- Antibodies (immunoglobulins) are Y-shaped proteins produced by plasma cells that recognize and bind to antigens. The main classes of antibodies are IgG, IgM, IgA, IgD, and IgE. - Antibodies have light and heavy chains that give them a flexible Y shape. The variable regions at the tips of the Y allow antibodies to bind to specific antigens. The constant regions mediate different effector functions. - Antibodies have different structures and functions. IgG is the most abundant in serum and provides long-term protection. IgM is the first antibody produced during infection and is effective at complement activation. IgA protects mucosal surfaces.