T Cells and the Cellular Response

Different Immune Responses - The humoral immune response is based on precipitation, but the cellular response involves a quite different mechanism for killing foreign cells. The cellular response plays a major role in tissue rejection and in destroying virus-infected cells. It may also destroy potential cancer cells before they have a chance to propagate. Although the mechanisms of the humoral and cellular processes are quite different, similar immunoglobulin molecules are involved in both cases (Figure 7.35).

Cytotoxic T Cells - The major participants in the cellular immune response are cytotoxic T cells, also referred to as killer T cells, which carry on their surfaces receptor molecules (T cell receptors) that are similar to the Fab fragments of antibody molecules. T cell receptors have a wide range of binding specificities, mostly directed toward short oligopeptide sequences. Oligopeptide sequences on foreign cells are bound by the major histocompatibility complex of T cells (MHC proteins; see Figure 7.35). When a killer T cell identifies (via its receptor) an appropriate antigen carried on the surface of another cell by an MHC protein, it releases a protein called perforin which forms pores in the plasma membrane of the cell being attacked, allowing critical ions to diffuse out and thereby killing the cell.

Common Structures - Both the antibodies of the humoral response and the molecules involved in the cellular response contain elements of common structure. The similarity is even greater than Figure 7.35 suggests. The domains in these molecules are built on a common motif, called the immunoglobulin fold, in which two antiparallel sheets lie face to face (Figure 7.36). This structure probably represents the primitive structural element in the evolution of the immune response. The immunoglobulin fold is also found in a number of other proteins.