Immunoglobulin M is a general purpose antibody produced by B-cells. It is 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

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