C3AR1 and infection: Interestingly, inhibition of C3aR significantly down-regulated the mRNA expression of the occludin gene at half of the time points during C. parvum infection, suggesting that C3a/C3aR signaling likely maintained the integrity of the intestinal barrier by upregulating the expression of the tight junction-related gene occludin, thus reducing the infection and propagation of C. parvum in mouse ilea.