Jeff, I think the test you are talking about is called quantitative immunoglobulins. This looks at the levels of IgG, IgM, and IgA. IgG is the most important. People look at IgM because these are the frontline immunoglobulins–showing ability to produce immunoglobulin recently. IgA is looked at because it is the immunoglobulin that crosses mucosal barriers–such as the nose to protect against sinusitis or into the lungs to help protect against pneumonia. The immunoglobulin levels may become low due to immunosuppressive treatment–such as steroids, or cyclosporin or immuran, etc or “chemotherapy drugs like cyclophosfamide or methotrexate, etc. Immunoglobulins will also be low after plasmapheresis.
If people are trying to tell if the immune system is working well enough, the amount of immunoglobulin matters as well as whether it is “specific” to a given infection. Some people have immunoglobulin, but it is not specific to be able to fight infections. Often people look at specific antibody to diptheria or tetanus to see that a person is able to make mature antibodies against something.
You mentioned autoimmune diseases–often people look at special tests such as ANA or anti-Hu antibodies, etc. For allergies, often people look at IgE levels to specific things. Usually looking for multiple myeloma is done with a serum protein electrophoresis (SPEP) in which one looks at the distribution of proteins and may see a large amount of one kind of protein (called the M-protein or monoclonal protein).
IgM is a really large immunoglobulin. You cannot replace IgM or IgA, but if someone is having lots of infections (especially viral infections), giving IgG can help if it was really low.
I hope this helps, WithHope