Can anyone give me any information about what IgG levels measure please?

Antibody. Immunoglobulin g, igg, is an antibody. Antibodies are proteins the immune system makes to fight off infections. Igg is the antibody that floats around in the blood stream and helps fight bacteria and viruses. Igg is typically measured to make sure the immune system is making enough antibodies or to diagnose certain infections. It is not an allergy test.