Level of complement activity and components C1, C4, C2, and C3 in complement response to bacterial challenge in malnourished rats.

In experimentally induced malnutrition in rats, there was no significant difference between the measured level of complement activity of the classical pathway (50% hemolytic complement [CH50]) and that of the alternative pathway (ACH50), although the levels of complement components C1, C4, C2, and C3 were depressed significantly. The complement activity showed a temporary elevation with a peak at 2 or 3 days after bacterial challenge with Staphylococcus aureus in rats, and we call this the complement response. After 3 days, CH50 and C3 in the malnourished rats and ACH50, CH50, and C3 in the well-nourished rats showed a significant increase, and C1, C4, and C2 in both groups tended to elevate. On the basis of these observations, the significance of the elevation of C3 in the complement response to bacterial infection showed a strong influence by enhancing the activation of both the classical and the alternative pathways, since C3 is known to be the junction of both complement pathways. In this way, C3 responded to an earlier stage than did the other components and may contribute to maintaining the body defense system against infection.