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Complement System

The complement system, specifically through the actions of proteins C3a and C3b, destroys microbes. – Comprises inactive proteins that circulate in the blood; of these proteins, the products of C3 cleavage (C3a and C3b) have multifold functions.

Three pathways lead to C3 cleavage: – In the classical pathway, C1 is "fixed" to antibody-antigen complexes, which initiates a cascade of events that lead to C3 cleavage. – The alternative pathway is triggered by spontaneously activated C3b. – The lectin pathway is triggered when lectins, such as, mannose-binding lectin, binds microbial sugars and marks them for phagocytosis.

Key Functions

Antibodies do not directly kill pathogens.

Neutralize microbes by blocking their extracellular receptors and inhibiting their attachment to host cells; antibody binding can also inhibit viral replication, stopping the spread of infection.

Antibodies can opsonize microbes, which means they bind to them and make it easier for phagocytes to recognize and engulf them.

Activate the complement cascade, leading to formation of membrane attack complexes (MAC) and, consequently, microbe lysis.

Agglutination occurs when antibodies bind multiple cell-bound antigens simultaneously, causing them to clump; precipitation, which we haven't shown, occurs when antibodies simultaneously bind multiple soluble antigens. In both cases, binding makes antigen capture and phagocytosis easier.