The alternate complement pathway in inflammatory bowel disease. Quantitation of the C3 proactivator (factor B) protein.

A component of the complement system's alternate pathway was investigated in ulcerative colitis and Crohn's disease. The mean C3PA (Factor B) titer in normals was 74 +/- 15%; in ulcerative colitis, 92 +/- 18%; and in Crohn's disease, 119+/- 24%. Significance was at the P less than 0.001 level when the mean values for the ulcerative colitis and the Crohn's disease groups were compared to normal subjects. Titers did not change significantly with exacerbation or amelioration of the diseases or when patient groups were analyzed according to the mode of treatment received.