IgG is used to measure the response to a vaccine (eg. PPV23) to assess for potential immunodeficiency in the case of recurrent infections. IgG is used to determine viral reactivation (eg. EBV). But IgG is not a determinant of “allergy”. Is anyone able to explain what sort of immune reaction IgG indicates? Specifically I’m interested in how an antibody titre (tetanus) can increase without an infection, a reactivation or a vaccination.