What does high IgG avidity indicate regarding an infection?

High IgG avidity indicates that the immune response to an infection has matured. IgG antibodies develop over time and typically demonstrate higher avidity after a certain period following an infection. When the avidity of IgG antibodies is high, it suggests that the individual has been exposed to the pathogen for a longer duration, which is indicative of an older infection rather than a recent one.

This phenomenon occurs because initially, the immune system produces IgM antibodies, which are less specific, and as the immune response continues, IgG antibodies are produced with higher affinity and specificity due to the somatic hypermutation and affinity maturation processes in B cells. Therefore, the presence of high avidity IgG indicates that the infection is not recent, as the body has had adequate time to mount a robust immune response.

In contrast, low avidity IgG would suggest a more recent infection, where the antibodies have not yet undergone maturation. This context helps clarify why an interpretation of high IgG avidity pointing to an older infection is consistent with our understanding of the immune response timeline.