Following exposure to an antigen the immune system may responde with a primary or secondary immune response.
Testing:
(1) IgM to antigen
(2) IgG to antigen
(3) titer of IgG antibody
Time frame for a primary immune response:
(1) Antigen exposure. A pathogenic organism may incubate and then spread.
(2) IgM antibody is formed after several days.
(3) IgG antibody appears after a few weeks. As IgG appears IgM production falls off.
(4) Eventually only IgG is present. Amount (titer) may persist or fall over time.
IgM
IgG
Interpretation
negative
negative
Not exposed.
Early, before IgM formed.
Immunodeficiency.
IgG below detection.
positive
negative
early infection (1 to 2 weeks after exposure)
positive
positive
2 to 12 weeks after exposure
negative
positive
4+ months after exposure
IgG titer peaks a few weeks after infection resolved. The titer may persist for years or fall off gradually of months or years. On re-exposure the IgG titer may rebound quickly (anamnestic or secondary immune response). Sometimes a nonspecific immune response will result in a transient increase in antibody production.
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