The C1q complex 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. 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.
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