at the top of the image. It is assumed that the globular ends are the sites for multivalent attachment to the complement fixing sites in immune complexed immunoglobulin. Patients with Lupus erythematosus often have deficient expression of C1q. Genetic deficiency of C1q is extremely rare (approximately 75 known cases) although the majority (>90%) of those have
SLE. C1q associates with C1r and C1s in order to yield the
C1 complex (
C1qr2s2), the first component of the serum
complement system. Deficiency of C1q has been associated with
lupus erythematosus and
glomerulonephritis. It is potentially
multivalent for attachment to the complement fixation sites of
immunoglobulin. The sites are on the CH2 domain of
IgG and, it is thought, on the CH4 domain of
IgM. IgG4 cannot bind C1q, but the other three IgG subclasses can. The appropriate
peptide sequence of the complement fixing site might become exposed following complexing of the immunoglobulin, or the sites might always be available, but might require multiple attachment by C1q with critical geometry in order to achieve the necessary
avidity. ==References==