T-lymphocytes, known colloquially as T-Cells, are specialised cells of the immune system, they're involved in the acquired immune response. There are two branches of T-cells, cytotoxic-T cells and T-helper cells. These cells can be distinguished by the expression of different receptors on their plasma membrane. Cytotoxic T cells express a CD8+ receptor, whereas T helper cells express a CD4+ receptor on their surface. Here we will discuss further only the T-helper CD4+ cells.
T-helper cells are derived from pluripotent hematopoietic stem cells and common lymphoid progenitors in the bone marrow. Progenitors migrate to the thymus where they continue their development. Thymocytes undergo positive selection, that is, if they are able to bind with MHC in the thymus, they will recieve survival signals. It is important that the T-cell receptor (TCR) can recognise self MHC + forregin peptide fragment. Those that do not recognise MHC or those that recognise MHC + self peptide are to be eliminated.
Following positive selection, T-cells undergo negative selection. That is the T-cell will be deleted if it recognises MHC-self-peptide on thymic dendritic cells. This induces clonal deletion. Less than 5% of T-cells will survive thymic selection.
- ↑ Murphy, K., Travers, P., Walport, M., & Janeway, C. (2011). Janeway's immunobiology (8th ed.). New York: Garland Science.