Immunoglobulin M

Immunoglobulin M is a general purpose antibody produced by B cells. It's the largest antibody produced by the immune system and the first antibody created in response to an infection. The primary site of Immunoglobulin M production is the spleen.
Physicians will often test for Immunoglobulin M if they suspect a recent infection as, after an infection has started to run its course, the level of Immunoglobulin M production drops.
Immunoglobulin M at Wikipedia