1. The mechanism of passive sensitization of chopped guinea-pig lung when incubated with rabbit antiserum alone and in the presence of non-antibody γ-globulin has been investigated.2. The effect of concentrations of antibody on the fraction of receptors occupied by molecules attached at one, two or three points has been calculated assuming mass action relations. If a single attachment is sufficient for sensitization the predicted log concentration—effect curve is S-shaped; if two or more attachments are needed the curve is bell-shaped. The effect of inhibiting non-antibody γ-globulin on the shape and position of the curves has also been calculated.3. The effect of incubating with a wide range of concentrations of antibody on the subsequent histamine release with antigen has been determined. Bell-shaped curves were obtained. Their shape and position were altered by inhibiting non-antibody in a manner predicted by the two-point attachment hypothesis.4. The first affinity constants for antibody and non-antibody γ-globulin have been calculated from the experimental concentration—effect curves. They are similar and have a value of about 107 l./mole. Approximate values for the second affinity constants and for the fraction of receptors effectively occupied at maximum sensitization have also been estimated.
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