house dust, animal dander, foods, medications, blood products, venom from insect bites, etc.

CD4+ vs CD8+ T-Cells

*CD4+ T cell-mediated inflammation*: -In response to repeat exposure of an antigen, CD4+ T cells secrete cytokines, leading to recruitment and activation of macrophages and neutrophils. This inflammatory response leads to tissue injury. -One classic example of CD4+ T cell-mediated inflammation is the delayed-type hypersensitivity reaction, a tissue reaction to antigens given to immune individuals. In this reaction, antigen administered into the skin of a previously immunized individual results in a detectable skin reaction within 24-48 hours (hence the term delayed, in contrast to immediate hypersensitivity reactions). *CD8+ T cell-mediated cytotoxicity*: -CD8+ cytotoxic T cells kill antigen-expressing target cells.

Reactive Plasma Cell Proliferation of a Tonsil (Histo Slide)

Arrow points to a few of the many plasma cells present

Goodpasture Syndrome

Didney: Left, schematic of the glomerulus with a capillary loop. Circulating autoantibodies to antigens intrinsic to the glomerular basement membrane results in antibody mediated injury. Right, the resulting anti-GBM immune complexes can be visualized with immunoflourescence microscopy as a *smooth, linear pattern of deposition (DOT!).*

Diagnostic Criteria for Anaphylaxis and Fatalities Due to Anaphylaxis

In a series of 164 fatalities due to anaphylaxis, the median time interval between onset of symptoms and respiratory or cardiac arrest was 5 minutes in iatrogenic anaphylaxis (meaning induced inadvertently by medical or surgical treatment, or by diagnostic procedures), 15 minutes in stinging insect venom-induced anaphylaxis, and 30 minutes in food-induced anaphylaxis.

Sequence of events in Immediate (Type I) Hypersensitivity.

Initiated by the introduction of an allergen, which stimulates TH2 responses and IgE production in genetically susceptible individuals. IgE binds to Fc receptors (FcεRI) on mast cells, and subsequent exposure to the allergen activates the mast cells to secrete the mediators that are responsible for the pathologic manifestations of immediate hypersensitivity. *In some individuals, immediate hypersensitivity reactions are triggered by temperature extremes and exercise (non-atopic allergy) and do not involve T helper cells or IgE. In these cases it is believed that the mast cells are abnormally sensitive to activation by various non-immune stimuli.* Image: *FOLLOW THE BOXES*

Normal Serum Protein Electrophoresis

Normal serum protein electrophoresis study (left) with normal immunofixation electrophoresis study (right). The gamma fraction (arrows) appears polyclonal, and no monoclonal protein is seen.

Tuberculosis, Caseating Granuloma

Note central necrosis (stars).

IgG Kappa Myeloma, Situ Hybridization Study

Patient with IgG kappa myeloma. Kappa (right) and lambda (left) in situ hybridization study on bone marrow sections show marked kappa light chain restriction.

Mechanisms of granuloma formation

Schematic illustration of the events that give rise to the formation of granulomas in cell-mediated (type IV) hypersensitivity reactions. Note the role played by activated T cells with the release of cytokines. A ganulomatous reaction can occur with certain persistent or nondegradable antigens (e.g. TB, fungi). Granulomas can also occur as a reaction to foreign bodies without eliciting an adaptive immune response.

Contact dermatitis, Slide Histo

The lesion shows an epidermal blister (vesicle) with dermal and epidermal mononuclear infiltrates.

Immune System Overview

Think of the immune system as an organ system. The components of the immune system include cellular components as well as non-cellular components (e.g. antibodies, cytokines). The function of the immune system is to protect against harmful micro-organisms as well as protect against cellular agents such as malignant cells.

Foreign Body Granulomatous Inflammation

Prostate (TURP, left); suture granulomas (above right and right)

Granulomatous Inflammation

Granulomatous inflammation is a *distinctive pattern of chronic inflammation* encountered in a limited number of infectious and non infectious conditions. It is a cellular attempt to contain an offending agent which is difficult to eradicate. Typically there is strong activation of T lymphocytes, leading to activated macrophages, resulting in tissue injury. A granuloma is a *focus of chronic inflammation consisting of a microscopic aggregation of macrophages that are transformed into epithelial-like cells (called histiocytes)*, with variable numbers of lymphocytes and plasma cells. The epithelial-like cells *(histiocytes) may also fuse to form multi-nucleated giant cells.* Granulomas are the *result of either immune reactions or are a reaction to foreign material*, and can be categorized by their morphologic appearance: Type I, *Foreign body granulomas*: see foreign material within histiocytes/giant cells, sometimes called "foreign body giant cell reaction" by pathologists Type II, *Caseating granulomas*: granulomas that induce cell mediated immune response with central necrosis; these are usually associated with infection (e.g. mycobacterial, fungal infections) Type III, *Non-caseating granulomas*: granulomas that induce cell mediated immune response without central necrosis (e.g. Sarcoidosis, Crohn's disease)

Immediate (Type 1) Hypersensitivity/Allergic Reaction, Treatment and Diagnosis

In some cases, it becomes desirable to identify the offending allergen. This can be done with skin prick test (histamine-mediated wheal and flare reaction) or with allergy blood tests (allergy screening test to measure IgE antibodies to specific allergens). Treatment typically involves: -Avoiding the offending allergen, if possible. -Use of various medications, such as antihistamines, corticosteroids, agents that inhibit release of histamine from mast cells, leukotriene modifiers (to name just a few). -In some cases, immunotherapy (desensitization therapy) is also used

Polyarteritis nodosa (PAN).

In this systemic vasculitis, there is segmental fibrinoid necrosis and thrombotic occlusion of the lumen of this small artery (white arrows). Note that part of the vessel wall at the upper right (small black arrow) is uninvolved. About 30% of patients with PAN have chronic hepatitis B and deposits of HBsAg-HBsAb immune complexes in affected vessels.

quantiFERON-TB test

In this test, the patients lymphocytes are incubated for 16-24 hours with a cocktail of TB antigens. If the individual has been previously exposed (sensitized) to TB, the T lymphocytes will release cytokines (IFN-gamma) which can be measured using an immunoassay.

Hypersensitivity Reactions

Individuals previously exposed to an antigen are said to be sensitized; upon repeat exposure(s), some individuals will develop an excessive, injurious pathologic immune reaction to the antigen. These *pathologic immune reactions are called hypersensitivity reactions*, and are the basis of the pathology associated with immune diseases. Some key general points about hypersensitivity reactions are listed below: -*Both exogenous (environmental) and endogenous self antigens may elicit hypersensitivity reactions* (e.g., allergic reactions, autoimmune diseases). -Hypersensitivity usually results from an *imbalance between the effector mechanisms of immune responses and the control mechanisms that serve to normally limit such responses.* -The development of hypersensitivity reactions (both allergic and autoimmune disorders) is often *associated with the inheritance of particular susceptibility genes (HLA and non-HLA genes).* The mechanisms of tissue injury in hypersensitivity reactions are similar to the effector mechanisms of defense against infectious pathogens. *The problem in hypersensitivity is that the reactions are poorly controlled, excessive, or misdirected* (e.g. against normally harmless environmental antigens or self antigens). *Diseases due to hypersensitivity reactions can be classified on the basis of the immunologic mechanism that mediates the disease*. However, multiple mechanisms may be occurring in some diseases.

Waldenstrom's macroglobulinemia Situ Hybridization Study

Kappa (left) and lambda (right) in situ hybridization study of patient's bone marrow shows marked kappa light chain restriction.

Kappa and Lambda Stain

Kappa (top) and lambda (bottom) in situ hybridization study in patient with reactive plasma cell proliferation. No light chain restriction is seen (polyclonal proliferation).

Kidney, an example of immune complex deposition pattern of injury in Type III Hypersensitivity

Left, circulating immune complexes are deposited in the glomerulus, resulting in a *granular pattern of deposition* on immunofluoresence microscopy (right). Granular deposition of immune complexes can be seen in SLE and poststreptococcal glomerulonephritis.

Immune Complex-Mediated (Type III) Hypersensitivity Pathogenesis and Fibrinoid Necrosis

Left: pathogenesis of systemic immune complex-mediated disease (type III hypersensitivity). The three sequential phases in the development of immune complex diseases are shown. Complement is consumed as part of the pathogenic process, and the *levels of C3 can be used to monitor disease activity* (low levels indicate active disease). Right: Immune complex vasculitis. The necrotic vessel wall is replaced by smudgy, pink "fibrinoid" material (*fibrinoid necrosis*).

Humoral immunity

Naive B lymphocytes recognize antigens, and under the influence of TH cells and other stimuli (not shown), the B cells are activated to proliferate and to differentiate into antibody-secreting plasma cells. Some of the activated B cells undergo heavy-chain class switching and affinity maturation, and some become long-lived memory cells. Antibodies of different heavy-chain classes (isotypes) perform different effector functions, shown on the right. Note that the antibodies shown are IgG; these and IgM activate complement; and the specialized functions of IgA (mucosal immunity) and IgE (mast cell and eosinophil activation) are not shown. *Emphasize: why polyclonal, loss of antibodies, loss of spleen and susceptibility to encapsulated organisms, complement deficiency, and monoclonal proliferations*

Waldenstrom's Macroglobulinemia, Flow Cytometry

Patient's bone marrow shows a population of CD20 positive B lymphocytes exhibiting marked kappa light chain restriction (arrows).

Mast cell mediators in Type 1 Hypersensitivity

Upon activation, mast cells release various classes of mediators that are responsible for the immediate and late-phase reactions. ECF, eosinophil chemotactic factor; NCF, neutrophil chemotactic factor); PAF, platelet-activating factor.