C3 and Huntington disease: IHC analysis of postmortem tissue from the caudate nucleus and cerebellum of patients with HD (Vonsattel grade 2 and grade 4) and age-matched controls also revealed increased association of both complement component C1Q, the initiator of the classical complement cascade (expressed by microglia; Extended Data Fig. 1e) and C3 (expressed by both astrocytes and microglia; Extended Data Fig. 1f,g) with corticostriatal synapses in the caudate nucleus of HD brains.