T-lymphocyte cells (T-cells) originally derive from stem cells of the bone marrow. At around the time of birth, lymphocytes derived in this way leave the marrow and pass to the thymus gland in the chest, where they multiply.

The lymphocytes are processed by the thymus gland, so that between them they carry the genetic information necessary to react with a multitude of possible antigens.

T-cell diversity is attained by a complex 5-stage genetic rearrangement that occurs at random in the developing T-cells in the thymus.
T-cells are tested for their ability to recognise and bond to antigens in the thymus.
It is thought that this occurs by positive and negative selection.

The postulation is that positive selection eliminates T-cells that do not bond tightly enough to antigen type molecules produced in the thymus and negative selection eliminates T-cells that bond too tightly to self type molecules found in the thymus.
This produces a mature T-cell that is effective against antigens but is also self tolerant.
It is known that approximately 95% of all developing T-cells die in the thymus.

Each lymphocyte (or possibly a small number of similar T-cells) is thus able to react with one antigen.  Antigens of the body's own cells are excluded so that the population of T-cells processed in this way is potentially capable of recognising 'self' and 'non-self' and only reacting against the 'non-self' molecules. Autoimmune diseases are those in which this recognition is defective, and certain of the body's cells may be recognised erroneously as 'non-self' and subsequently destroyed.

Processing of T-cells occurs largely during foetal life and early childhood, after which the thymus gland shrinks in size. In adult life the active portion of the thymus is largely replaced with fatty tissue.

Mice experimentally deprived of the thymus gland immediately after birth, before they have begun to process T-cells, are incapable of cell-mediated immune responses. They fail to reject transplanted tissue. In addition, the humoral immune system works less efficiently, indicating some co-operation between the two systems.

The T-cells, each processed to 'recognise' and interact with a specific antigen, circulate permanently between the blood and lymphatic systems.

There are three types of T-cells involved.
Helper T-cells also known as T-helper cells, TH cells or T4 cells (these are the type of cell attacked by HIV).
Killer T-cells also known as cytotoxic T-cells, TC cells or T8 cells
Memory T-cells another type of T8 cell.

On recognition of the antigen by a helper T-cell, one or a group of lymphocytes takes up residence in secondary lymphoid tissue (e.g. lymph glands, Peyer's patches, MALT, spleen, bone marrow) and divides to form two types of cells, memory cells, which are lymphocytes processed in the same way as themselves, and killer cells, which interact with the antigen.

T-cell receptor sites sit on the surface of T cells and provide the specificity in antigen binding
They have two components that give each different type a unique surface morphology and it is this that allows bonding to antigens.
They are a Ti component which recognises antigens and a CD3 component that transfers a signal into the cell body.