B-cell growth and differentiation factors.

The number of recombinant factors which have been shown to regulate B cell activation, proliferation, and differentiation now stands at nine (IL-1, IL-2, IL-4, IL-5, IL-6 (BSF-2), BCGFLOW, IFN-alpha and gamma, and TNF). Several others have been described but are not yet fully characterized or available as recombinant gene products and will not be discussed. Recent work with these factors, especially IL-4, has revealed a remarkable diversity of function. Most of them seem able to act at more than one stage of B cell activation, proliferation and differentiation, thus invalidating the concept of a single unique factor controlling each discrete step of normal B cell responses. In addition, these factors are not B cell specific, but have a wide range of cell types as targets. The functional diversity of these factors has important implications for their potential as therapeutic agents, and for the design of standardization protocols.