Which immunoglobulin is secreted by plasma cells in the blood?

• A. IgA
• B. IgG
• C. IgM
• D. IgD

Answer: (B)

The antibody most abundant in the bloodstream is IgG. After B cells undergo class switching and differentiate into plasma cells, many of these cells secrete IgG into the circulation, making it the dominant serum immunoglobulin. IgG provides versatile systemic protection, including opsonization, neutralization, and activation of the complement system.

IgM is produced early in a response and often exists as pentamers in blood, but it’s less abundant than IgG in the long term. IgA is mainly found in secretions like mucosal surfaces and breast milk, not the bloodstream. IgD mainly functions as a B cell receptor and is not a major secreted antibody in blood.