Which immune marker represents past exposure to disease?

• A. Immunoglobulin G (IgG)
• B. Immunoglobulin M (IgM)
• C. Immunoglobulin A (IgA)
• D. Interferon-gamma

Answer: (A)

Immunoglobulin G (IgG) is the marker that represents past exposure because it is produced after the initial infection and remains in the bloodstream for a long time, often years, signaling prior contact or immunity. This class switches on after the early IgM response and provides long-term protection, so a detectable IgG titer typically indicates previous exposure or vaccination. In contrast, IgM shows up first during an active or very recent infection and fades as IgG rises, so its presence points to current or recent infection rather than past exposure. IgA is mainly associated with mucosal immunity and secretions, so it’s not the standard indicator of past systemic exposure. Interferon-gamma is a cytokine involved in cellular immune responses, not an antibody, so it isn’t used as a marker of prior exposure.