The complement system of the duck

Antibody (Ab)-dependent and-independent activation of the duck complement (C') system were studied. Ab-independent C' activity exhibited characteristics similar to those of the mammalian alternative C' pathway (ACP), including the selective lysis of rabbit erythrocytes (RRBC), a requirement for Mg2+, but not Ca2+, depletion of activity by zymosan, and lack of sensitivity to the mammalian C1 inhibitor carrageenan. Measurement of C' activity using antisera against sheep erythrocytes (SRBC) revealed that duck Abs activate C' by a pathway resembling the mammalian classical pathway (CCP) requiring both Ca2+ and Mg2+. Ab-dependent and-independent activities were further distinguishable by their kinetics of lysis and sensitivities to heat. Duck Abs were also found to activate C' in normal and carrageenan-treated serum by a mechanism that requires only Mg2+, and thus resembles the ACP. However, this Ab-dependent ACP-like activity exhibits patterns of ionic strength dependence and ontogeny which are clearly different from those of the conventional ACP and CCP. These findings indicate that duck C' can be activated by three mechanisms: Ab-mediated activation of the CCP, and Ab-mediated and Ab-independent activation of the ACP. Duck Ab responses to SRBC and RRBC were followed by direct agglutination, antiglobulin agglutination, and activation of the CCP and ACP. While the C'-activating abilities of duck anti-SRBC Abs persisted through a 3-month programme of inoculation, the anti-RRBC response lost its ability to activate C' after 2 weeks.