Complement System and Classical Pathway Vocabulary

Vocabulary flashcards covering key terms and definitions from the complement system lecture notes.

Complement

A defense system of soluble proteins that kill/lyse foreign organisms; activated by antigen–antibody complexes.

Classical Pathway

One activation route of the complement system initiated by antigen–antibody complexes.

Antigen

Foreign macromolecule, often pathogen surface protein(s), that elicits an immune response.

Antibody

Immunoglobulin produced by the immune system in response to a specific antigen.

Antigen–Antibody Complex

Complex that triggers the complement cascade and initiates the classical pathway.

C1 Complex

Recognition unit of the classical pathway, composed of C1q, C1r, and C1s; binds antibodies.

C1q

Subcomponent that binds the Fc region of antibodies (2IgG or 1 IgM) to activate the C1 complex; changes C1r from inactive to active

C1r

Subcomponent activated by C1q that subsequently activates C1s.

C1s

Subcomponent activated by C1r; cleaves C4 and C2 in the activation unit.

IgG

Immunoglobulin G; activation of the classical pathway requires two IgG molecules bound to antigen.

IgM

Immunoglobulin M; one bound IgM (often pentameric) can activate C1.

Requirements for Recognition Unit Binding

2 adjacent antigen-antibody complexes within binding range; stability that activates C1 in the classical pathway.

Activation Actions of C1 Complex

cleaves C4 into C4a and C4b

cleaves C2 into C2a and C2b

C4

Complement component cleaved by C1s into C4a and C4b; C4b participates in activation.

C4a

Fragment released from C4; not used in complement

C4b

Fragment that participates in activation by forming part of the C3 convertase with C2a.

C2

Complement component cleaved by C1s into C2a and C2b.

C2a

Fragment that participates in forming the C3 convertase with C4b.

C2b

Fragment produced from C2 cleavage; not used in complement, acts as anaphylactoxin

C3 Convertase

C4b2a; formed by C4b and C2a; cleaves C3 to produce C3a and C3b.

C3

Central complement component cleaved into C3a and C3b

C3a

Anaphylactoxin produced from C3; promotes inflammation; not used in complement

C3b

forms part of C5 convertase; also opsonin that makes C3b sites on surface of target cells

Immune Adherence Phenomenon

Increased immune response of lymphocytes, PMNs, macrophages, and monocytes to attack the target cell; occurs after C3b sites are on the surface

C5 Convertase

C4b2a3b; formed when C3b associates with the C3 convertase; cleaves C5.

C5

Complement component cleaved by C5 convertase into C5a and C5b.

C5a

fragment of C5; anaphylactoxin that mediates inflammation; not used in complement

C5b

Initiates assembly of the membrane attack complex by embedding in the pathogen membrane then recruiting C6–C9.

MAC (Membrane Attack Complex)

Pore-forming complex that disrupts pathogen membranes and causes lysis.

C6

MAC component that binds with C5b–C7 to form the pore; first to bind to MAC

C7

MAC component; second to bind to MAC

C8

MAC component that assists pore formation in the membrane; penetrates membrane before recruitment of C9; third to bind to MAC

Poly C9

MAC component that forms a transmembrane channel, which disrupts membrane integrity and causes lysis; last to bind to MAC

Recognition Unit

First complement step; C1q binds to antibody Fc and activates C1r/C1s.

Activation Unit

Second complement step; formation of C3 and C5 convertases

MAC Formation

Final assembly of MAC (C5b–C9) that creates a membrane pore resulting in pathogenic cell lysis