Unformatted text preview: The principal antibody of the secondary immune response is IgG. This antibody is the most common in the bloodstream and is found in many secretions, such as spinal, synovial, lymph, and peritoneal fluids. IgG crosses the placenta and protects the fetus and newborn. IgG therefore provides a natural type of passive immunity. IgG also forms in the primary antibody reaction after a large amount of IgM has already formed. IgG has the “typical” antibody structure of four protein chains. The third class of antibody is IgA. This antibody is found in external secretions such as those at the mucosal surfaces of the respiratory, gastrointestinal, and urogenital tracts. It is also present in the tears, saliva, bile, urine, and colostrum, and it is transferred in the breast milk. IgA consists of two antibody units held together by a J chain and secretory component. IgA resists infections at the body antibody units held together by a J chain and secretory component....
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- Fall '07