Hypocomplementaemia due to a genetic deficiency of beta 1H globulin.

An 8-month-old Asian boy who presented with the haemolytic uraemic syndrome was found to have a low haemolytic complement, and in particular a very low C3 level, with a normal C4 level. These abnormalities persisted after recovery and were not associated with the presence of circulating C3 nephritic factor. A clinically healthy 3-year-old brother was found to have an identical complement profile, which indicated increased alternative pathway activation. Both brothers had normal levels of the C3b inactivator, but very low levels of beta 1H globulin (less than 10% of a reference standard serum). The parents, who were first cousins, had half-normal levels of beta 1H globulin, and low levels were found in other members of the family, indicating that the defect was inherited.