Antibodies

IgG

free in plasma; about 80% of circulating antibodies

most abundant; crosses placenta and provides passive immunization to fetus

IgM

surface of b cell; free in plasma

antigen receptor on b cell membrane; first class of antibodies released by b cells during primary response

IgD

surface of b cell

cell surface receptor of mature b cell; important in b cell activation

IgA

monomer found in plasma; polymers in saliva, tears, milk, and other body secretions

protects mucosal surfaces; prevents attachment of pathogens to epithelial cells

IgE

secreted by plasma cells in skin and tissues lining gastrointestinal and respiratory tracts

found on mast cells and basophils; when bound to antigens, triggers release of histamine from mast cell or basophil that contributes to inflammation and some allergic responses