Chapter 22 Lymphatic system and Immune Response

What happens to foreign substances in lymph that enter a lymph node? (FIG 22.6 Q)

Foreign substances in lymph that enter a lymph node may be phagocytized by macrophages or attacked by lymphocytes that mount immune responses.

What chemicals are responsible for killing ingested microbes? (FIG 22.9 Q)

Lysozyme, digestive enzymes, and oxidants can kill microbes ingested during phagocytosis.

How are the activities of natural killer cells and phagocytes similar and different?

Natural killer (NK) cells are lymphocytes (a subclass of white blood cells) that recognize infected or tumorogenic cells and kill them. Phagocytic cells clean up behind them.

What happens once a Cytotoxic T Cell is activated?

Once a cytotoxic T cell is activated, it forms a clone of active cytotoxic T cells and memory cytotoxic T cells.

What happens once a helper T cell is activated?

Once a helper T cell is activated, it forms a clone of active helper T cells and memory helper T cells.

What physical and chemical factors provide protection from disease in the skin and mucous membranes?

PHYSICAL FACTORS: 1) EPIDERMIS - due to is many closely packed keratinised cells, provides physical barrier to the entrance of microbes; 2) EPIDERMAL CELLS - the periodic shedding of epidermal cells helps remove microbes from skin surface, preventing bacteria from penetrating epidermis; 3) EPITHELIAL LAYER OF MUCUS MEMBRANES - (which line body cavities) secretes mucus that lubricates and moistens cavity surface. Due to viscosity of mucus, it traps microbes & foreign substances; 4) MUCUS COATED HAIRS OF NOSE - trap & filter microbes, dust & pollutants from inhaled air; 5) CILIA OF UPPER RESPIRATORY TRACT - waving action of cilia propels inhaled dust & microbes that have become trapped in mucus toward throat; 6) LACRIMAL APPARATUS OF EYES - manufactures & drains away tears in response to irritants; 7) LYSOZOME OF TEARS, SALIVA, PERSPIRATION, NASAL SECRETIONS & TISSUE FLUIDS - breaks down the cell walls of certain bacteria; 8) SALIVA- produced by salivary glands, washes microbes from surfaces of teeth & from mucus membrane of the mouth. Flow of saliva reduces colonisation of the mouth by microbes. 9) FLOW OF URINE - retards microbial colonisation of the urinary system; 10) VAGINAL SECRETIONS - move microbes out of female body; 11) DEFECATION & VOMITING - expel microbes. CHEMICAL FACTORS 12) SEBACEOUS GLANDS - secrete sebum that forms a protective film over surface of skin; 13) PERSPIRATION - helps flush away microbes from surface of skin; 14) GASTRIC JUICE - Produced by glands of stomach - it is a mixture of hydrochloric acid, enzymes & mucus. Acidity of Gastric Juice destroys many bacteria & most bacteria toxins.

What do positive selection, negative selection, and anergy accomplish?

POSITIVE SELECTION: pre-T cells recognize T cell receptors and are part of an antigen MHC complex, these T cells survive. NEGATIVE SELECTION: T cells interact with dendritic cells located at the junction of the cortex and medulla in the thymus, T cells that recognize self antigens are deleted or put into ANERGY (prolonged state of inactivity) when they remain alive but are unresponsive to antigenic stimulation.

What causes each of the following signs and symptoms: redness, pay, heat & swelling? (FIG 22.10 Q)

REDNESS- results from increased blood flow due to vasodilation; PAIN- from injury of nerve fibers, irritation by microbial toxins, kinins, and prostaglandins, and pressure due to edema; HEAT- from increased blood flow and heat released by locally increased metabolic reactions; SWELLING - from leakage of fluid from capillaries due to increased permeability.

What tissue contains stem cells that develop into lymphocytes? (FIG 22.1 Question)

Red bone marrow contains stem cells that develop into lymphocytes.

What is the Route of Lymph flow through a Lymph Node? (Fig 22.6)

Route of lymph flow through lymph node: 1. Afferent lymphatic vessel--> 2. Subcapsular sinus--> 3. Trabecular sinus--> 4. Medullary sinus--> 5. Efferent lymphatic vessel (Tortora 806)

What are the main signs, symptoms, and stages of inflammation?

SIGNS: Redness, heat, swelling, pain, loss of function depending on site of injury & extent of the injury. 3 BASIC STAGES: 1) VASODILATION & INCREASED PERMEABILITY OF BLOOD VESSELS: 2) EMIGRATION OF PHAGOCYTES FROM BLOOD INTO INTERSTITIAL FLUID; 3) TISSUE REPAIR.

How many different kinds of antibodies will be secreted by the plasma cells in the clone shown in FIG 22.18?

Since all of the plasma cells in this figure are part of the same clone, they secrete just one kind of antibody.

Which type of lymphocytes mature in the thymus? (FIG 22.5 Q)

T Cells mature in the Thymus.

How do the five classes of antibodies differ in structure and function?

Table 22.3 page 827 Tortora - 5 CLASSES OF ANTIBODIES - CHARACTERISTICS & FUNCTIONS 1. IgG = Most abundant, about 80% of all antibodies in blood; found in blood, lymph, and intestines; monomer (one-unit) structure. Protects against bacteria and viruses by enhancing phagocytosis, neutralizing toxins, and triggering complement system. Is the only class of antibody to cross placenta from mother to fetus, conferring considerable immune protection in newborns. 2. IgA = Found mainly in sweat, tears, saliva, mucus, breast milk, and gastrointestinal secretions. Smaller quantities are present in blood and lymph. Makes up 10-15% of all antibodies in blood; occurs as monomers and dimers (two units). Levels decrease during stress, lowering resistance to infection. Provides localized protection of mucous membranes against bacteria and viruses. 3. IgM = About 5-10% of all antibodies in blood; also found in lymph. Occurs as pentamers (five units); first antibody class to be secreted by plasma cells after initial exposure to any antigen. Activates complement and causes agglutination and lysis of microbes. Also present as monomers on surfaces of B cells, where they serve as antigen receptors. In blood plasma, anti-A and anti-B antibodies of ABO blood group, which bind to A and B antigens during incompatible blood transfusions, are also IgM antibodies (see Figure 19.12). 4. IgD = Mainly found on surfaces of B cells as antigen receptors, where it occurs as monomers; involved in activation of B cells. About 0.2% of all antibodies in blood. 5. IgE = Less than 0.1% of all antibodies in blood; occurs as monomers; located on mast cells and basophils. Involved in allergic and hypersensitivity reactions; provides protection against parasitic worms.

What is the function of the CD8 protein of a cytotoxic T cell? (FIG 22.16 Q)

The CD8 protein of a cytotoxic T cell binds to the MHC-I molecule of an infected body cell to help anchor the T-cell receptor (TCR)-antigen interaction so that antigen recognition can occur.

What is the role of the Thymus in immunity?

The Thymus serves a vital role in the training & development of T-Lymphocytes (T-Cells) (a type of white blood cell). T Cells are involved in adaptive immunity. Mature T Cells leave the Thymus via the blood and are carried to lymph nodes, the spleen, and other lymphatic tissues where they populate parts of these organs/tissues.

When is the bilobed thymus largest and when does the function proportion of the bilobed thymus atrophy?

The bilobed thymus is largest at puberty and then the functional portion atrophies with age.

Which pathway for activation of complement involves antibodies? Explain why. (FIG 22.20 Q)

The classical pathway for the activation of complement is linked to antibody-mediated immunity because Ag-Ab complexes ac- tivate C1.

In what ways does the complement system augment antibody-mediated immune responses?

The classical pathway starts when antibodies bind to antigens (microbes). The antigen-antibody complex binds and activates C1. Eventually C3 is activated and the C3 fragments initiate phagocytosis, cytolysis, and inflammation.

What functions do the Tonsils serve?

The five tonsils form a ring at the junction of the oral cavity, nasal cavity, and throat. They are strategically placed to participate in immune responses against inhaled or ingested foreign substances.

Which lymphatic vessels empty into the cisterna chyli, and which duct receives lymph from the cisterna chill? (FIG 22.3 Q)

The left and right lumbar trunks and the intestinal trunk empty into the cisterna chyli, which then drains into the thoracic duct.

What does the Lymphatic system consist of?

The lymphatic system consists of lymph, lymphatic vessels, lymphatic tissues, and red bone marrow.

What are the major types of Phagocytes?

The major types of phagocytes are neutrophils and macrophages. (Tortora 813)

How is the secondary response to an antigen different from the primary response?

The secondary response is much more accelerated and intense. Antibodies produced during this phase have a higher affinity for the antigen than those produced during the primary response, this they are more successful in disposing of it.

What is the sequence of fluid flow of the lymphatic system to the cardiovascular system?

The sequence of fluid flow is blood capillaries (blood) n interstitial spaces (interstitial fluid) n lymphatic capillaries (lymph) n lymphatic vessels (lymph) n lymphatic ducts (lymph) n junction of the internal jugular and subclavian veins (blood).

What do complement proteins enhance when they have been activated?

When activated, complement proteins enhance phagocytosis, cytolysis, and inflammation.

After birth, what are the main functions of the spleen? (Fig 22.7 Q)

White pulp of the spleen functions in immunity; red pulp of the spleen performs functions related to blood cells.

What are the functions of the Lymphatic system?

1. Drains excess interstitial fluid. 2. Transports dietary lipids from the gastrointestinal tract to the blood. 3. Protect against invasion through immune responses.

What internal defenses provide protection against microbes that penetrate the skin and mucous membranes?

1) INTERNAL ANTIMICROBIAL SUBSTANCES - That discourage microbial growth: -INTERFERONS (INFs): Protect uninfected host cells from viral infection. -COMPLEMENT SYSTEM: Causes cytolysis of microbes; promotes phagocytosis; contributes to inflammation. -IRON BINDING PROTEINS: Inhibit growth of certain bacteria by reducing amount of available iron. -ANTIMICROBIAL PROTEINS: Have broad-spectrum antimicrobial activities and attract dendritic cells and mast cells. 2) PHAGOCYTES - ingest microbes or other particles such as debris. two types: NEUTROPHILS & MACROPHAGES. 3) NATURAL KILLER CELLS - Kill infected target cells by releasing granules that contain perforin and granzymes; phagocytes then kill released microbes. 4) INFLAMATION- traps microbes, toxins or foreign material and initiates tissue repair. 5) FEVER - intensifies effects of interferons. Inhibits bacterial growth & speeds up tissue repair. (Table 22.1 Tortora 814)

How do Lymphatic Vessels differ in structure from veins?

1) Lymph vessels being as lymphatic capillaries which are closed at one end & located in space between cells; 2) Lymphatic capillaries are slightly larger than blood capillaries; 3) Lymph capillaries have unique structure that permits interstitial fluid to flow INTO them but NOT OUT OF THEM; 4) Lymph capillaries begin in the tissues & carry the lymph that forms there toward a larger lymphatic vessel, whereas Blood Capillaries link two larger blood vessels that form part of a circuit; 5) Lymphatic capillaries unite to form larger & larger lymph vessels, WHEREAS Blood Capillaries unite to form venues & veins; 6) Lymph vessels resemble veins in structure BUT have thinner walls and more valves; 7) From lymphatic vessels, lymph eventually passes into one OF two main channels. The: a) Thoracic Duct; or (b) Right Lymphatic Duct.

How are cell-mediated and antibody-mediated immune responses similar and different?

ANTIBODY MEDIATED IMMUNITY (AMI) = refers to destruction antigens by antibodies. - B cells transform in Plasma cells; - Plasma cells then synthesise and secrete specific proteins called antibodies. Works mainly against antigens dissolved in body Fluids and extracellular pathogens, primarily bacteria, that multiply in body fluids but rarely enter cells. Specificity = Millions of different B cells, each capable of responding to a specific antigen. CELL MEDIATED IMMUNITY (CMI) = refers to destruction of antigens by T cells. - Killer T cells attack antigens; - Helper T Cells co-stimulate T & B cells. Works effectively against intracellular pathogens, such as fungi, parasites & viruses; some cancer cells; and foreign tissue transplants. CMI always involves cells attacking cells.

How does Lymph return to the blood stream?

All lymph returns to the bloodstream through the thoracic (left) lymphatic duct and right lymphatic duct.

An antibody combines only with the ** (fill in blank) on the antigen that triggered its production.

An antibody combines only with the EPITOPE on the antigen that triggered its production.

What do Plasma cells secrete?

Antibodies.

How do antigens arrive at lymphatic tissues?

Antigens that get past the innate defenses generally follow one of three routes into lymphatic tissue: (1) Most antigens that enter the bloodstream (for example, through an injured blood vessel) are trapped as they flow through the spleen. (2) Antigens that pen- etrate the skin enter lymphatic vessels and lodge in lymph nodes. (3) Antigens that penetrate mucous membranes are entrapped by mucosa-associated lymphatic tissue (MALT). (Tortora 817) The body undergoes Antigen recognition - B & T Cells must recognise a foreign antigen before beginning their immune response. Once recognised, the Antigens are Processed. The process will depend on the type of antigen which is required to be eliminated from the body. EXOGENOUS ANTIGENS = foreign antigens present in fluid OUTSIDE BODY CELLS. *After processing an antigen, the antigen-presenting cell migrates to lymphatic tissue to present the antigen to T cells. Within lymphatic tissue, a small number of T cells that have compatibly shaped receptors recognize and bind to the antigen fragment- MHC-II complex, triggering an adaptive immune response. The presentation of exogenous antigen together with MHC-II molecules by antigen-presenting cells informs T cells that intruders are present in the body and that combative action should begin. (Tortora 819) ENDOGENOUS ANTIGENS = Foreign antigens present WITHIN BODY CELLS. The display of an endogenous antigen bound to an MHC-I molecule signals that a cell has been infected and needs help. (Tortora 819)

According to the graph in Figure 22.21, how much more IgG is circulating in the blood in the secondary response than in the primary response? (Hint: Notice that each mark on the antibody titer axis represents a 10-fold increase.)

At peak secretion, approximately 1000 times more IgG is produced in the secondary response than in the primary response.

How are interstitial fluid and lymph fluid similar?

Both fluids are chemically similar to blood plasma. NB: Interstitial fluid passes into lymphatic vessels & there it becomes lymphatic fluid. (Tortora 800).

What are cytokines, where do they arise, and how do they function?

CYTOKINES: Are small protein hormones that stimulate or inhibit many normal cell functions, such as cell growth and differentiation. (Tortora 820) ARISE BY: Lymphocytes and antigen-presenting cells secrete cytokines, as do fibroblasts, endothelial cells, monocytes, hepato- cytes, and kidney cells. GENERAL FUNCTIONS: Some cytokines stimulate proliferation of progenitor blood cells in red bone marrow. Others regulate activi- ties of cells involved in innate defenses or adaptive immune responses. a) Stimulate or inhibit many normal cell functions, i.e cell growth & differentiation; b) Stimulate proliferation of progenitor blood cells in red bone marrow; c) regulate activities of cells involved in innate defences or adaptive immune responses. TYPES OF CYTOKINES - ORIGINS & FUNCTIONS: (TABLE 22.2) 1) INTERLEUKIN-1 (IL-1) = Produced by macrophages; promotes proliferation of helper T cells; acts on hypothalamus to cause fever. 2) INTERLEUKIN-2 (IL-2) = Secreted by helper T cells; costimulates proliferation of helper T cells, cytotoxic T cells, and B cells; activates NK cells. 3) INTERLEUKIN-4 (IL-4) = Produced by helper T cells; costimulator for B cells; causes plasma cells to secrete IgE antibodies (see Table 22.3); promotes growth of T cells. 4) INTERLEUKIN-5 (IL-5) = Produced by some helper T cells and mast cells; costimulator for B cells; causes plasma cells to secrete IgA antibodies." 5) INTERLEUKIN-6 (IL-6) =Produced by helper T cells; enhances B cell proliferation, B cell differentiation into plasma cells, and secretion of antibodies by plasma cells." 6) TUMOUR NECROSIS FACTOR (TNF) = Produced mainly by macrophages; stimulates accumulation of neutrophils and macrophages at sites of inflammation and stimulates their killing of microbes." 7) INTERFERONS (INFs) = Produced by virus-infected cells to inhibit viral replication in uninfected cells; activate cytotoxic T cells and natural killer cells, inhibit cell division, and suppress the formation of tutors. 8) MACROPHAGE MIGRATION INHIBITING FACTOR = "Produced by cytotoxic T cells; prevents macrophages from leaving site of infection.

In addition to cells infected by microbes, what other types of target cells are attacked by cytotoxic T cells? (FIG 22.17 Q)

Cytotoxic T cells attack some tumor cells and transplanted tissue cells, as well as cells infected by microbes.

How do cytotoxic T cells kill infected target cells?

Cytotoxic T cells have two principal mechanisms for killing infected target cells. 1. RELEASING GRANZYMES: Cytotoxic T cells, using receptors on their surfaces, recognize and bind to infected target cells that have microbial antigens displayed on their surface. The cytotoxic T cell then releases granzymes, protein-digesting enzymes that trigger apoptosis (Figure 22.17a). Once the infected cell is destroyed, the released microbes are killed by phagocytes. 2. RELEASING PROTEINS FROM GRANULES: Cytotoxic T cells bind to infected body cells and release two proteins from their granules: perforin and granulysin. -PERFORIN - inserts into the plasma membrane of the target cell and creates channels in the membrane. This allows ECF to flow into the target cell and cause cytolysis.(Figure 22.17b). - GRANULYSIN - Enters through the channels and destroys the microbes by creating holes in their plasma membranes. Other Mechanisms: 3. SECRETING LYMPHOTOXIN: That activates enzymes in the target cell causing its DNA to fragment. 4. SECRETE GAMMA-INTERFERON: To activate phagocytic cells. (refer to slides 37 & 38 of Session 9 Lecture).

What type of enzyme do Cytotoxic T Cells release? What does this type of enzyme trigger in the infected target cells?

Cytotoxic T cells release granzymes that trigger apoptosis and perforin that triggers cytolysis of infected target cells.

What types of cells are APCs, and where in the body are they found? (FIG 22.13 Question)

Dendritic cells, macrophages, and B cells. Location: Strategically located in places where antigens are likely to penetrate the innate defenses and enter the body, such as: a) EPIDERMIS & DERMIS of the skin (intraepidermal macrophages are a type of dendritic cell); b) MUCOUS MEMBRANES that line the respiratory, gastrointestinal, urinary, and reproductive tracts; and c) LYMPH NODES. After processing and presenting an antigen, APCs migrate from tissues via lymphatic ves- sels to lymph nodes.

What is the difference between an epitope and a happen? (Fig 22.12 Question).

EPITOPES= are small immunogenic parts of a larger antigen; HAPTENS= are small molecules that become immunogenic only when they attach to a body protein. (Tortora 839)

What are some examples of endogenous antigens? (FIG 22.14 Question)

Endogenous antigens include viral proteins, toxins from intracellular bacteria, and abnormal proteins synthesized by a cancerous cell.

What are the first and second signals in activation of a T cell? (FIG 22.15 Q)

FIRST SIGNAL in T Cell activation = Antigen recognition by a TCR with CD4 or CD8 proteins. SECOND SIGNAL = Costimulation (by a costimulator), such as a cytokine or another pair of plasma membrane molecules.

What are the functions of helper, cytotoxic, and memory T cells?

HELPER T CELLS - Function to empower macrophages so that they can destroy intravesicular pathogens (bacteria). CYTOTOXIC T CELLS - Cytotoxic T (or CD8) cells are activated in your lymph nodes by dendritic cells. Once activated they are sent out to the site of infection and they bind to the cells that express the MHC class I that are presenting the foreign antigen. They will then release perforin (punch holes in the infected cell) and granzymes (induces apoptosis). The infected cells are now destroyed. MEMORY T CELLS - Memory cells are made at this point for future protection and FasR & CTLA-4 is released from the helper T cells to disable the cytotoxic T cells.

Which type of T cell participates in both cell- mediated and antibody- mediated immune responses? (Fig 22.11 question)

Helper T cells participate in both cell-mediated and antibody- mediated immune responses." (Tortora 839)

What is immunocompetence, and which body cells display it?

Immunocompetence is the ability to carry out adaptive immune responses. Before T cells leave the thymus or B cells leave red bone mar- row, they develop immunocompetence. This means that B cells and T cells begin to make several distinctive proteins that are inserted into their plasma membranes. Some of these proteins function as antigen receptors—molecules capable of recognizing specific antigens (Figure 22.11). (Tortora 816)

What is Immunological memory the basis for?

Immunological memory is the basis for successful immunization by vaccination.

How is immunological surveillance useful?

Immunological surveillance is the immune system's response to finding, recognising and destroying cells with tumour antigens. Carried out by Cytotoxic cells, macrophages & Natural killer cells. USEFUL: because it can destroy any cancer cells carrying the Tumour Antigen.

Does inhalation promote or hinder the flow of lymph? (FIG 22.4 Q)

Inhalation promotes the movement of lymph from abdominal lymphatic vessels toward the thoracic region because the pressure in the vessels of the thoracic region is lower than the pressure in the abdominal region when a person inhales.

How do the major histocompatibility complex class I and class II self-antigens function?

Located in the plasma membrane of body cells are "self-antigens," the major histocompatibility complex (MHC) antigens. These are Glycoproteins located in plasma membrane of all body cells & they are specifically unique to you unless you have an identical twin. Class I MHC (MHC-I) molecules are built into the plasma membranes of all body cells except red blood cells. Function: If a cell is infected with a virus, MHC-1 contains bits of virus marking the cell so that T Cells recognise that there is a problem in that cell. (slide 16 Session 9 Lecture) Class II MHC (MHC-II) molecules appear only on the membrane surface of antigen-presenting cells (Dendritic cells, macrophages, B Cells). Function: If antigen presenting cells ingest foreign bodies, they will display some as part of their MHC-II. (Slide 17, Session 9 Lecture). (Tortora 818)

Is lymph more similar to blood plasma or to interstitial fluid? Why? (FIG 22.2 Q)

Lymph is more similar to interstitial fluid than to blood plasma because the protein content of lymph is low.

What functions do the Lymph Nodes Serve?

Lymph nodes FILTER LYMPH. Lymph nodes may contain B cells (that develop into plasma cells), T cells, dendritic cells and macrophages. As lymph flows through the node, foreign substances are trapped by reticular fibers within the spaces between cells. Macrophages in the nodes destroy some of the foreign substances via phagocytosis. Lymphocytes in nodes destroy other foreign substances via a variety of immune responses. The filtered Lymph leaves the other end of the node through one or two efferent (away) lymphatic vessels.

Where and how are Lymph nodes found in the body?

Lymph nodes are present throughout the body, usually clustered in groups.

Where are lymphatic capillaries found? Where are they not found?

Lymphatic capillaries are found throughout the body EXCEPT in avascular tissues, the central nervous system, portions of the spleen, and bone marrow.

What are Lymphatic tissues derived from?

Lymphatic tissues are derived from mesoderm. (Tortora 810)

When do lymphatic tissues begin to develop? (FIG 22.8 Q)

Lymphatic tissues begin to develop by the end of the fifth week of gestation.

What functions does the Spleen serve?

The spleen consists of two different kinds of tissue called white pulp and red pulp (Figure 22.7 b, c). WHITE PULP: is lymphatic tissue, consisting mostly of lymphocytes and macrophages arranged around branches of the splenic artery called central arteries. RED PULP consists of blood-filled venous sinuses and cords of splenic tissue called splenic cords or Billroth's cords. Splenic cords consist of red blood cells, macro- phages, lymphocytes, plasma cells, and granulocytes. Veins are closely associated with the red pulp. Blood flowing into the spleen through the splenic artery enters the central arteries of the WHITE PULP. Within the white pulp, B cells and T cells carry out immune functions, similar to lymph nodes, while spleen macrophages destroy blood-borne pathogens by phagocytosis. Within the RED PULP, the spleen performs three functions related to blood cells: (1) removal by macrophages of ruptured, worn out, or defective blood cells and platelets; (2) storage of platelets, up to one-third of the body's supply; and (3) production of blood cells (hemopoiesis) during fetal life." (Tortora 808)

What is the largest single mass of lymphatic tissue in the body?

The spleen is the largest single mass of lymphatic tissue in the body.

What are the two types of adaptive immunity?

The two types of adaptive immunity are: 1) cell-mediated immunity; and 2) antibody- mediated immunity.

What is the function of the variable regions in an antibody molecule? (FIG 22.19 Q)

The variable regions recognize and bind to a specific antigen.

What is the difference between interstitial fluid and lymphatic fluid?

There are 2 major differences: 1) Location - i) Interstitial Fluid: is found between cells. ii) Lymphatic Fluid : is located within lymphatic vessels and lymphatic tissue. ( Tortora 800).

How are T cells affected by aging?

They become less responsive to antigens, and fewer of them to respond to infections -age related atrophy of the thymus -decreased production of thymic hormones -B cells are less responsive, antibody levels do not increase as rapidly in response to a challenge by an antigen resulting in increase susceptibility to infections