Antibodies Microbiology for Pharm D.pptx

Best for Pharm D student

1. ANTIBODIES Dr. Anam Zamin

2. INTRODUCTION Antibodies are globulin proteins (immunoglobulins [Ig]) that react specifically with the antigen that stimulated their production. They make up about 20% of the protein in blood plasma. Antibodies are gamma globulins. There are five classes of antibodies: IgG, IgM, IgA, IgD, and IgE. Antibodies are subdivided into these five classes based on differences in their heavy chains.

3. Functions of antibodies The most important functions of antibodies are: To neutralize toxins and viruses To opsonize microbes so they are more easily phagocytosed To activate complement system To prevent the attachment of microbes to mucosal surfaces. Antibodies also have a catalytic (enzymatic) capability

4. Antibodies An antibody (Ab), also known as an immunoglobulin (Ig). Is a large Y-shape protein produced by plasma cells that is used by the immune system to identify and neutralize foreign objects such as bacteria and viruses. The antibody recognizes a unique part of the foreign target, called an antigen. Each tip of the "Y" of an antibody contains a paratope (a structure analogous to a lock) that is specific for one particular epitope (similarly analogous to a key) on an antigen, allowing these two structures to bind together with precision.

5. IMMUNOGLOBULIN STRUCTURE Immunoglobulins are glycoproteins made up of light (L) and heavy (H) polypeptide chains. The terms light and heavy refer to molecular weight; light chains have a molecular weight of about 25,000 Da , whereas heavy chains have a molecular weight of 50,000 Da to 70,000 Da. The simplest antibody molecule has a Y shape and consists of four polypeptide chains: two H chains and two L chains. The four chains are linked by disulfide bonds. An individual antibody molecule always consists of identical H chains and identical L chains.

6. L and H chain s are subd i v ided in t o variable and constant regions. The regions are composed of three-dimensionally folded, repeating segments called domains. A n L c h ain cons i st s of o n e vari a bl e (VL) and one constant (CL) domain. Mos t H c h ains co n s i s t of o n e vari a bl e (VH) and t h ree constant (CH) domains. Each domain is approximately 110 amino acids long. The variable regions of both the light and heavy chain are responsible for antigen-binding. The constant region of the heavy chain is responsible for various biologic functions (e.g., complement activation and binding to cell surface receptors). The constant region of the light chain has no t known biologic function s.

7. L chains belong to one of two types, κ ( kappa) or λ (lambda), on the basis of amino acid differences in their constant regions. Both types occur in all classes of immunoglobulins (IgG, IgM, etc.), but any one immunoglobulin molecule contains only one type of L chain. H chains are distinct for each of the five immunoglobulin classes and are designated γ, α, μ, ε, and δ. The amino-terminal portion of each L chain participates in the antigen-binding site. The amino-terminal portion of each H chain participates in the antigen-binding site; the carboxy terminal forms the Fc fragment, which has the biologic activities.

8. Antibody isotopes Five different antibody isotopes are known in mammals, which perform different roles, and help direct the appropriate immune response for each different type of foreign object they encounter.

9. IgA Immunoglobulin A ( IgA , also referred to as sIgA ) is an antibody that plays a critical role in mucosal immunity. This accumulates up to 15% of the total immunoglobulin produced in the entire body. IgA is the main immunoglobulin in secretions such as colostrum, saliva, tears, and respiratory, intestinal, and genital tract secretions. It prevents attachment of microorganisms (e.g., bacteria and viruses) to mucous membranes.

10. IgA has two subclasses (IgA1 and IgA2) and can exist in a dimeric form called secretory IgA ( sIgA ). IgA is a poor activator of the complement system. Each secretory IgA molecule consists of two H2L2 units plus one molecule each of J (joining) chain and secretory component. The secretory component is a polypeptide synthesized by epithelial cells . It also protects IgA from being degraded in the intestinal tract. In serum, some IgA exists as monomeric H2L2.

11. IgG There are four subclasses, IgG1–IgG4, based on antigenic differences in the H chains and on the number and location of disulfide bonds. IgG1 makes up most (65%) of the total IgG. IgG2 antibody is directed against polysaccharide antigens and is an important host defense against encapsulated bacteria. IgG is the predominant antibody in the secondary response and constitutes an important defense against bacteria and viruses. IgG is the only antibody to cross the placenta . It is therefore the most abundant immunoglobulin in newborns.

12. IgG is one of the two immunoglobulins that can activate complement; IgM is the other. IgG is the immunoglobulin that opsonizes. It can opsonize (i.e., enhance phagocytosis) because there are receptors for the H chain on the surface of phagocytes.

13. IgM IgM is the main immunoglobulin produced early in the primary response. It is present as a monomer on the surface of virtually all B cells, where it functions as an antigen-binding receptor. In serum, it is a pentamer composed of five H2L2 units plus one molecule of J (joining) chain. IgM has a μ heavy chain. Because the pentamer has 10 antigen-binding sites, it is the most efficient immunoglobulin in agglutination, complement fixation (activation), and other antibody reactions and is important in defense against bacteria and viruses. It can be produced by the fetus in certain infections.

14. IgD This immunoglobulin has no known antibody function but may function as an antigen receptor; it is present on the surface of many B lymphocytes. It is present in small amounts in serum.

15. IgE IgE is medically important for two reasons: (1) it mediates immediate (anaphylactic) hypersensitivity, and (2) it participates in host defenses against certain parasites (e.g., helminths [worms]). The Fc region of IgE binds to the surface of mast cells and basophils. Bound IgE serves as a receptor for antigen (allergen). When the antigen-binding sites of adjacent IgEs are cross- linked by allergens, several mediators are released by the cells, and immediate (anaphylactic) hypersensitivity reactions occur. IgE does not fix complement and does not cross the placenta.

16. M O NO C LONAL AN T IBO D IES Antibodies that arise in an animal in response to typical antigens are heterogeneous, because they are formed by several different clones of plasma cells (i.e., they are polyclonal). Antibodies that arise from a single clone of cells (e.g., in a plasma cell tumor [myeloma]) are homogeneous (i.e., they are monoclonal). Monoclonal antibodies also can be made in the laboratory by fusing a myeloma cell with an antibody-producing cell. Such hybridomas produce virtually unlimited quantities of monoclonal antibodies that are useful in diagnostic tests and in research. Monoclonal antibodies are now used in a variety of clinical situations, such as immunosuppression related to organ transplants, treatment of autoimmune disease, treatment of cancer, and the prevention of infectious disease