Nature of the Immune System IV. The Immune Response Terry Kotrla, MS, MT(ASCP)BB

Antigens and Antibodies An antigen is any substance which is recognized as foreign by the body and is capable, under appropriate conditions, of provoking a specific immune response. It is capable of: Stimulating the formation of antibody and the development of cell-mediated immunity. Reacting specifically with the antibodies or T lymphocytes produced.

Physical Nature of Antigens Foreign nature The immune system of an individual can normally distinguish between body components ("self") and foreign substances ("non-self"). The body is tolerant of its own components and does not initiate immune response against these. Under certain circumstances this natural tolerance may be disturbed, permitting the individual to react against himself, as is seen in autoimmune disease. The greater the “foreignness” or difference from self, the greater the immune response.

Physical Nature of Antigens Molecular size The higher the molecular weight, the better the molecule will function as an antigen. The larger the size, the greater the number of antigenic sites and the greater the variety and amount of antibody production. Molecules with a molecular weight of less than 10,000 daltons have no or weak antigenicity.

Physical Nature of Antigens Molecular complexity and rigidity The more complex an antigen is, the more effective it will be. Complex proteins are better antigen than large repeating polymers such as lipids, carbohydrates, and nucleic acids, which are relatively poor antigens. Specific regions of limited size function at antigenic sites, it’s thought that 2 antigenic determinants per molecule are required to stimulate antibody production. Haptens are substances, usually of low molecular weight, that can combine with antibody but cannot initiate an immune response unless it is coupled to a larger carrier molecule.

Physical Nature of Antigens Genetic factors Not all individuals within a species will show the same response to a substance - some are responders and some non-responders. There is also a wide variation between species.

Physical Nature of Antigens Route of administration and dose Route of administration (oral, skin, intramuscular, IV, peritoneal, etc.) for stimulation of the immune response is very important. Recognition may not occur if the dose is to small. If the dose is too large it may cause "immune paralysis" and also fail to elicit an immune response.

Antigenic Determinants or Epitopes Structures on antigens that are recognized as foreign by the immune system. Number of antigenic determinants on a molecule varies with molecular size. An immune response is directed against specific determinants, and resultant antibodies will specifically bind to them.

Antigen-Antibody Binding Binding of antigenic determinant to the antibody binding can be likened to a "lock and key". Antibodies of different degrees of specificity may be produced in the immune response to a given antigen. "Poor fit" of an antigen with an antibody is in response to the antigen reacting with an antibody produced in response to an entirely different antigen. This phenomenon is called cross reactivity.

Humoral Immunity Results in production of proteins called “immunoglobulins” or “antibodies”. Body exposed to “foreign” material termed “antigen” which may be harmful to body: virus, bacteria, etc. Antigen has bypassed other protective mechanisms, ie, first and second line of defense.

Dynamics of Antibody Production Primary immune response Latent period Gradual rise in antibody production taking days to weeks Plateau reached Antibody level declines

Dynamics of Antibody Production Initial antibody produced in IgM Lasts 10-12 days Followed by production of IgG Lasts 4-5 days Without continued antigenic challenge antibody levels drop off, although IgG may continue to be produced.

Secondary Response Second exposure to SAME antigen. Memory cells are a beautiful thing. Recognition of antigen is immediate. Results in immediate production of protective antibody, mainly IgG but may see some IgM

Humoral Immune Response

Primary versus Secondary

Dynamics of Antibody Production

Cellular Events Antigen is “processed” by T lymphocytes and macrophages. Possess special receptors on surface. Termed “antigen presenter cell” APC. Antigen presented to B cell

Basic Antibody Structure Two identical heavy chains Gamma Delta Alpha Mu Epsilon

Basic Antibody Structure Two identical light chains Kappa OR Lambda

Antibody Structure

Basic Antibody Structure

Basic Structure of Immunoglobulins

Papain Cleavage Breaks disulfide bonds at hinge region Results in 2 “fragment antigen binding” (Fab) fragments. Contains variable region of antibody molecule Variable region is part of antibody molecule which binds to antigen.

Papain Cleavage

Pepsin Breaks antibody above disulfide bond. Two F(ab’)2 molecules The rest fragments Has the ability to bind with antigen and cause agglutination or precipitation

Cleavage of Antibodies by Enzymes

Papain and Pepsin Cleavage

IgG Most abundant Single structural unit Gamma heavy chains Found intravascularly AND extravascularly Coats organisms to enhance phagocytosis (opsonization)

IgG Crosses placenta – provides baby with immunity for first few weeks of infant’s life. Capable of binding complement which will result in cell lysis FOUR subclasses – IgG1, IgG2, IgG3 and IgG4

IgG

IgA Alpha heavy chains Found in secretions Produced by lymphoid tissue Important role in respiratory, urinary and bowel infections. 15-10% of Ig pool

Secretory IgA Exists as TWO basic structural units, a DIMER Produced by cells lining the mucous membranes.

Secretory IgA

IgA Does NOT cross the placenta. Does NOT bind complement. Present in LARGE quantities in breast milk which transfers across gut of infant.

IgM Mu heavy chains Largest of all Ig – PENTAMER 10% of Ig pool Due to large size restricted to intravascular space. FIXES COMPLEMENT. Does NOT cross placenta. Of greatest importance in primary immune response.

IgM

IgM

IgE Epsilon heavy chains Trace plasma protein Single structural unit Fc region binds strongly to mast cells. Mediates release of histamines and heparin>allergic reactions Increased in allergies and parasitic infections. Does NOT fix complement Does NOT cross the placenta

IgE

IgD Delta heavy chains. Single structural unit. Accounts for less than 1% of Ig pool. Primarily a cell bound Ig found on the surface of B lymphocytes. Despite studies extending for more than 4 decades, a specific role for serum IgD has not been defined while for IgD bound to the membrane of many B lymphocytes, several functions have been proposed. Does NOT cross the placenta. Does NOT fix complement.