IL-4 plays a key role in the contact sensitivity skin reaction. This has several implications. First, the view that contact sensitivity (CS) is only mediated by cells with a Th1 profile of cytokine secretion needs modification, in the light of the essential role of IL-4 at the effector stage. Second, the concept of a single cell involved in the systemic transfer of CS is no longer tenable, as it is known that both alpha beta and gamma delta cells are required. Studies with the cell lines (which contain both alpha beta and a few gamma delta cells) suggest that this double requirement may involve the action of IL-4 on gamma delta cells, which bear receptors for IL-4. Finally, the view that T cell lines only transfer CS when injected locally, but not when injected intravenously (systemic transfer), is correct but incomplete, as T cell lines actually give systemic transfer of CS, providing the cell line or the recipient is treated with IL-4.