Target cells: The function of the effector T cells is always determined on the basis of changes which they cause in antigen-bearing target cells. These can be B cells which they stimulate to produce antibodies, macrophages which they stimulate to kill bacteria or tumour cells, or other marked cells which are destroyed by cytotoxic T cells.

Th1 cells: This subgroup of T helper cells directs the defence against pathogens such as viruses and bacteria.

Th2 cells direct the defences against worm parasites. In this case, IgE antibodies are formed which, together with mastocytes, act to eliminate the bothersome parasites.

T Helper cells: This group of T lymphocytes is equipped with CD4 receptors. If they recognize foreign antigens with the receptors, they coordinate the appropriate defence measures.

Thymus gland: The thymus gland, found directly behind the sternum in the upper part of the thorax, is where T cells develop. The thymus gland consists of two lobes, each of them made up of numerous lobuli which are each surrounded by a cortex. The cortex is where the proliferation of the precursor cells, the rearrangement of the genes for the T cell receptor and the selection of the developing T cells, in particular the positive selection of the epithelial cells of the thymus cortex, take place. The thymus stroma consists of epithelial and connective tissue cells. These two types of cells provide the required micro-environment for the development of T cells.

Tolerance: The inability to react to an antigen is called tolerance. Tolerance of the body's own antigens is a key property of the immune system. If this tolerance does not exist, the immune system may destroy endogenous tissue, which is what happens in cases of autoimmune diseases.

Tonsils: Lymphatic tissue in the throat as in the area of the oesophagus and trachea. The tonsils contain defence cells which can effectively fight any invading pathogens.

Transplantation: The transfer of organs or tissue from one person to another is known as transplantation. The transplanted organs can be rejected by the recipient's immune system if he is not tolerant to the antigens of the foreign tissue or if immunosuppressives are not used.

Tuberculin test: The tuberculin test involves the subcutaneous injection of a purified protein derivative of mycobacterium tuberculosis, the tuberculosis pathogen. This derivative, also called PPD (from purified protein derivative), triggers a hypersensitive reaction of the delayed type in people who have had tuberculosis or who have been immunized against it.

Tumour: Cell group caused by excessive cell growth.

Tumour antigens: Surface structures on tumour cells which can trigger an immune reaction.

Tumour necrosis factor alpha (TNF) is a messenger substance which triggers inflammation.

T cells or T lymphocytes are lymphocytes which mature in the thymus gland.

T cell receptor: The T cell receptor is a heterodimer made up of one highly variable a chain and one highly variable b chain, which are connected to each other by disulfide bridges and stored in the cell membrane in complexes with the CD3 chains. A subgroup of the T cells bears a receptor on the surface which consists of variable g and d chains in the complex with CD3.