IgM

When your immune system meets a pathogen for the first time, IgM is the first antibody it makes. It is a large molecule, five antibodies joined together, that gets going within days and can grab a lot at once, but not very precisely. After a week or two IgG takes over: more targeted, smaller, and it stays in the blood for months to years as memory. IgM then fades away again. That timing is the reason IgM against one pathogen is used to say whether an infection is recent: IgM present and IgG not yet means just started. This test measures something else: all your IgM together, against anything at all. That total says how much your immune system is making, not against what. Your report shows IgM in g/L, and the range for adults is the same at every lab: 0.4 to 2.3 g/L. It is almost always measured together with IgG and IgA.

Total IgM is rarely measured on its own. It belongs to the trio IgG, IgA and IgM used to see whether the immune system is complete, in people who keep getting infections, and it belongs to the question of whether an excess of antibodies is of one kind, which can point at a proliferation of antibody cells. For anyone healthy without complaints, it is a value that rarely yields anything. Where it does say something is as the first antibody. A high total IgM means the immune system has just been set to work or has been at work for a long time: a recent infection, a chronic infection, a lasting inflammation. It falls again by itself once the stimulus is gone. A strongly raised IgM that persists is the exception and calls for investigation. A low IgM is usually a variant without consequences in a healthy person. It only counts when IgG is low too, because then it is the immune system as a whole that falls short, or with immunosuppressant medicines and protein loss. What you can do for your antibodies yourself is limited and general: sleep, not smoking, exercise, and vaccinations, which build exactly the memory IgG then holds on to.

A high IgM comes, in order of how often it occurs, from: a recent infection, which has just set the immune system to work; a lasting infection or inflammation, from hepatitis to a chronic bowel inflammation; an autoimmune disease; rarely an autoimmune liver disease that keeps IgM high; and rarely a proliferation of the cells that make IgM. The higher and the more lasting, the more the last two come into view. A slightly raised IgM after a past infection is the most common outcome and falls within weeks. A low IgM usually means nothing in a healthy person; a small share of people naturally make little IgM without being troubled by it. It counts when IgG and IgA are low too, with medicines that suppress the immune system, and with protein loss through kidneys or gut. Very rarely it is an inborn deficiency with recurring infections. IgM belongs beside IgG and IgA. With a high value CRP belongs beside it, which shows whether there is inflammation now, and the liver values. And check that it is total IgM: an IgM test against one pathogen carries the same name and has its own numbers.