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Chapter 1: The Immune System

T-cells and NK-cells Figure 1

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 2 chains, usually α and β. Similar to the BCR, each one of these chains includes a variable and a constant domain.

T-cells are able to recognise Ag (through their TCR) only when the Ag is bound to a major histocompatibility complex (MHC) molecule.

T-cells and NK-cells Figure 2

Source: Wikipedia (link correct at time of publication)

After migrating to the secondary lymphoid organs, naive T-cells are exposed to Ag 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.

There are 2 classes of MHC molecules: class I and class II. Th recognises Ag in the context of class II MHC, whereas Tc recognises Ag bound to class I MHC.

T-cells and NK-cells Figure 3

Source: French and Yokoyama Arthritis Res Ther 2004 6:8-14   doi:10.1186/ar1034 (link correct at time of publication; June 2015)

Activated Th cells divide and produce a clone of effector cells, which in turn secrete CK, activating other components of the immune response.

Once activated, Tc induce apoptosis of dysfunctional cells (i.e. infected) by enzymatic or signalling processes. Natural killer (NK) cells have a similar function.

Memory T-cells are produced after Ag exposure. They remain quiescent and provide an enhanced response after repeated exposure to the Ag.

Revision Questions

  1. What is the structure of the T-cell receptor?
  2. How can T-helper and T-cytotoxic cells be easily distinguished?
  3. What is the main function of cytotoxic T-cells?
B-Cell Diversity Immune System Activity

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