Much progress has been made over the last several decades in unraveling the complex nature of our immune system. The ability of our immune system to protect us is dependent upon the ability of immune cells to communicate with one another in order to coordinate activities. Cells are able to communicate with one another through cell-to-cell contact or by secreting small signaling proteins called cytokines. Cell-to-cell contact and/or reaction to cytokines is mediated through a diverse variety of membrane bound receptors and ligands that are expressed on the surface of the cell at the right moment. Ligands are molecules that bind to receptors initiating a signal. Literally hundreds of these receptors and ligands have been identified by immunologists all over the world. Today each receptor is referenced using an international language called the “Cluster of Differentiation” or CD. A number identifying the order in which the receptor was discovered follows the letters CD. For example, all T lymphocytes express the receptor CD3. However, a special type of T lymphocyte called a “Helper” T Cell, also expresses the receptor referred to as CD4.