The cells of the adaptive immune system are special types of leukocytes, called lymphocytes. There are two main types of lymphocytes, B cells and T cells, which are derived from hematopoietic stem cells in the bone marrow. T cells are involved in cell-mediated immune response, whereas B cells are involved in the humoral immune response. The surface of each lymphatic cell has receptors that enable them to recognize foreign substances.
There are two major subtypes of T cells: the helper T cell and the killer T cell. Helper T cells are the major driving force and the main regulators of the immune defence. These cells do not kill infected cells or clear pathogens directly. Their primary task is to activate B cells and killer T cells. For helper T cells to activate any other cell, they themselves need to be activated. This happens through a process called antigen presentation. When the receptor of a helper T cell recognizes the antigen, the T cell is activated. Once activated, helper T cells start to divide and to produce proteins that activate B and T cells as well as other immune cells.
Killer T cells specialise in attacking infected cells of the body and do this by directly attacking other cells carrying foreign or abnormal antigens on their surfaces. Once they have been activated by a helper T cell they travel throughout the body and each killer T cell uses its receptors to search every cell that it meets to check for a virus or bacterium. They can also kill cancer cells. T cell killing of host cells is particularly important in preventing the replication of viruses.
The major function of B cells is the production of antibodies in response to foreign proteins of bacteria, viruses, and tumour cells. B cells search for antigen matching its receptors and when it finds such antigen it connects to it and a triggering signal inside of the B cell is set off. Once the B cell receives protein produced by the helper T cell it is fully activated. It can then...
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