IgG antibodies indicate which of the following?

• A. Past exposure
• B. Acute infection
• C. Vaccination response
• D. No exposure

Answer: (A)

IgG antibodies are the long-lasting, high-affinity responders that the body makes after an exposure to a pathogen or after vaccination. Their presence shows that the immune system has encountered the antigen before and has developed memory, which translates to immunity for future encounters. Because IgG persists for months to years, a positive IgG result is interpreted as evidence of past exposure or vaccine-induced immunity, not an active infection. In contrast, acute infection is typically signaled by IgM antibodies, which appear early and then wane as IgG rises. A detected IgG could also reflect a response to vaccination, but it does not by itself indicate current disease. If there were no exposure, IgG would usually be negative.