Ans. C. IgM. (Ref. H - 17th/Table 308-11 - pg. 708) IgM>IgG Immunoglobulin isotypes Mature B lymphocytes express IgM and IgD on their surfaces. They may differentiate in germinal centers of lymph nodes by isotype switching (gene rearrangement; mediated by cytokines and CD40 ligand) into plasma cells that secrete IgA, IgE, or IgG. # IgG: Main antibody in 2deg (delayed) response to an antigen. Most abundant isotype in serum. Fixes complement, crosses the placenta (provides infants with passive immunity), opsonizes bacteria, # neutralizes bacterial toxins and viruses. # IgA: Prevents attachment of bacteria and viruses to mucous membranes; does not fix complement. Monomer (in circulation) or dimer (when secreted). Crosses epithelial cells by transcytosis. Most # produced antibody overall, but released into secretions (tears, saliva, mucus) and early breast milk (known as colostrum). Picks up secretory component from epithelial cells before secretion. # IgM: Produced in the 1deg (immediate) response to an antigen. Fixes complement but does not cross the placenta. Antigen receptor on the surface of B cells. Monomer on B cell or pen tamer when # secreted. Shape of pentamer allows it to efficiently trap free antigens out of tissue while humoral response evolves. # IgD: Unclear function. Found on the surface of many B cells and in serum. # IgE Binds mast cells and basophils; cross-links when exposed to allergen, mediating immediate (type I) hypersensitivity through release of inflammatory mediators such as histamine. Mediates immunity to worms by activating eosinophils. Lowest concentration in serum. Antibody structure and function: Variable part of L and H chains recognizes antigens. Fc portion of IgM and IgG fixes complement. Heavy chain contributes to Fc and Fab fractions. Light chain contributes only to Fab fraction. Fab: wAntigen-binding fragment f Determines idiotype: unique antigen-binding pocket; only 1 antigenic specificity expressed per B cell Fc: f Constant f Carboxy terminal f Complement binding f Carbohydrate side chains f Determines isotype (IgM, IgD, etc.) Antibody diversity is generated by: f Random "recombination" of VJ (light-chain) or V(D)J (heavy-chain) genes / Random combination of heavy chains with light chains / Somatic hypermutation (following antigen stimulation) / Addition of nucleotides to DNA during recombination by terminal deoxynucleotidyl transferase. PHYSICAL, CHEMICAL, AND BIOLOGIC PROPERTIES OF HUMAN IMMUNOGLOBULINS Property IgG IgA IgM IgD IgE Usual molecular form Monomer Monomer, dimer Pentamer, hexamer Monomer Monomer Other chains None J chain, SC J chain None None Subclasses Gl, G2, G3, G4 Al, A2 None None None Heavy chain allotypes Gm (=30) No Al, A2m (2) None None None Molecular mass, kDa 150 160, 400 950, 1150 175 190 Sedimentation constant, Sw20 6.6S 7S, US 19S 7S 8S Carbohydrate content, % 3 7 10 9 13 Serum level in adult, mg/mL 9.512.5 1.5-2.6 0.7-1.7 0.04 0.0003 Percentage of total serum Ig 75-85 7-15 5-10 0.3 0.019 Serum half-life, days 23 6 5 3 2.5 Synthesis rate, mg/ kg per day 33 65 7 0.4 0.016 Antibody valence 2 2,4 10, 12 2 2 Classical complement activation +(G1, 2?, 3) ++ ' - Alternate complement activation +(G4) + - + - Binding cells via Fc Macrophages, neutrophils, large granular lymphocytes Lymphocytes Lymphocytes None Mast cells, basophils, B cells Biologic properties Placental transfer, secondary Ab for most antipathogen responses Secretory immunoglobulin Primary Ab responses. Phylogenitically oldest Ig is IgM and it is the one to appear earliest in fetus at 20 wks of gestation. Marker for mature B cells Allergy, antiparasite responses. Homocytotropism is vital property.