Human Complement System: Activation & Regulation Part 1

Vocabulary flashcards covering key terms related to activation, pathways, components, regulation, and function of the human complement system.

Complement system

A large group of serum and tissue proteins that activate in a cascade to destroy microbes, promote phagocytosis, inflammation, and membrane perforation.

Alternative pathway

One activation route that begins on pathogen surfaces without antibodies, initiating the complement cascade.

Lectin pathway

Activation route initiated by mannose-binding lectin binding to pathogen surfaces.

Classical pathway

Activation route triggered by C-reactive protein or antibodies bound to antigen on a pathogen.

Mannose-binding lectin (MBL)

Lectin that recognizes mannose on microbes to trigger the lectin pathway.

C-reactive protein (CRP)

Acute-phase protein that initiates the classical pathway by binding to pathogens; not a complement component.

C1q

Recognition subcomponent that binds CRP or antibody-antigen complexes to start classical activation.

C1r

Serine protease in C1 that activates C1s.

C1s

Serine protease that cleaves C4 and C2 to form the classical C3 convertase.

MASP-1

Lectin pathway serine protease associated with MBL that helps propagate activation.

MASP-2

Lectin pathway protease that cleaves C4 and C2 to form the classical C3 convertase.

Factor B

Component of the alternative pathway that forms part of the C3 convertase when bound to C3b.

Factor D

Protease that cleaves Factor B when bound to C3b to form the alternative C3 convertase.

C3 convertase

Enzyme complex that cleaves C3 into C3a and C3b; forms in each pathway (e.g., C3bBb in the alternative pathway and C4b2a in classical/lectin).

C3b

Opsonin that covalently binds to microbes and promotes phagocytosis.

C2a

Fragment of C2 that combines with C4b to form the classical C3 convertase.

C4b2a

Classical C3 convertase consisting of C4b and C2a.

C5 convertase

Enzyme complex that cleaves C5 into C5a and C5b; formed after extended C3 convertase activity.

C4b2a3b

Classical C5 convertase (C4b2a with an extra C3b) that generates C5a and C5b.

C3a

Anaphylatoxin released during C3 cleavage; promotes inflammation.

C4a

Anaphylatoxin generated in classical/lectin pathway activation; promotes inflammation.

C5a

Potent anaphylatoxin driving inflammation and recruiting immune cells.

Membrane Attack Complex (MAC)

Pore-forming complex assembled from C5b, C6, C7, C8, and C9 that lyses target membranes.

C5b

Initiator of MAC assembly that binds C6.

C6

MAC component that binds to C5b to stabilize the pore.

C7

MAC component that enables insertion of the pore into the membrane.

C8

MAC component that participates in pore formation.

C9

MAC component that polymerizes to complete the pore.

CR1 (Complement Receptor 1)

Receptor that promotes phagocytosis and immune complex clearance and helps regulate complement.

DAF (Decay-Accelerating Factor)

Regulatory protein that accelerates decay of C3/C5 convertases to limit complement activation.