ANTIBODIES Jiri Mestecky, M.D., Ph.D. - Lecturer Distribution in body fluids: secretions plasma (serum), tears, saliva, milk, genitourinary, and intestinal Cells producing antibodies and their tissue distribution: plasma cells > lymphoblasts > B cells (surface) intestines > bone marrow > spleen-lymph nodes Structure: a) -globulins - immunoglobulins b) 2 Heavy (H) and 2 Light (L) chains connected by disulfide bridges and non-covalent (hydrogen) bridges (see Fig. 1) c) Enzymatic digestion Fab, Fc hinge region, papain, pepsin d) shape of the molecule Y (see Fig. 2) 1
Primary (amino acid sequence) structures: myeloma proteins, Bence-Jones proteins Variable and constant regions of L and H chains Domains (see Fig. 3) Antigen-binding site (see Fig. 4) Functions of Antibodies: 1) a) Prevention of adherence of microorganisms b) Prevention of uptake of undigested food antigens 2) Neutralization of biologically active antigens (toxins, viruses) 3) Enhancement of activities of innate factors 4) Activation of complement lysis of microorganisms 2
5) Promotion of phagocytosis 6) a) Antibody-dependent cell-mediated cytotoxicity b) Fc receptors Acquisition of Antibodies (pp. 159-163) Active Natural postinfections Artificial immunization (vaccines) Passive natural artificial prenatal postnatal antibodies injected or ingested 3
Ig ISOTYPES (classes and subclasses) IgG - immunoglobulin G (see Fig. 5) Structure mol. wt. 150-160.000-7S 2 H chains 2 L chains, molecular formula 2 2 or 2 2 2-3% carbohydrate (on H chains) Four subclasses IgG1-66% of total IgG in serum IgG2-23% IgG3-7% IgG4-4% Structural differences in interheavy chain disulfide bonds serum level 7-15 g/l half-life 21-25 days, intravascular - 45% bivalent (2 Ag-binding sites) binds Complement promotes phagocytosis (Fc receptors on phagocytes) crosses placental barrier (humans) secretory Ig in the milk of cows, horses, pigs, sheep (absorbed from the gut of newborn animals) typical of secondary antibody response 4
IgG subclasses - differences in biological functions: C binding: IgG3>>IgG1>IgG2>IgG4 (neg.) Placental transfer: IgG1=IgG3=IgG4>>IgG2 Ab activity: IgG2 - antipolysaccharide Ab IgG1, IgG3 - antiprotein IgA - immunoglobulin A (see Fig. 6) Structure: mol. wt. 160.000 - monomer 7S 335.000 - dimer 9S 400.000 - Secretory IgA 11S & 15.5S monomer 2 H chains - 2 2 or 2 L chains -, 2 2 dimer 4 chains ( 2 2 ). 2 J or 4 L chains ( 2 2 ). 2 J 1 J (joining) chain Secretory IgA 4-8 H chains 4-8 L chains ( 2 2 ) 2-4. J. SC 1 J or 1 Secretory Component SC ( 2 2 ) 2-4. J. SC 5
J chain SC (see Fig. 7) carbohydrate 7-11% (on, J chains and SC) Two subclasses IgA1 serum 85% external secretions 40-70% IgA2 serum 15% external secretions 30-60% serum levels 0.5-3.5 g/l external secretions - highly variable 0.1-20 g/l half-life - 5-6 days when the short half-life and large amount of IgA in external secretions are considered, in humans more IgA is produced per day than all other Ig classes combined. 2 Ag binding sites in a monomer, 4 in dimer does not bind complement does not efficiently promote phagocytosis does not cross placental barrier is not absorbed from the intestine resistant to proteolysis by digestive enzymes IgA1 sensitive to bacterial proteases from Strep. pneumoniae, gonococcus, meningococcus and other bacteria. binds to SC on surfaces of epithelial cells - selective transport of polymeric, J chain-containing IgA into external secretions! Also, binds to hepatocytes. neutralizes viruses prevents absorption of antigens from mucosal surfaces prevents attachment (adherence) of bacteria to epithelial cells adult serum IgA levels reached in puberty adult levels of secretory IgA reached in 6 mo. - 1 yr. 6
IgM - immunoglobulin M Structure mol. wt. 970,000 19S pentamer: 10 H chains μ 10 L chains, (μ 2 2 ). 5 J or 1 J chain (μ 2 2 ). 5 J also present in low concentrations in human external secretions as secretory IgM - (μ 2 2 ). 5 J. SC. Secretory IgM in external secretions of fishes & amphibia. carbohydrates ~10% (on H and J chains) Phylogenetically oldest Ig! Proteolytic fragments - Fab and (Fcμ) 5 serum levels 0.5-2 g/l half-life 6-10 days 10 Ag binding sites binds Complement extremely efficiently promotes phagocytosis does not cross placental barrier typical of the primary Ab response IgD - immunoglobulin D Structure mol. wt. 180.000 7S 2 H chains - 2 2 or 2 L chain -, 2 2 carbohydrates 9-14% (on H chains) serum levels 0.03-0.1 g/l half-life 2-3 days bivalent (2 Ag binding sites) does not bind complement does not promote phagocytosis does not cross placental barrier extremely sensitive to proteolytic enzymes function of serum IgD unknown present on surfaces of B lymphocytes - important in the triggering of lymphocyte differentiation 7
IgE- immunoglobulin E Structure mol. wt. 190.000 8S 2 H chains - 2 2 or 2 L chains -, 2 2 carbohydrate 12% (on chain) serum level 0.0000 5 g/l half-life 2 days bivalent (2 Ag-binding sites) does not bind complement does not promote phagocytosis does not cross placental barrier binds through Fc to receptors on basophils and mast cells. When cross-linked by Ag - these cells degranulate - release active components responsible for clinical symptoms of Type I hypersensitivity (anaphylaxis, hay fever, drug allergies, etc.) plays a role in the defense against parasites 8