Reversible C3 hypocomplementaemia in megaloblastic anaemia due to vitamin B12 deficiency.

Serum C3 and C4 levels have been determined in patients with Addisonian pernicious anaemia (PA) and megaloblastic anaemia due to vitamin B12 deficiency from other causes, before and after treatment, in order to study the interaction between vitamin B12 deficiency and complement and the role of complement in the pathogenesis of the gastric lesion of PA. C3 levels are significantly reduced in vitamin B12 deficiency and return to normal on treatment; C3 levels correlate with the degree of anaemia but not with serum vitamin B12 levels at diagnosis. C4 levels are normal. These observations suggest that the observed C3 hypocomplementaemia is not a consequence of immune mechanisms, but may be due to altered synthesis of C3 complement component.