Complement System and Cytotoxicity – Core Vocabulary

A concise set of vocabulary flashcards covering the main proteins, pathways, enzymes, and biological outcomes of complement activation with emphasis on cytotoxicity.

Complement System

A group of plasma proteins that work in a cascade to enhance innate and adaptive immunity by promoting inflammation, opsonization, and direct lysis of pathogens.

Classical Pathway

Complement activation route triggered by antigen–antibody (IgG or IgM) complexes, beginning with the C1q r₂ s₂ complex.

Lectin Pathway

Antibody-independent complement route initiated when mannose-binding lectin (MBL) or ficolins bind microbial carbohydrates, activating MASP proteases.

Alternative Pathway

Complement route that is constantly tick-over–activated and is amplified when C3b binds directly to microbial surfaces, involving factors B, D, and properdin.

C1 Complex (C1q r₂ s₂)

The initiating complex of the classical pathway; C1q recognizes antibodies, while C1r and C1s are serine proteases that cleave C4 and C2.

C3 Convertase

An enzyme complex that cleaves C3 into C3a and C3b; forms are C4b2a (classical/lectin) and C3bBb (alternative).

C5 Convertase

A complement complex that cleaves C5 into C5a and C5b; forms include C4b2a3b (classical/lectin) and C3bBbC3b (alternative).

C3b

Large cleavage fragment of C3 that covalently binds pathogen surfaces, acting as an opsonin and part of C5 convertase.

C3a

Small peptide released from C3; functions as an anaphylatoxin that promotes mast-cell degranulation and chemotaxis.

C5a

Potent anaphylatoxin produced from C5; a strong chemoattractant for neutrophils and activator of inflammation.

Membrane Attack Complex (MAC)

The lytic pore (C5b-9) formed on target membranes that leads to osmotic lysis of bacteria and some viruses.

Opsonization

Process by which molecules such as C3b coat a pathogen, enhancing its recognition and uptake by phagocytes.

Chemotaxis

Directed migration of immune cells toward higher concentrations of complement-derived peptides (e.g., C5a, C3a).

Anaphylatoxin

Complement fragments (C3a, C4a, C5a) that promote vasodilation, increased vascular permeability, and degranulation of mast cells.

Mannose-Binding Lectin (MBL)

A soluble pattern-recognition molecule that binds mannose residues on microbes and initiates the lectin pathway.

MASP-2

MBL-associated serine protease that cleaves C4 and C2 during lectin pathway activation.

Factor B

Complement protein that binds C3b on microbial surfaces and is cleaved by factor D to form part of the alternative C3 convertase (C3bBb).

Factor D

Serine protease that cleaves factor B once it is bound to C3b, enabling formation of C3bBb.

Properdin (Factor P)

Stabilizing protein that binds and prolongs the half-life of the alternative pathway C3 convertase (C3bBb).

Serine Protease

An enzyme with a serine residue at its active site; many complement components (C1r, C1s, MASP-2, factor D) are serine proteases.

Complement Receptor 1 (CR1)

Phagocyte and erythrocyte receptor that recognizes C3b and C4b, facilitating clearance of opsonized particles.

Factor H

Regulatory protein that inhibits the alternative pathway by displacing Bb from C3b and promoting C3b cleavage by factor I.

C4b2a

The classical/lectin pathway C3 convertase composed of C4b and C2a.

C3bBb

The alternative pathway C3 convertase made of C3b and Bb.

C4b2a3b

Classical/lectin pathway C5 convertase formed when C3b associates with C4b2a.

C3bBbC3b

Alternative pathway C5 convertase created when an additional C3b binds C3bBb.

Terminal Pathway

Final common sequence (C5b, C6, C7, C8, C9) leading to assembly of the membrane attack complex.

C5b-9

Composite designation for MAC components that insert into membranes to create lytic pores.

Anaphylaxis (Complement-mediated)

Acute inflammatory reaction driven by excess anaphylatoxins, causing smooth-muscle contraction and vascular leakage.

Complement-Mediated Cytotoxicity

Process by which complement activation results in direct lysis of pathogens or antibody-coated cells via MAC formation.