Immunoglobulins of the human amniotic fluid.

Abstract

Except for the description of a secretory immunoglobulin (S-Ig) of a low size, no recent study has investigated the molecular status of antibodies in the human amniotic fluid. After separation with a high performance chromatography, we analyzed the different isotypes of amniotic Igs by immunoblotting and ELISA. 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 detected. IgA is monomeric, with a low level of secretory IgA and with various amounts of free secretory component (SC). The presence of a low level of SC-containing immunoglobulin of a low size is confirmed during the last trimester of pregnancy. This molecule contains no alpha chain but includes a Fabgamma fragment noncovalently associated with SC. IgG, IgA, and SC are detected in the fetal urine and, therefore, can reach the amniotic fluid by this route. 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 autoantibodies is discussed.