Immunoglobulin M

Immunoglobulin M, or IgM is a basic antibody that is produced by B cells.

Chronic fatigue syndrome
One study found elevated serum concentrations of both IgA and IgM to lipopolysaccharides of gram-negative bacteria. Serum IgA levels correlated with illness severity as measured by the FibroFatigue scale

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Immunoglobulin M, Wikipedia