The phagocytic process and the role of complement in host defense.

Three lines of defense play an integral role in host defense. The mechanical barrier (skin and mucous membrane: first line), non-specific defense (phagocytic cells non-specific humoral factors) and the specific line of defense. Phagocytic cells act in phases: chemotaxis, attachment of the bacteria to the membrane of the phagocytic cells, ingestion, killing and digestion. Attachment of the particle to receptors is only optimal when the bacteria are loaded with antibodies (IgG) and/or activated complement factors (C3b, C3Bbi), because the phagocyte has receptors for these opsonins (Fc, and C receptors). The late complement factors are also able to kill and lyse certain bacteria without phagocytes. The killing mechanism of the phagocyte is complex. It is oxygen dependent and oxygen independent. Presumably toxic oxygen species are most important to kill bacteria by phagocytes.