In response to a second injection of rabbits with a dinitrophenylated antigen, given 2 months to 2 yr after the first injection, there was rapid synthesis of large amounts of antibody high in relative affinity for the dinitrophenyl (DNP) determinant. The antibodies formed 3 days after restimulation were already high in affinity.

Amounts of antigen too small to elicit detectable antibody production may prime the animal for a partial secondary response characterized by the formation of antibody of intermediate affinity after a second antigenic stimulus.

Investigations into the specificity requirements for the secondary response indicated that variation in the carrier protein and in the haptenic determinant could be tolerated. Thus, after immunization with DNP-bovine γ-globulin, DNP-hemocyanin elicited the vigorous production of high affinity anti-DNP antibodies. However, DNP serum albumin was much less effective: it elicited a secondary response in some animals primed with DNP-bovine γ-globulin only when the interval between injections was increased from 10 to 28 wk. A secondary response was also evoked when the haptenic determinent of the second immunogen differed slightly from that of the one injected initially (i.e., 2,4,6-trinitrophenyl versus 2,4-dinitrophenyl).

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