Book excerpt: T cells play a vital role mediating adaptive immunity, a specific acquired resistance to an infectious agent produced by the introduction of an antigen. There are a variety of T cell types with different functions. They are called T cells, because they are derived from the thymus gland. This volume discusses how T cells are regulated through the operation of signaling mechanisms. Topics covered include positive and negative selection, early events in T cell receptor engagement, and various T cell subsets.