Our observation that key complement factors involved in the complement activation pathways (e.g. C2, C3, C4, factor B), complement control and host protection (CD55, CD46, CFI, CFH), are increased in expression by epithelial cells exposed to infectious challenge, suggests that local complement production may be needed in the intestine as an early defence mechanism against encounters with microorganisms due to a dysfunctional barrier or infection. The gene discussed is C4A; the disease is infection.