The complement system can be activated through several different methods.
The classical pathway involves the binding of C1q to two Fc regions on antibody that has opsonised the target pathogen. C1q activates the conversion of C1 to C1a and C1b, triggering the complement cascade (C1, 2, 4, 3, 5, 6, 7, 8, 9). This leads to the recruitment of other immune cells (phagocytes, helper T cells) and the formation of the membrane attack complex (MAC, by the polymerisation of C9).
The MB-lectin pathway involves lectin (a bacterial sugar) binding to mannose, catalysing the conversion of C3 to C3a and C3b to activate complement.
The alternative pathway is activated by the presence of LPS, a bacterial toxin. This only works with Gram-negative bacteria, which is beneficial as the MAC is only effective against Gram-negative bacteria.