T cells

These are lymphocytes which have migrated from the bone marrow and matured in the Thymus. They comprise about 75% of circulating lymphocytes (1). T cells are the predominant cell in the paracortical region of lymph nodes (c.f. B cells).

• T cells are divided into two distinct populations, which behave either as cytotoxic T cells or helper T cells (TH cells)
  • TH cells may be sub-divided into TH1 and TH2
    • TH1 are pro-inflammatory T cells and stimulate macrophages
    • TH2 orchestrate B cell differentiation and maturation and hence are involved in the production of humoral immunity (antibody mediated)
• T cells are identified via their tendency to form rosettes of sheep red cells around them, when mixed with sheep red cells. They also can be identified by using monoclonal antibodies that react with surface antigens
• T cells express cell surface proteins, described by cluster determination (CD) numbers
  • TH cells express CD4 molecules on their cell surface
    • enables the lymphocyte to bind to a MHC class II molecule
  • T cell receptor is unique in that it is only able to identify antigen when it is associated with a MHC molecule on the surface of the cell
  • cytotoxic T cells are primarily involved in the destruction of infected cells, notably viruse
    • unlike TH cells, cytotoxic cells possess CD8 cell surface markers, which bind to antigenic peptides expressed on MHC class I molecules
  • T cells are also clonally selected and this confers to the production of T memory cells
• T and B cells both are able to recirculate around the body migrating from blood to tissue and vice versa
  • ability to recirculate increases the efficiency with which cells of the immune system can home onto the invading antigen.
• T cell deficiency is associated with susceptibility to viral, fungal and chronic bacterial infection

Reference:

• von Andrian UH, Mackay CR. T-cell function and migration. NEJM 343: 1020-1034.