Complement Terry Kotrla, MS, MT(ASCP)BB Fall 2007

Introduction Complement refers to a complex set of 14 distinct serum proteins (nine components) that are involved in two separate pathways of activation. –Components numbered in order of discovery. –Sequence of activation is not in numerical order. –Components circulate in inactive precursor form, develop activity upon activation. Complement proteins designated by “C” followed by numbers and letters.

Two major functions: Promote the inflammatory response. Alter biological membranes to cause direct cell lysis or enhanced susceptibility to phagocytosis.

Complement

General Properties of Complement Primary role is cell lysis. Activity of complement destroyed by heating sera to 56 C for 30 minutes. IgM and IgG are the only immunoglobulin capable of activating complement (classical pathway). Complement activation can be initiated by complex polysaccharides or enzymes (alternative or properdin pathway). Portions of the complement system contribute to chemotaxis, opsonization, immune adherence, anaphylatoxin formation, virus neutralization, and other physiologic functions

Sequential interaction of complement components Cleavage of components generates a small fragment which is released, and a larger molecule which attaches to cell surface and continues in reaction sequence. Sequence of activation referred to as a cascade reaction.

Three Pathways

Classical Pathway – C1: The Recognition Unit C1 consists of 3 subunits: C1q, C1r, and C1s. C1q molecule consists of a collagenous region with six globular head groups globe end serves as recognition unit When antibody binds to antigen, binding sites for the globular head groups of C1q are exposed on the Fc region of the antibody.

Classical Pathway – C1: The Recognition Unit For C1q to initiate the cascade it must attach to at least 2 Fc fragments, requires at least 2 molecules of IgG or one molecule of IgM. C1q binding causes C1r to enzymatically activate C1s.

Classical Pathway – C1: The Recognition Unit

The Activation Unit (C4b2a3b) C1s cleaves C4 into C4a and C4b C1s cleaves C2 into C2a and C2b C4b2a (C4b2b in some texts) is enzymatically active and can cleave many molecules of C3 into C3a and C3b.

Membrane Attack Unit In the presence of C5b, molecules of C6, C7, C8 and a variable number of C9 molecules assemble themselves into aggregates. This molecular complex causes a change in membrane permeability. Exact cause of lysis unknown, one theory is change in lipid membrane causes exchange of ions and water molecules across membrane. Cells can lyse without C9 but it’s slower.

Membrane Attack Unit

Order of Activation in Classical Pathway C1, C4, C2, C3, C5, C6, C7, C8, C9

Alternative Pathway (Properdin Pathway) Cleavage of C3 and activation of the remainder of the complement cascade occurs independently of antibody. Triggers for the alternative pathway include –bacterial cell walls –bacterial lipopolysaccharide –fungal cell walls –some virus infected cells –and rabbit erythrocytes

Alternative Pathway (Properdin Pathway) Molecules of C3 undergo cleavage at continuous low level in normal plasma. At least 4 serum proteins (factor B, factor D, properdin (P), and initiating factor (IF) function in this pathway. C3b attaches to appropriate site (activating surface) which is actually a protective surface

Alternative Pathway (Properdin Pathway) Action by the 4 serum proteins on C3b proceeds to the C3 activator stage without participation of C1, C4 or C2. Activation sequence: C3, C5, C6, C7, C8, C9.

Regulation of the Complement Cascade Modulating mechanisms are necessary to regulate complement activation and control production of biologically active split products

Regulation of the Complement Cascade First means of control is extreme lability of activated complement –If activated complement does not combine within milliseconds the activity is lost or decreased. –Active fragments rapidly cleared from the body.

Regulation of the Complement Cascade Second type of control involves specific control proteins –C1 inhibitor blocks activity of C1r and C1s. –Factor I in activator in the presence of certain cofactors inactivates C3b and C4b. –A number of proteins act to control membrane attack unit

References