General Info: IgM, IgG, and IgA are the isotypes found mainly in blood, lymph, and extracellular spaceshinge regions are present in G, A, and D (not M or E)light chain isotypes: kappa and lambdaIgG 150 kDmonomersfour different subclasses: (IgG1, IgG2, IgG3, IgG4)found in vascular/extravascular spaces as well as secretions most abundant immunoglobulin in bloodbulk of immunity to most blood-borne infectious agentswith IgM, functions to prevent blood-borne infection (septicemia)only antibody to cross placenta and provide passive immunity to developing and newborn baby babies have level of IgG protection in plasma equal to mother transport across placenta is also mediated by FcRB decreases gradually; IgG levels are lowest in infants aged 3-12 monthsactively transported from blood into extracellular spaces by FcRn (Brambell receptor) receptor present on endothelial cells 2 molecules of FcRB bind to Fc region of one IgG IgG delivery to extracellular spaces in connective tissues --> protects IgG from degradation of serum proteinsused to neutralize microbial toxins and animal venomsIgA monomers and dimers monomeric IgA enters extracellular spaces; helps IgG protect against bacteria and virus particles with IgM and IgG, provide Ag-binding within fluids and tissues of the body dimeric IgA made in secondary LT underlying mucosal surfaces GI tract, eyes, nose, throat, mammary glands, respiratory, urinary, and genital tracts made in lamina propia (connective tissue underlying basement membrane of mucosal epithelium) transported to other side of epithelium by receptors on basolateral surface of epithelial cells When transported across epithelial barriers, bound by J chain, bound by poly-Ig receptor on basolateral surface of epithelial After transcytosis, released into apical surface poly-ig receptor cleaves and releases IgA; IgA still bound to secretory component (fragment of poly-ig receptor)present in external secretions (colostrum, milk, saliva) baby receives IgA from mother in milk --> transferred to baby's gut --> bind to microorganisms in gut preventing attachment to …..gut epithelium --> passes in fecesalso contains secretory chain and J chainfirst line of defense against microbes at mucosal surfaces (respiratory, gastrointestinal, and genitourinary tracts)Antibody Isotypesmore IgA is made than any other isotype (highest concentrations throughout the body)used to neutralize microbial toxins and animal venomsIgM 900 kDpentamersfirst antibody produced by B cells during immune responsecomposed of 5 four-chain units with 10 combining siteshigh avidity very efficient early on, before IgG has been producedpentameric nature --> strong binding but large size (reduced ability to leave blood and also no IgM Fc receptors on phagocytic cells or leukocytes --> IgM can't directly recruit these cells)with IgM, functions to prevent blood-borne infection (septicemia)10 binding sites --> low affinity, high avidityIgD monomerspresent in low quantities in circulationfunctions primarily as antigen receptor on B cells B cells express both IgM and IgD that are specific for same antigenrelatively unknownIgE monomersvery low levelsenhances acute inflammation and provides protections from wormsbinds to high-affinity Fc receptors on mast cells, basophils, and activated eosinophils cross-linking of receptors by antigen causes release of histamine from mast cells--> inflammation --> recruits cells FcεRI has high affinity for IgEallergic reactions IgE binds to receptors on mast cells that are specific for Fc regions (constant regions) triggers release of