The S-IgM test checks the levels of antibodies called immunoglobulins in your bloodstream. Antibodies are proteins made by your immune cells to fight off pathogens like bacteria, viruses, and mold but also to food proteins. The immunoglobulin test can show whether there's a problem with your immune system.
IgM antibodies are your body’s first line of defense against infection. The body makes IgM antibodies on first infection with bacteria and viruses. Levels then fall as IgG antibodies take over.