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Infection and immunity
Infection and immunity

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4.10  T cells in adaptive immunity

There are two types of T cells with different roles in adaptive immunity. The cytotoxic T cells release destructive chemicals onto their target’s outer surface in much the same way as the cytotoxic leukocytes do in an innate immune response. But there is one crucial difference. Cytotoxic T cells are programmed to kill the body’s own cells that have become infected by viruses or by the few types of bacteria and single-celled pathogens that can ‘hide’ inside the cells of their host (Mycobacterium tuberculosis, the bacteria that cause TB, can do this). Without the cytotoxic T cells, we would be particularly susceptible to infectious diseases caused by these pathogens.

The other T cell type is called the helper T cells. They send activation signals to all the other leukocytes involved in inflammation, phagocytosis, cytotoxicity or production of antibodies by B cells (Figure 51). Recognition of a pathogen by binding to it is only the first step. The other leukocytes cannot take action against the pathogens they encounter without activation signals from the helper T cells.

Described image
Figure 11  Helper T cells activate all the other leukocytes responsible for innate and adaptive immune responses against pathogens.

If you have seen documentaries or read reports about HIV/AIDS, you have possibly heard that HIV (the human immunodeficiency virus) infects and ultimately destroys the helper T cells.

  • Why does the destruction of helper T cells by HIV leave a person susceptible to many other infections?

  • When the number of helper T cells declines to a low level, they cannot activate the other leukocytes to act effectively against other pathogens, so the person becomes susceptible to infections they could otherwise have overcome. Eventually the person develops AIDS – acquired immune deficiency syndrome – because their immune responses have become so deficient.

Next we explain how our defences against pathogens can be enhanced by vaccination, and consider some of the problems that limit the ability of vaccines to protect us from infection.