Quick Review Cards for Medical Laboratory Science

Complement Immunology Review 385

Definition

Functions

Classical pathway*

Alternative pathway*

Lectin pathway*

Present in highest
concentration in
plasma

Key component of
all pathways

Ions required

Group of >30 proteins involved in phagocytosis & clearance of foreign antigens. Most are inactive enzyme

precursors that are converted to active enzymes in precise order (cascade).

Inflammation, opsonization, chemotaxis, cell lysis.

Triggered by ag/ab rxn. IgM is most efficient activator. Single molecule attached to 2 adjacent ags can

initiate cascade. IgG1, 2, & 3 can activate complement but at least 2 molecules required.

Recognition unit: C1 (first to bind).

Activation unit: C4, C2, C3.

Membrane attack complex: C5, C6, C7, C8, C9 (cell lysis).

Ab independent. Activated by bacteria, fungi, viruses, tumor cells, some parasites.

Ab independent. Initiated by mannose-binding lectin (MBL). Nonspecific recognition of sugars on microor-

ganisms. Important defense mechanism in infancy.

C3

C3

Ca2+, Mg2+

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