42.2 Adaptive immune response  (Page 3/39)

Naïve T cells can express one of two different molecules, CD4 or CD8, on their surface, as shown in [link] , and are accordingly classified as CD4 ⁺ or CD8 ⁺ cells. These molecules are important because they regulate how a T cell will interact with and respond to an APC. Naïve CD4 ⁺ cells bind APCs via their antigen-embedded MHC II molecules and are stimulated to become helper T (T _H ) lymphocytes , cells that go on to stimulate B cells (or cytotoxic T cells) directly or secrete cytokines to inform more and various target cells about the pathogenic threat. In contrast, CD8 ⁺ cells engage antigen-embedded MHC I molecules on APCs and are stimulated to become cytotoxic T lymphocytes (CTLs) , which directly kill infected cells by apoptosis and emit cytokines to amplify the immune response. The two populations of T cells have different mechanisms of immune protection, but both bind MHC molecules via their antigen receptors called T cell receptors (TCRs). The CD4 or CD8 surface molecules differentiate whether the TCR will engage an MHC II or an MHC I molecule. Because they assist in binding specificity, the CD4 and CD8 molecules are described as coreceptors.

Art connection

Which of the following statements about T cells is false?

1. Helper T cells release cytokines while cytotoxic T cells kill the infected cell.
2. Helper T cells are CD4 ⁺ , while cytotoxic T cells are CD8 ⁺ .
3. MHC II is a receptor found on most body cells, while MHC I is a receptor found on immune cells only.
4. The T cell receptor is found on both CD4 ⁺ and CD8 ⁺ T cells.

Consider the innumerable possible antigens that an individual will be exposed to during a lifetime. The mammalian adaptive immune system is adept in responding appropriately to each antigen. Mammals have an enormous diversity of T cell populations, resulting from the diversity of TCRs. Each TCR consists of two polypeptide chains that span the T cell membrane, as illustrated in [link] ; the chains are linked by a disulfide bridge. Each polypeptide chain is comprised of a constant domain and a variable domain: a domain, in this sense, is a specific region of a protein that may be regulatory or structural. The intracellular domain is involved in intracellular signaling. A single T cell will express thousands of identical copies of one specific TCR variant on its cell surface. The specificity of the adaptive immune system occurs because it synthesizes millions of different T cell populations, each expressing a TCR that differs in its variable domain. This TCR diversity is achieved by the mutation and recombination of genes that encode these receptors in stem cell precursors of T cells. The binding between an antigen-displaying MHC molecule and a complementary TCR “match” indicates that the adaptive immune system needs to activate and produce that specific T cell because its structure is appropriate to recognize and destroy the invading pathogen.