Which immunoglobulin is deficient in selective IgA deficiency?

• A. IgA
• B. IgG
• C. IgM
• D. IgE

Answer: (A)

Selective IgA deficiency is defined by the lack or very low levels of IgA, while other immunoglobulins, especially IgG and IgM, are normal. IgA is the antibody that predominates in mucosal secretions (tears, saliva, mucus, and gut/respiratory tract) as secretory IgA, forming a first line of defense against pathogens at these surfaces. When IgA is deficient, mucosal immunity is weakened, leading to more frequent mucosal infections and, in some people, reactions to blood products containing IgA due to anti-IgA antibodies. Because IgG and IgM are typically normal, this pattern distinguishes selective IgA deficiency from deficiencies involving other immunoglobulins.