In conclusion, we demonstrated, for the first time, that (1) TBI-induced long-term memory deficits in aged animals are dependent on accumulation of early complement cascade components (C1q, C3, and CR3) in the brain; (2) there is a progressive increase in synaptic engulfment activity by microglia; and (3) inhibition of the classical complement cascade, either through deletion of C3 or inhibition of C1q, provides protection against cognitive decline (see Figure 7). This evidence concerns the gene C3 and Mental deterioration.