IL-4 assay

[Jens Rainer Hansen]

I have set up an human IL-4 assay using human PBMC (1e6 cells/ml) and PHA
(80-40-20-10 mcg/ml) as 
the stimulator for 72-48 and 24 hours. The amount of IL-4 was determined
using an IL-4 ELISA, but only max. approx. 300 pg/ml was meassured which
also is what I have seen other's have found in the litterature. Only a
small fraktion of the cells are producing IL-4. Is it possible to get the
IL-4 production enhanced ? 
Do you have some experience or some ideas concerning this question? 

Jens Rainer Hansen