IL-4 was first described in 1982 as a cofactor in the proliferation of resting B-cells stimulated through the cross-linkage of their membrane IgM by anti-IgM antibodies . It was also described as a T-cell factor that induced B-cell differentiation into plasma cells secreting IgG1 . Hence its early names were B-cell stimulation factor-I (BSF-I), B-cell differentiation factor-I (BCDF-I) and B-cell growth factor-I (BCGF-I).

IL-4 exerts different effects on B-cells at different stages in the cell cycle. On resting B-cells, IL-4 acts as an activating factor, inducing them to enlarge in size and increase class II MHC expression. Following activation by an antigen or mitogen, IL-4 acts as a growth factor, driving DNA replication in the B-cells. In the case of proliferating B cells, IL-4 acts as a differentiation factor by regulating class switch to Cepsilon and Cgamma1, i.e., the production of the IgE and IgG1 subclasses. In this role it has been termed a "switch-inducing" factor.

IL-4 also plays a major role in T-cell development. It is thought to be influential in promoting differentiation of T helper cells into TH2 cells during an immune response. IL-4 can also act as a mast cell growth factor. IL4 actions (except increases in MHC II) are "neutralized" by IFNgamma , which is made by TH1 cells .