Structural requirements of rabbit IgG F(ab')2 fragment for activation of the complement system through the alternative pathway--II. Ionic and indole groups

Immunoglobulin fragments, F(ab')2, that are cytotoxic to enzyme-treated cells

Bivalent immunoglobulin fragments of IgG, F(ab')2, prepared from normal murine sera were found to be cytotoxic to neuraminidase-treated cells. The fragments were cytotoxic to both allogenic and syngeneic targets (with respect to the source of the sera), suggesting that the antigen bound by the F(ab')2 is not related to the major histocompatibility locus of mice (H-2).

The biological properties of immunoglobulin G and its split products [F(ab')2 and Fab]

Antibodies of the IgG class possess antibacterial, antiviral and toxin neutralizing properties and for this reason are administered prophylactically and therapeutically. In the case of the immunoglobulin preparations commercially available for i.v. application a basic distinction must be made between unsplit immunoglobulins and those antibody preparations obtained by enzymatic digestion, such as F(ab')2 or Fab antibodies. This survey deals with the largely experimental evidence describing the biological properties of these preparations. Administration of antibodies in the presence of the corresponding antigens leads to the formation of immune complexes in the organism. These immune complexes can activate, either directly or indirectly, the cellular and humoral systems which are involved in phagocytosis and the elimination of antigens, in the regulation of the body's own antibody production and in inflammatory reactions. As a result of their inability to interact with Fc receptors, immune complexes with F(ab')2 or F(ab) antibodies appear to be less active in the release of inflammation mediators from leucocytes and thrombocytes than immune complexes with unsplit immunoglobulins. These, on the other hand, can antigen-specifically and non-antigen-specifically suppress the immune system which is not the case for immune complexes with F(ab')2 or Fab antibodies. There are indications that these split products also occur in vivo due to the action of tissue and leucocyte proteases. Unlike Fab preparations, F(ab')2 antibodies have antibacterial and antiviral potencies similar to unsplit immunoglobulins, which is probably due to the ability of F(ab')2 molecules to activate complement, not by the classical but by the alternative pathway. Like Fab preparations, F(ab')2 molecules appear to be superior to unsplit IgG in the elimination of haptens. On account of the relatively long period of time unsplit immunoglobulins remain in the blood, they are well suited for prophylactic treatment and substitution over longer periods. The extent to which indications, obtained predominantly from experimental studies, of a reduced release of inflammation mediators, a lack of immune suppression and a lack of augmentation of IgG catabolism would advocate the use of F(ab')2 split products, especially for therapeutic purposes, can only be ascertained after prospective and comparative studies have been carried out.