Influence of albumin-bound bilirubin on the antibody-dependent cellular cytotoxicity of human cells in vitro.

In neonatal hyperbilirubinaemia the distribution of unconjugated bilirubin (BR) between albumin and cellular binding determines its toxicity. The relationship between the ratio of BR/albumin and the impairment of cellular functions was studied employing a system of antibody-dependent cellular cytotoxicity (ADCC) with erythrocytes as target cells. The cytotoxic activity of peripheral blood monocytes, lymphocytes and granulocytes is inhibited in a concentration-related manner depending on the molar ratio BR/albumin. The occupation of albumin binding sites by BR is related to the extent of inhibition of ADCC activity suggesting that there is a competition between albumin and cellular binding sites for free bilirubin. This competition is governed by the respective affinity and concentration of the binding sites. Bilirubin binding to the cellular (silent) membrane receptors leads to the chain of events resulting in impairment of cellular functions, i.e. ADCC.