migration to the follicles in the lymph node. Also coming to the lymph node are

antigen-laden dendritic cells, which present the antigen to T-cells. This in turn

causes T-cell activation followed by migration into the follicle. In the follicle,

activated T-cells stimulate B-cells to initiate a process of antibody maturation in-

volving genetic rearrangements, that results in the production of higher-affinity

binders. They also trigger the formation of memory cells that can be re-activated

upon re-infection. These T-cells are thus called T-helper cells.

3.3.2.3

Cytotoxic T-Cells

The other important type of T-cells are the cytotoxic T-cells. This type of T-cell

attacks abnormal cells, such as those infected by a virus. Intracellular proteolytic

fragments of a pathogen or abnormal protein are displayed on the surface of the cell

in question, and are recognized by T-cells expressing receptors that specifically

recognize those fragments. As shown in Figure 3.12, upon activation by antigen-

binding, cytotoxic T-cells produce perforin, a molecule that will kill the cell by

making holes in it. They also produce granzymes, molecules that trigger apoptosis

in the target cell. Cytotoxic T-cells induce cell death very much like NK cells, but

their action is much more specific because T-cells recognize their targets with the

help of very specific receptors. Figure 3.13 shows the difference in function of

the two types of T-cells, one that supports antibody formation in B cells (T-helper)

and the other that triggers cell death (killer T) in abnormal cells such as those that

are virally infected or cancerous [7].

3.4

VACCINES

The aim of vaccination is to induce protection from the pathogen, without making the

individual experience the sickness. Therefore, the strategy is to initiate the innate

FIGURE 3.11 T-cell receptor. The T-cell receptor is made up of two chains, α and β held

together by a disulfide bond. Both chains consist of variable and constant regions, with the

variable region harboring an antigen-binding site. Both chains span the plasma membrane

and have a short cytoplasmic domain.

Introduction to basic immunology

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