Microbiology Practice Exam

The presence of IgG in the body typically indicates past exposure to an antigen. IgG is one of the five classes of immunoglobulins and plays a crucial role in the immune response. It is the most abundant antibody in the blood and is primarily produced in response to infections and vaccinations.

When the body encounters a pathogen for the first time, it produces IgM antibodies initially. After some time, as the immune system is activated and adapted, it starts producing IgG antibodies. Thus, high levels of IgG often signify that the body has been exposed to a particular antigen previously, whether through an infection or vaccination, and that it has mounted an immune response to it.

In contrast, other choices focus on different contexts of antibody presence. Current infections are more often indicated by the presence of IgM antibodies, which are produced early in the immune response. Immediate allergic responses are typically mediated by IgE antibodies, not IgG. While vaccination can lead to the production of IgG, the presence of IgG itself does not exclusively indicate vaccination, as it can also result from natural infection. Hence, the best representation of IgG's role is its association with past exposure to an antigen.