The Interleukin-2 Receptor (IL-2R) test measures the level of IL-2 receptors in the blood. Interleukin-2 (IL-2) is an important cytokine in the immune system that stimulates the growth and division of T cells, the main type of immune cell in the body. The IL-2 Receptor (IL-2R) is a protein that recognizes and binds to IL-2 on the surface of T cells, helping to regulate the immune response.