Use of sheep erythrocytes as solid phase supports in double antibody immunoassay systems.

There is a growing trend to use antibodies coupled to insoluble support materials to facilitate the separation of antigen-antibody complexes from antigen in radioimmunoassay (RIA) procedures. Materials most widely employed are polymers, such as polystyrene and agarose. Usually a suspension of fine beads is used. Alternatively, antibodies may be attached to the walls of the polystyrene tubes in which the assay is to be carried out. Erythrocytes have a number of properties that make them suitable for use as insoluble supports, and they have been extensively used over many years in passive haemagglutination procedures. They do not appear to have gained popularity as reagents for RIA, although Luner (1975) has described an insulin assay in which he used antibodies coupled to cells by the chromic chloride technique. Erythrocytes have a small size that gives them a large surface area to volume ratio and, in addition, permits them to stay in suspension for a reasonable length of time without continual agitation. Their uniformity of size results in even settling. They are inexpensive and readily obtainable. A further advantage possessed by red cells is that they are themselves antigenic. Consequently, it is possible to use an immunochemical procedure for coupling antibodies to them that is less damaging than chemical procedures. For example, to couple donkey anti-rabbit gammaglobulin antibodies to sheep red cells (SRBC), the cells are first treated with rabbit anti-SRBC antiserum to produce a coating of rabbit gammaglobulin (anti-SRBC antibody). The coated cells are then exposed to an excess of gammaglobulin antiserum to give a second coating with antibodies that are capable of binding further molecules of rabbit gammaglobulin. We have used antibody-coated SRBC for over a year and have found the technique to have advantages over conventional assay procedures employing antibody in solution. In this report we shall describe their use in an RIA for growth hormone (HGH) as a model system.