C3 and early-onset autosomal dominant Alzheimer disease: In addition, studies have demonstrated that the classical pathway complement proteins, C1q, C3, C4 in particular, are involved in the pathology of neurological diseases, including Alzheimer’s disease, through the facilitation Aβ clearance or synapse engulfment by reactive microglia (Loeffler, 2004; Propson et al., 2021).