The cytokine responsiveness is directly related to the number of receptors on the surface of the cell or the intracellular pathways up-regulated by the cells. The immune system in general and hematopoiesis are controlled through the up- and down-regulation of both cytokines (ligands) released by cells and cytokine receptors on the surface of cells. Cytokine responsiveness changes as cells make their way through the maturation process. Transcription factors play a key role in this regulation. It makes sense as certain cytokines lead to different cellular events at different times along the way.
