When guinea pigs are passively sensitized by the injection of rabbit immune serum, quantitative studies show that the anchored, or cellular, antibody is capable of combining with varying quantities of the antigen. This observation holds true whether the antigen be complex, such as horse serum, or simple, such as crystalline egg albumin. It holds true whether the experiments are conducted on the living animal, or on the isolated uterus. Moreover, it holds true, also, of native antibody, as shown by experiments in active anaphylaxis.
Partially combined cellular antibody manifests a marked diminution in its affinity for fresh antigen. This diminution is inverse, but only very roughly, to the degree of saturation by antigen. Considerable variation in the amount of the desensitizing dose of antigen may produce in practically the same degree a loss of reactivity, or of avidity, toward fresh increments of antigen.
The minimal anaphylactic dose after partial desensitization shows an enormous increase over that in the undesensitized animals. This increase cannot possibly be accounted for on the theory of neutralization of part of the cellular antibody, leaving the remainder free to act.
Partially saturated antibody shows not a diminished, but a qualitatively altered reactivity.
Native guinea-pig anti-antibody attenuates alien (rabbit's) sensitizing antibody. In relatively large amounts the former may completely abolish the reactivity of the latter; in smaller amounts it lowers the reactivity in such a fashion that very large amounts of antigen are required to induce an anaphylactic response.
Partially neutralized antigen shows, not a diminution, but a qualitive alteration of its reactive function.
The combination of cellular antibody with antigen in varying proportions suggests an analogy with colloidal reactions, or adsorption phenomena. A very essential point of difference is the specific affinity of the two reagents.
The Danyz-Dungern phenomenon illustrates the fact that toxins and antitoxins in vitro likewise combine in varying proportions.
The alteration of the reactive capacity of cellular antibody by anti-antibody also suggests the analogy of colloidal reactions. It is entirely similar to the effect of diphtheria antitoxin upon diphtheria toxin.
The same antibody, when in solution as precipitin, combines with antigen quantitatively, and in strictly constant proportions, to form precipitate; when attached to the cell, as sensitizing antibody, it combines with antigen in varying proportions. The living cell, therefore, modifies its properties.
There is no general law governing the mode of reaction of antibody. Depending upon circumstances, it may combine with antigen according either to chemical or to physical (colloidal) analogies.