Complement fixing, complement fixation test, CFT.

The binding of complement to antibodies in an antigen:antibody complex. A serological test for detection of antibodies or antigens.

The classical pathway of complement activation is triggered by activation of the C1 complex. This is achieved by the binding of C1q to the C_H3 domains of IgM or the C_H2 domains of IgG. To initiate the complement cascade a single C1q molecule must bind to at least two immunoglobulin Fc portions simultaneously. Because IgM is a pentamer (5 Fc portions) a single molecule can suffice, whereas multiple IgG molecules (as in an immune complex) are needed. The process of binding to C1, triggering activation of the classical pathway, and promoting assembly of the terminal membrane attack complex (MAC) is the process of complement fixation. Because complement reacts with almost any antigen-antibody complex, complement fixation is harnessed as a clinical test. The basic principle of a...