Soluble low affinity Fc receptors for IgE in the serum of allergic and nonallergic children.

IgE-binding factors are thought to have regulatory activity in in vitro IgE synthesis. To obtain evidence of the participation of IgE-binding factors in in vivo IgE synthesis, the serum level of low affinity Fc receptors for IgE (sFc epsilon RII) (IgE-BFs) was examined in 41 nonallergic children and in 37 allergic children whose serum IgE levels were significantly higher than those of nonallergic children. The serum level of sFc epsilon RII showed a marked age-dependent variation. It was highest in infants and then decreased gradually with age. The serum level of sFc epsilon RII in allergic children was significantly higher than that of nonallergic children in early childhood (1128.0 +/- 323.8 vs 777.3 +/- 227.0 pg/ml, p less than 0.01 in infants (less than 1 y) and 851.8 +/- 270.0 vs 579.4 +/- 197.1 pg/ml, p less than 0.05 in children aged 1-2 y) but not in older children (3-15 y). Three allergic infants (less than 1 y) with serum sFc epsilon RII levels higher than the mean + 1 SD (1451.8 pg/ml) of all allergic infants (less than 1 y) had serum IgE levels (geometric mean 125.9 IU/ml) significantly higher than the other seven allergic infants (less than 1 y) (geometric mean 5.6 IU/ml, p less than 0.05). A close positive correlation between the serum level of sFc epsilon RII and the absolute number of Fc epsilon RII(+) peripheral blood lymphocytes was observed (Spearman's rank correlation coefficient = 0.79, p less than 0.001 in 27 allergic and Spearman's rank correlation coefficient = 0.72, p less than 0.001 in 19 nonallergic children). In conclusion, serum sFc epsilon RII may be derived mainly from Fc epsilon RII(+) lymphocytes, and may have relationship to the increased production of IgE in early childhood (0-2 y).