IgM antibodies are:

• A. The most common antibody
• B. The first antibody produced
• C. Antibodies that are primarily in secretions
• D. Antibodies that cross the placenta

Answer: (B)

The main idea is that IgM is the first antibody produced during a primary immune response. When naive B cells first encounter an antigen, they differentiate into plasma cells that secrete IgM before switching to other isotypes. IgM is a pentamer, which gives it high avidity and makes it efficient at activating the classical complement pathway to control pathogens early on. As the response develops, B cells undergo class switching to antibodies like IgG, IgA, or IgE, so IgM levels decline and other isotypes become more prevalent.

Keep in mind that IgG is the most common antibody in serum, IgA is the main antibody in secretions, and IgG is the one that crosses the placenta.