Chapter 1: The Immune System
T-cell structure - Figure 10
Source: Hunt R., Professor at University of South Carolina School of Medicine. (link correct at time of publication; June 2015)
T lymphocytes arise in the BM but soon migrate to the thymus, where they mature to express the Ag-binding T-cell receptor (TCR) on their membrane.
The TCR is a dimer composed of two chains, usually α and β. Similar to the BCR, each one of these chains includes a variable and a constant domain. (Fig 1.10)
T cells are able to recognise Ags (through their TCR) only when the Ag is bound to a major histocompatibility complex (MHC) molecule.
T-cell maturation - Figure 11
Figure abbreviations: MHC, major histocompatibility complex; TCR, T-cell receptor.
Source: Wikipedia (link correct at time of publication)
After migrating to the secondary lymphoid organs, naïve T cells are exposed to Ags which bind to the TCR. TCR activation induces proliferation and differentiation.
T cells mature to distinct T-helper (Th) and T-cytotoxic (Tc) populations characterised by expression of CD4 and CD8, respectively. (Fig 1.11)
There are two classes of MHC molecules: class I and class II. Th cells recognise Ags in the context of class II MHC, whereas Tc cells recognise Ags bound to class I MHC.
Natural killer cell activation - Figure 12
Figure abbreviations: MHC, major histocompatibility complex; NK, natural killer.
Source: French and Yokoyama Arthritis Res Ther 2004 6:8-14
Activated Th cells divide and produce a clone of effector cells, which in turn secrete CKs, activating other components of the immune response.
Once activated, Tc cells induce apoptosis of dysfunctional cells (i.e. infected) by enzymatic or signalling processes. NK cells have a similar function. (Fig 1.12)
Memory T cells are produced after Ag exposure. They remain quiescent and provide an enhanced response after repeated exposure to the Ags.
Revision Questions
- What is the structure of the TCR?
- How can Th and Tc cells be easily distinguished from one another?
- What is the main function of Tc cells?
