Isotyping of component C4 of human complement using differences in the functional activity of isotypes C4A and C4B

The difference in the functional activity of the isotypes A and B of compon ent C4 of human complement was used to determine their ratio and to detect the inherited deficiency of the isotypes. ELISA methods were developed for the quantitative assay of component C4 (conventional sandwich method) and i ts functional activity. When determining the functional activity, the class ic pathway of the complement and therefore of component C4 was activated by activators sorbed on ELISA microplates (immunoglobulin IgG3 or Liposacchar ide of the Shigella sonnei cell walls, which activates the complement by bi nding component C1). The nascent fragment C4b is covalently bound to the ta rget activator; C4Ab binds better to the target protein (immunoglobulin), a nd C4Bb to the target carbohydrate (liposaccharide). Therefore, when immuno globulin is a target activator, isotype C4A is bound and determined; and wh en the complement is activated by liposaccharide, isotype C4B is determined . The ratio of the activities determined by the two methods indicates a def iciency in the individual isotypes of component C4 or its absence. The rabb it polyclonal monospecific antibodies against the human component C4 and th e conjugates of these antibodies with horseradish peroxidase were used in t he methods described.