Studies on the sensitivity to complement‐mediated lysis of erythrocytes (Inab phenotype) with a deficiency of DAF (decay accelerating factor)

No episodes of clinically significant in vivo haemolysis have been reported in individuals with a novel form of decay accelerating factor (DAF) deficiency (Inab phenotype), nor do functional in vitro assays for complement-mediated haemolysis show the extreme sensitivity to lysis characteristic of paroxysmal nocturnal haemoglobinuria (PNH) erythrocytes. DAF appears to be totally deficient in the Inab erythrocytes as judged by immunochemical and functional assays. Unlike PNH, the only other described DAF deficiency (where several other phosphatidylinositol (PI)-linked membrane proteins are also absent), the only protein lacking from Inab erythrocytes appears to be DAF. The Inab phenotype seems to be an inherited specific defect in DAF whereas PNH is an acquired defect in the mechanism of insertion of PI-linked proteins into cell membranes. These findings support the view that susceptibility of PNH erythrocytes to in vivo and in vitro complement-mediated haemolysis is not due simply to DAF deficiency but to either the combined lack of several membrane proteins or to deficiency of other regulatory proteins such as the membrane attack complex inhibitor/homologous restriction factor (MIP/HRF). The findings also raise questions as to the role of erythrocyte DAF.