PROBLEM: Except for the description of a secretory immunoglobulin (S-Ig) of
a low size, no recent study has investigated the molecular status of antib
odies in the human amniotic fluid.
METHOD: After separation with a high performance chromatography, we analyze
d the different isotypes of amniotic Igs by immunoblotting and ELISA.
RESULTS: IgG is found to be the major isotype and to contain mother-derived
tetanus antitoxins. IgA is much less abundant, whereas no IgM can be detec
ted. IgA is monomeric, with a low level of secretory IgA and with various a
mounts of free secretory component (SC). The presence of a low level of SC-
containing immunoglobulin of a low size is confirmed during the last trimes
ter of pregnancy. This molecule contains no alpha chain but includes a Fab
gamma fragment noncovalently associated with SC. IgG, IgA, and SC are detec
ted in the fetal urine and, therefore, can reach the amniotic fluid by this
CONLUSION: In addition to the predominant maternal IgG, the amniotic fluid
contains different molecular forms of fetal immunoglobulins. Their function
as an immune barrier against infection and against mother-derived autoanti
bodies is discussed.