Complement System Lecture Flashcards

Vocabulary-style flashcards covering the general properties, components, activation stages, and specific pathways of the complement system.

Complement (C)

A group of proteins, constituting about $5\%$ of normal serum proteins, that are found in an inactive form but augment immune responses when activated.

Inactivated serum

Serum that has lost its complement activity after being heated at $56^{\circ}C$ for $30\,minutes$, causing the proteins to denature.

Properdin system

A group of serum proteins associated with the complement system that are named using letter symbols, such as factor-B.

Zymogens

The inactive form in which all complement proteins are synthesized before they are activated by proteolysis.

Fragment ‘b’

The larger fragment produced during proteolysis (e.g., C3b) that participates in the cascade reaction, with the exception of C2a.

Fragment ‘a’

The smaller fragment produced during proteolysis (e.g., C3a) that typically diffuses away to mediate functions other than the main cascade.

Classical pathway

An antibody-dependent complement activation pathway triggered by the formation of antigen-antibody complexes.

Alternative pathway

An antibody-independent complement activation pathway triggered directly by the antigen.

Lectin pathway

A recently described antibody-independent pathway that resembles the classical pathway mechanism.

C1q

A hexameric subunit of C1 having six globular heads that react with the Fc portion of IgM or IgG bound to an antigen.

C1s esterase

The activated form of the C1s subunit that cleaves C4 into C4a (an anaphylatoxin) and C4b.

C3 convertase (Classical)

The enzymatic complex C14b2a, formed in the presence of magnesium ions, which hydrolyses C3 molecules.

C5 convertase (Classical)

The complex C14b2a3b, formed when C3b remains attached to C3 convertase, which functions to cleave C5.

Membrane attack complex (MAC)

The C5b6789 complex that destroys target cells by creating tubular channels of $10\,nm$ diameter in the cell membrane.

Complement-mediated cytotoxicity

The process of cytolysis where a target cell is destroyed via the action of the membrane attack complex.