FIG 2.

Complement pathways. The classical, lectin, and complement pathways are shown with the associated activation triggers from meningococcus (pink shapes). For the classical pathway, antigen antibody complexes are recognized by C1 to trigger the cascade. The lectin pathway is activated via mannose binding lectins (MBL) interacting directly with the bacterial surface. The alternative pathway is also activated by direct interaction between the bacterial surface and C3. All pathways lead to the conversion of C5 into C5b and accumulation of C6 to C9 on the bacterial cell surface forming the membrane attack complex (MAC), which lyses the cell. In the case presented, C6 is deficient, marked with “X”; as a result, the MAC cannot form.