Frequency of low molecular weight IgM in human cord blood.

Thirty cord blood sera from healthy neonates and five sera from still-born infants (two with suspected infections and high IgM) were assessed for the presence of low molecular weight (LMW) IgM using two independent sensitive techniques, viz. filtration chromatography and immunoblotting. The first technique revealed this LMW moiety in 4 of 22 sera, all from healthy full-term infants, and it constituted 4-25% of the total IgM. LMW IgM was not found in any of the 30 sera using the immunoblotting technique or in 15 healthy adult sera, but was found consistently in rheumatoid sera used as positive controls.