Immunoglobulin M (IgM)

(i) IgM is a pentamer consisting of 5 immuno globulin subunits and one molecule of J chain, which joins the Fc region of the basic subunits.

(ii) It constitutes about 5-8 percent of total serum immunoglobulins. The normal level in serum is 0.5-2 mg/ml.

(iii) Half life is about five days.

(iv) It is heavy molecule (19S) with a molecular weight 900,000 to 1000000 hence also called the ‘millionaire molecule‘.

(v) IgM is mainly distributed intravascularly (80%).

(vi) It is the earliest synthesised immunoglobulin by foetus.

(vii) It appears early in response to infection before IgG.

IgM antibodies are short lived, and disappear earlier than IgG. Hence, its presence in serum indicates recent infection

(viii) It can not cross the placenta, presence of IgM antibody in serum of newborn indicates congenital infection.

(ix) It is very effective antibody in agglutination and complement fixation. It is more efficient than IgG in these reactions.

(x) IgM provides protection against blood invasion by microorganisms.

Source – textbook of microbiology for dental students c p baveja