This test measures the level of types of antibodies in the blood. Our immune system produces antibodies to protect us from bacteria, viruses, and allergens. Sometimes, our body may mistakenly treat healthy organs and tissues like foreign invaders and make antibodies against healthy organs and tissue. This is called as an autoimmune disease. The different types of antibodies are Immunoglobulin A (IgA), Immunoglobulin G (IgG), Immunoglobulin M (IgM), Immunoglobulin E (IgE), Immunoglobulin D (IgD).