A previously unexposed adult receives a novel protein antigen for the first time. Which immunoglobulin isotype appears FIRST in detectable quantities in the serum during this primary humoral response?

• A IgM ✓
• B IgG
• C IgA
• D IgE

Explanation

Naive A cells carry surface IgM and IgD, and upon first antigen encounter they differentiate into plasma cells secreting IgM before any class switch recombination occurs. IgM therefore dominates the early primary response, appearing within days. IgG rises later and predominates in the secondary (anamnestic) response because switched memory A cells respond faster and undergo further class switching. This kills option B, the most tempting distractor, since IgG is the major circulating antibody but not the first produced.

Reference: Robbins and Cotran Pathologic Basis of Disease, 10th ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

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