T cell

The T cell (also called T lymphocyte) is one of the most import cells of the adaptive immune system. Because it is part of the adaptive immune system, it takes a while for the T cell to become fully activated and differentiated (mostly +/- 7 days after an antigen/pathogen enters the body). This is due to the fact that T cells aren't always exactly at the location where the pathogen enters the body. This is why a dendritic cell, which are positioned right beneath the skin surface, catches a (part of the) pathogen as soon as it enters, tears it into pieces, travels to the lymph nodes and presents the small pieces to the T cell. Even then, the T cell has to undergo a process of further differentiation and activation, but after that it can finally leave the lymph node. The T cell will travel through the bloodstream to the site of inflammation, where it leaves the bloodstream, invades into the inflamed tissue and tries to kill the pathogen.

This is of course a very brief summary of how the T cell works. There are different types of T cells, each with their own activation-process and function:

• CD4+ T helper cell
  • Th1 cell
  • Th2 cell
  • Th17 cell
  • TFH cell
• CD8+ cytotoxic T cell
• Regulatory T cell

Th1 cells help assist other lymphocytes (mostly macrophages) to become highly microbicidal. Some pathogens like Mycobacterium tuberculosis and Mycobacterium leprae tend to grow inside a macrophage and have a very specific mechanism to prevent being killed by the phagolysosome: they prevent the fusion between the phagosome (where the mycobacterium is located) and the lysosome (where the proteases are located). That way the phagosome doesn't acidify, the mycobacterium isn't broken down and the macrophage can't present little fragments of the mycobacterium to the T cells. There are however receptors present in the phagosome that sense the present of a pathogen (because of DNA/RNA). That way the Th1 cell can help to macrophage become more microbicidal and make sure that it fuses the phagosome and the lysosome, so the mycobacterium dies.

The phagosome needs 2 signals for this process to start: interferon gamma and CD40-ligand or lipopolysaccharide (LPS). The second signal (CD40-ligand or LPS) makes the phagosome more susceptible to interferon gamma. Th1 cells have a CD40-ligand on their cell membrane and they can produce interferon gamma, but this process can also happen by CD8+ T cells where they produce the interferon gamma, while the lipopolysaccharide comes from a gram negative bacterium.

Th1 cells are particularly formed in the presence of interleukin 12 (IL-12), which is mostly likely produced by the dendritic cells, and by interferon gamma, which is most likely formed by the natural killer (NK) cells. The effector function of the Th1 cells is to strengthen the NK cells, macrophages and CD8+ cytotoxic T cells and to make sure the B cells produce IgG antibodies.