Most acute infections elicit a predictable immune response. There are some important exceptions, however:
- Some superficial infections may fail to induce an antibody response despite significant illness.
- Infections in immunocompromised individuals, including certain healthy infants, may not result in a significant antibody response.
- Acute infections and immunizations may be thwarted in the presence of passively acquired antibody, circumventing the production of new patient antibody (e.g., transplacental IgG antibody may prevent the production of antibody to the measles vaccine if administered to infants too early).
Antibodies in the IgM class usually appear early in the infection before the appearance of the IgG class. The presence of IgM antibody is usually transient and suggests current or recent and not necessarily primary infection. Recurrent or reactivated infections have occasionally elicited an IgM response primarily among the herpes virus group (CMV, HSV, EBV). IgM antibody usually appears 7-10 days after a primary infection and reaches maximum levels within 2-3 weeks. The duration of the IgM response is variable, depending on the infecting organism and the patient. Interpretation of a positive IgM result must be made with caution and in conjunction with clinical findings.
IgG antibody usually appears after the initial IgM response and reaches peak levels 3-4 weeks later. IgG antibody may persist for life. Individuals who have a mild infection or are treated early in the course of the disease may revert to an apparent negative IgG status over time. The detection of IgG antibody suggests past exposure, infection or immunization to the organism. With many diseases like rubella or measles, in the absence of a current or recent infection, the presence of IgG is consistent with immunity to the disease.
Antibody/serology tests are designed to detect multiple or specific classes of immunoglobulins (e.g., total antibody vs. specific IgG or IgM).