Name the four major antibody isotypes and one key function of each.

• A. IgG: placental transfer; IgM: first responder; IgA: mucosal immunity; IgE: allergic responses
• B. IgG: mucosal immunity; IgM: placental transfer; IgA: first responder; IgE: opsonization
• C. IgG: allergic responses; IgM: mucosal immunity; IgA: placental transfer; IgE: agglutination
• D. IgG: opsonization; IgM: immune tolerance; IgA: intracellular signaling; IgE: anti-viral

Answer: (A)

Antibody isotypes have specialized roles that fit where they’re most effective. IgG is the most abundant in serum and uniquely crosses the placenta, giving the fetus passive protection. IgM is the first antibody produced during a new infection and, as a pentamer, is superb at rapid agglutination and efficient activation of the complement system. IgA operates mainly at mucosal surfaces and in secretions, forming secretory IgA to shield the gut, respiratory tract, and other mucosal sites. IgE plays a key role in allergic responses and in defense against parasites by sensitizing mast cells and basophils. The pairing here matches these distinctive functions: IgG with placental transfer; IgM as the first responder; IgA in mucosal immunity; IgE in allergic responses.