Figure 1.

Overview of the complement system. The complement system is a collection of plasma and membrane-bound proteins which form part of the innate immune response against invading pathogens as well as performing other immunological roles. The system can be divided into three pathways (classical, lectin, alternative). The classical pathway is typically antibody-dependent and relies on the binding of C1q protein. The lectin pathway is antibody-independent and utilises lectins such as mannose-binding lectin (MBL) and ficolin-1 (FCN-1) to bind glycosylated regions on the surface antigens of pathogens. The alternative pathway is activated from spontaneous hydrolysis of the C3 protein and an absence of regulatory proteins on the microbial surface. In the context of viral infection, complement activation aims to limit infection through the promotion of inflammation and chemotaxis, the opsonisation of virions, the aggregation of virions, the direct lysis of virion and infected cell membranes, and by aiding the development of the adaptive immune response. (Ab), antibody; (Ag), antigen; (DC), dendritic cells; (FB), factor B; (FD), factor D; (FCN-1), ficolin-1; (Inf.), infected; (MBL), mannose-binding lectin; (P), properdin.