C3-independent immune haemolysis: mechanism of membrane attack complex formation.

The isolated active complex of C5 and C6, C&56bar; was found to bind to EAC 142 in the absence of C3 or C7, and to form a unique intermediate, EAC 142561 which is susceptible to lysis by the addition of C7, C8 and C9. Further studies revealed that C&56bar; alone could bind to EAC 142 but not to E, EA, EAC I or EAC4, nor to EAC14 in the absence of C7, that the C&56bar; binding to EAC 142 was highly dependent on temperature and on the ionic strength of the buffer, and that the degree of EAC 14256 formation from EAC 142 and C&56bar; depended on the amount of C2 on EAC 142 and on the amount of added C&56bar;. These findings suggest that C2 or C&42bar; on EAC 142 may be an acceptor for C&56bar; In addition, C&56bar; appears to bind to EAC 142 much more efficiently than to unsensitized erythrocytes, even in the presence of C7. Thus, binding of C&56bar; to EAC 142 is likely to be an initial step of membrane attack complex formation in C3-independent immune haemolysis. [ABSTRACT FROM AUTHOR]