Effector functions of a mouse IgG that lacks the entire CH1 domain. C1q binding and complement fixation in the absence of antigen.

C1q binding and complement fixation were examined of a short-chain IgG2a antibody that lacks the entire CH1 domain. This short-chain antibody has been reported to have a low level of constitutive complement-activating activity in the absence of Ag. Two-dimensional SDS/PAGE and cation-exchange chromatography have demonstrated that two types of IgG2a proteins are secreted by the short-chain IgG2a antibody producing cell line. It has been shown that 1) the difference between these two types of the IgG2a proteins is whether the two L chains are linked by a disulfide bridge or not, and 2) C1q-binding and complement-activating activities are expressed only when the inter L chain disulfide bridge does not exist. A molecular model is presented for the active form of the short-chain IgG2a antibody.