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T lymphocytes (T cells), the mediators of cellular immunity, get their name from the organ site where the final maturation stages of bone marrow‐derived pre‐T cells take place, namely the thymus. Fully mature T cells emerge from the thymus as naïve T cells. All T cells express T cell receptors (TCR) in association with CD3, which, together with other transmembrane signal transduction molecules (ζ homodimers), enable cells to communicate with the nucleus when the T cell binds to its specific antigen (Figure 2.10). T‐cell activation and signaling mechanisms are discussed in detail in Chapter 10.

As shown in Figure 2.6, T cells differentiate from common lymphoid progenitor cells into a variety of functionally distinct T‐cell subsets. As noted earlier, the bone marrow environment where T cells first develop is not equipped to promote the final maturation stages of T cells. That process takes place within the thymus.

Phenotypically, fully mature T cells, all of which express the cluster of differentiation (CD) molecule CD3 (see Figure 2.10), can be broadly divided into cells expressing CD4 or CD8. Cytotoxic T cells (T_C) capable of killing target cells expressing antigens recognized by their TCRs express both CD3 and CD8 on their membranes whereas all other T‐cell subsets express CD3 and CD4. The CD4 cells are further subdivided into the different subsets shown in Figure 2.10 including T_H1, T_H2, T_H17, and T_{Follicular Helper} (T_FH) cells. As discussed below, each of these subsets has characteristic functional properties mainly due to their associated cytokine and transcription factor profiles.

Figure 2.9. T‐cell‐dependent antigen activation of B cells. The antigen stimulates a member of a B‐cell clone expressing a BCR with antigen specificity. Activation requires T_H cell help which provides proliferation and differentiation factors (cytokines) to facilitate B cell proliferation and differentiation into antibody‐secreting plasma cells and memory cells.