Veterinary Immunology and Serology

THE CLASSICAL PATHWAY

• Free immunoglobulin molecules can not bind or activate complement components.
• When immunoglobulin binds to the antigen, the complement-binding site is exposed (because of conformational changes).
• In classical pathway, first complement component C₁ binds to CH³ domain of IgM or CH² domain of IgG molecule. The C₁ molecule is composed of three separate proteins C_1q, C_1r and C_1s bound together by calcium (Ca++) dependent bonds.
• The C_1q subunit is made up of an umbrella like radial array of six chains that are connected to central stalk by a collagen like arm and each has globular head, which recognizes and binds to Fc region of immunoglobulin heavy chain.
• Each Fc region of an immunoglobulin has one C_1q binding site and for activation of C₁q at least two heavy chain (Fc region) must bind.
• Since IgG has one Fc region, at least two molecule of IgG must be brought close together before C_1q can bind and this is possible when they bind to a multivalent antigen.
• IgM being a pentamer, one single molecule can activate C₁.Thus IgM is more efficient complement fixing antibody than IgG.
• C₁r and C₁s are serine esterases and they form a tetramer complex containing two molecule of each and located between C₁q strands. Binding of C_1q to two or more Fc regions leads to enzymatic activation of C₁r that cleaves and activates C₁s.
• The activation is normally prevented by a protein C₁ inhibitor (C₁ – INH), it also removes C_1r and C_1s from the complex but this inhibition is overcome when immunoglobulin is bound to antigen.
• Activated C₁s cleaves the next protein in the cascade, C₄ to generate C_4b (the small fragment C_4a leaves the major fragment C _4b and the removal of C_4a activates and expose a thioester bond on the C₄b molecule that generates a reactive carbonyl (=C=0) group and binds C₄b to target cell surface (i.e. antigen).
• The C₂ glycoprotein binds C_4b to form C_4b2 in presence of Mg⁺⁺ ions. Activated C_1s splits bound C₂ into C_2a (larger fragment) and C_2b (smaller, soluble fragment). The C₂ must be bound to C₄ before it is cleaved and this is called substrate modulation. The C_4b2a complex is the classical pathway C₃ convertase. The C_4b2a protease breaks down C₃ into C_3a and C_3b. The small fragment C_3a is removed and C_3b form covalent bonds with target cell surface or with the antibody where complement activation was initiated. Once C_3b is deposited, it can bind to factor B and generate more C₃ convertase by the alternative pathway. Thus a single molecule of C_4b2a complex can lead to the deposition of hundred or thousands of molecules of C_3b on the cell surface where complement is activated. C_4b2a3b complex function as the classical pathway C₅ convertase and cleaves C₅ and initiate the terminal steps of complement of activation.
• The classical pathway was first identified and characterized.