How are T cells made

4: Transplantation and Autoimmune Diseases

Image with caption
Production of T cells (model)

T cells play the main role in the rejection of donor organs. They coordinate the defense measures in a similar way to an infection and cause antibodies and killer T cells to destroy the transplant. This unwanted immune reaction is related to the task of the T-cell troop: to examine body tissue for suspicious features, but only intervene in an emergency. This ability is acquired by the TCells in Thymus, a central organ of the lymphatic system.

T cells Like all other white blood cells (leukocytes), they are formed from stem cells in the bone marrow. Like blanks, these progenitor cells go through several developmental steps and finally mature in the thymus. A T-cell has nothing but identical receptors. This means that it only recognizes a characteristic molecule that the target cell shows on its surface like an ID. If the T cell receptor and molecule like lock and key fit, the target cell has been identified.

The immune system must be armed against all conceivable opponents. It prepares for this difficult task during embryonic development. First of all, suitable precursor T cells are produced for each potential molecular shape.

The required variety of T cell receptors is created by a random process in which the corresponding genes are varied. In principle, there is a suitable lock for every possible molecular key.

Test station thymus

Before the immature T cells are fit for duty, they are subjected to a strict test in the thymus:

Every T cell is checked to see whether it would even find target cells, which can be identified by molecules. If the receptor of a T cell does not match any MHC molecule, it is, so to speak, blind and is discarded.

All T cells are also tested to see whether they recognize harmless self-antigens. In this case, the immune system could be directed against your own tissue in an uncontrolled manner. That is why such dangerous T cells are normally also destroyed.

Image with caption
Animated film "The Rejection of Foreign Organs"

The T-cell test in the thymus is shown in a simplified form as an animated film ("The rejection of foreign organs"). MHC-I molecules are shown as a square, MHC-II molecules as a circle.

The rigorous selection in the thymus ensures that the body's own cells are spared as far as possible. All that remains are mature T cells that specialize in fighting foreign antigens and tissues. While this protects us from pathogens, it also leads to considerable complications when organs are transplanted. About 20% of T cells recognize foreign tissue.

MHC molecules that form the identity documents of the cells. The abbreviation MHC stands for Major Histocompatibility Complex. The MHC molecules are also known as HLA, human leukocyte antigens. They can be easily identified on the surface of white blood cells. There you will find variants of MHC molecules that are typical for everyone. They differ from individual to individual. They are only identical in identical twins. In this exceptional case, an organ transplant does not lead to a rejection reaction.