Opsonization of yeast by human serum IgA anti-mannan antibodies and phagocytosis by human polymorphonuclear leucocytes.

The ability of sera from 72 patients with liver disease to opsonize yeast for phagocytosis by normal polymorphonuclear leucocytes has been studied. Seven showed defective opsonization. The opsonic activity of all but two sera was decreased markedly by heating at 56 degrees C for 1 h. When the two sera with heat stable opsonic activity were fractionated by gel filtration and by ion exchange chromatography, the activity copurified with IgA, not with IgG. The purified IgA, radiolabelled with 125I was shown to bind in a saturable manner to the yeast. Both sera had high levels of anti-yeast mannan IgA detected by an ELISA. In one case most of the anti-mannan activity was due to monomeric IgA, in the other it was dimeric. This was consistent with the observation of an apparent molecular weight of the opsonin of approximately 180 kD in one serum and 300-400 kD in the other.