Activation of T Lymphocytes

OVERVIEW OF T LYMPHOCYTE ACTIVATION, 203 SIGNALS FOR T LYMPHOCYTE ACTIVATION, 205

Recognition of Antigen, 205

Role of Costimulators in T Cell Activation, 206

FUNCTIONAL RESPONSES OF T LYMPHOCYTES, 210

Changes in Surface Molecules During T Cell Activation, 210 IL-2 Secretion and IL-2 Receptor Expression, 211 Clonal Expansion of T Cells, 213 Differentiation of CD4+ T Cells into TH1, TH2, and TH17 Effector Cells, 214

Differentiation of CD8+ T Cells into Cytotoxic T Lymphocytes, 219 Development of Memory T Cells, 220

DECLINE OF T CELL RESPONSES, 222

SUMMARY, 222

The goal of T cell activation is to generate, from a small pool of naive lymphocytes with predetermined receptors for any antigen, a large number of functional effector cells that can eliminate that antigen and a population of memory cells that remain for long periods to rapidly react against the antigen in case it is reintroduced. A fundamental characteristic of the T cell response, like all adaptive immune responses, is that it is highly specific for the antigen that elicits the response. Both the initial activation of naive T cells and the effector phases of T cellmediated adaptive immune responses are triggered by recognition of the antigen by antigen receptors of T lymphocytes. In Chapter 6, we described the specificity of T cells for peptide fragments, derived from protein antigens, that are bound to and displayed by self major histocompatibility complex (MHC) molecules. In Chapter 7, we described the antigen receptors and other molecules of T cells that are involved in the activation of T cells by antigens. In this chapter, we describe the biology of T cell activation. We begin with a brief overview of T cell activation, discuss the role of costimulators and other signals provided by antigen-presenting cells (APCs) in T cell activation, and describe the sequence of proliferation and differentiation that occurs in the responses of CD4+ and CD8+ T cells to foreign antigens. The functions of differentiated effector cells in host defense are described in Chapter 10. Thus, Chapters 9 and 10 together cover the biology of T lymphocytes.

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