Defence Against Pathogens
How does cell-mediated immunity work?
Cell-mediated immunity provides resistance to the intracellular phase of bacterial and viral infections. These pathogens, such as the bacteria responsible for tuberculosis and Legionnaire’s disease, specialise in invading and replicating inside their hosts’ own cells, making them particularly difficult to overcome.
Cell-mediated immunity is important in ‘fighting’ whole cells, such as providing resistance to fungi and parasites, and in rejecting foreign-tissue transplants – it also appears to be important in fighting cancer cells.
The T-lymphocytes are responsible for cellular immunity – they occur in the same lymphoid tissue as B-cells but occupy different areas of the tissue. Like B-cells, there are thousands of types of T-cells, and each types responds to only one particular antigen.
When a foreign antigen such as a virus or a bacterium enters the body, the antigen-presenting cells present the antigen to the particular type of T-cells which become activated or sensitised. The sensitised T-cells enlarge and divide, each giving rise to a clone, a group of identical T-cells. Some cells of the clone remain in the lymphoid tissue as memory cells, which are able to quickly recognise the original invading antigen.
If infection with the same antigen should occur again, these memory cells can initiate a faster response.
What are the other T-cell types?
The T-cells that do not become memory cells develop further, producing three different types of T-cell.
1. Killer T-cells (also known as cytotoxic T cells) – These cells migrate to the site of infection and deal with the invading antigen – they attach to the invading cells and secrete a chemical that will destroy the antigen, and then go in search of more antigens.
2. Helper T-cells – These cells play an important role in both humoral and cellular immunity, as they bind to the antigen on antigen-presenting cells and stimulate the secretion of cytokines which:
Attract lymphocytes to the infection site which become sensitised and activated, thus intensifying the response
Attract macrophages to the place of infection so that the macrophages can destroy the antigens by phagocytosis
Intensify the phagocytic activity of macrophages
Promote the action of killer T-cells
3. Suppressor T-cells – These cells act when the immune activity becomes excessive or the infection has been dealt with successfully, and they release substances that inhibit T-cell and B-cell activity, slowing down the immune response.