Serum IgG levels and complement activity in hypogammaglobulinaemic patients under substitution therapy

Haemolytic activity of the classical and alternative pathways of complement as well as serum levels of C1q, Factor B, Factor H, C3, C4, C3d,g and IgG were determined in 15 hypogammaglobulinaemic patients on immunoglobulin replacement therapy. Alternative pathway activity (AP) and C1q were defective in the presence of low IgG levels and normalized on achievement of normal IgG levels; for both variables the correlation with serum IgG was highly significant. Classical pathway activity (CP), C3, C4 and Factor H serum levels were normal independently of IgG levels; Factor B and C3d, g serum levels were elevated in hypogammaglobulinaemic patients regardless of IgG levels. The present report supports the hypothesis that IgG serum levels influence complement function.