C4A and thanatophoric dysplasia: The system consists of the classical, lectin, and alternative pathways that converge on the cleavage of C3 into its active fragments C3a and C3b, resulting in phagocytosis, inflammation, and cell lysis by formation of the membrane attack complex (reviewed in Woo et al., 2019 In Huntington’s disease patients who often manifest dyskinesia reminiscent of TD, complement components of the classical immune pathway, including C4, were found in their striatum, but not so in control brains (Singhrao et al., 1999).