Lymphocytes bearing Fc receptors for IgE. VIII. Affinity of mouse IgE for Fc(ε)R on mouse B lymphocytes

Fc(ε)R(+) lymphocytes were demonstrated in BALB/c, C57BL/6 and SJL strains of mice by a rosetting technique. The proportion of Fc(ε)R(+) lymphocytes in their spleens was 25 to 33% of the total cells. The majority of the Fc(ε)R(+) cells in the spleen of BALB/c mice were B cells, which also bore Fc(γ)R. However, after infection of mice with Nippostrongylus brasiliensis, a significant proportion of T cells also bore Fc(ε)R. The average number of Fc(ε)R per Fc(ε)R(+) cell in normal BALB/c spleens was 5100. These receptors were saturated with IgE after incubation of the cells with 5 μg/ml mouse IgE. The forward rate constant (k₁) of the IgE binding to Fc(ε)R on normal B cells was 1.66 x 10⁴ M^-1 sec^-1, while the dissociation rate of IgE from the Fc(ε)R was 1.7 x 10^-4 sec^-1. The equilibrium constant between mouse IgE and Fc(ε)R on normal B cells was approximately 0.95 x 10⁸ M^-1. However, Fc(ε)R on mouse lymphocytes appeared to be heterogeneous with respect to their affinity for IgE. After infection of mice with Nippostrongylus brasiliensis, the number of Fc(ε)R per receptor-bearing lymphocyte increased several-fold. The Fc(ε)R newly expressed after infection appeared to have a lower affinity for IgE than those present originally.