Further studies of the C3g component of the alpha 2D fragment of human C3.

Recent studies employing sheep red cells, rabbit anti-sheep cell IgM, guinea-pig R3 reagent and purified human C3, factor I, factor H and human serum have led to the discovery of a new C3b breakdown product antigen, C3g, which is present on the alpha 2D fragment of C3 but is removed by trypsinization. The present studies were undertaken to confirm the above findings in entirely human red cell and serum systems, including studies on cells from nine patients with a variety of complement-mediated autoimmune haemolytic anemias. The results confirm the recently reported findings in every respect. The presence of both C3d and C3g on all patient's red cells so far tested supports the concept that red cells coated by C3d alone are likely to be a laboratory artefact and that the alpha 2D fragment (C3d,g) is the natural end product of cell bound C3b breakdown in vivo.