Interaction of Ross River virus with the complement system.

In the absence of virus-specific antibody, Ross River virus failed to activate either the classical or alternative complement pathways. Instead, it inhibited the cleavage of C3 via both pathways. The virus did not appear to act by disrupting C3bBb complexes or by preventing cleavage of factor B by factor D. Instead Ross River virus was found to interfere with the actual cleavage of C3 by activated factor B (C3bBb) of the alternative pathway and C4b2a of the classical pathway.