T-cell immunity is a reaction in the body’s immune system by which the immune system recognizes a foreign invader, called an antigen, and responds by destroying it. There are two types of white blood cells: B-cells and T-cells. T-cell immunity uses white blood cells, or lymphocytes, called T-cells, or T-lymphocytes, to destroy antigens. This particular immune response is distinct from other immune responses, such as compliment system's protein response or the body's use of phagocytes to eat the foreign bodies, though these elements take part in the act of T-cell immunity.
There are several types of T-cells at work in the body: helper T-cells, killer T-cells, suppressor T-cells and memory T-cells. Though T-cells, like other lymphocytes, form in the bone marrow in the body, T-cells move to the thymus after development. This function also gives it its name, "T-cell."
T-cell immunity requires other components of the immune system to be evoked, beginning with a process called antigen presentation. First, a phagocyte, such as a macrophage, captures and devours the invader. Then it travels to a lymph node to convey information about the invader to a helper T-cell by presenting pieces of the antigen on its surface. Each helper T-cell’s receptors only recognize one type of antigen, so the phagocyte must find the right helper T-cell to recognize it and elicit a response. When a T-cell finally recognizes the antigen, it begins dividing and creating proteins called cytokines to notify the rest of the immune system — killer T-cells and B-cells — to continue the immune response.
Killer T-cells are also known as cytotoxic T-lymphocytes. As their name suggests, they react by attacking and killing infected cells that would go unnoticed by other components of the immune system. Its receptors inspect every nearby cell, and thus attacks any cell that shows signs of infection by using an enzyme that eventually kills the cell in question. Types of infections that can befall a cell — and thus a reaction from a killer T-cell — include viruses, bacterium and even cancer.
Once the antigen is dealt with, other T-cells spring into action. The suppressor T-cells, for example, work to curb additional killer T-cells from forming unnecessarily. In addition, memory T-cells remember that specific antigen to elicit a faster response if the invader ever returns to the body.
T-cell immunity is also known as cell-mediated immunity, and can itself be disrupted by such viruses as human immunodeficiency virus (HIV). These types of viruses specifically target and kill the helper T-cells to severely weaken even basic immune response. This action by HIV, for example, is what causes the body to eventually succumb to acquired immune deficiency syndrome (AIDS).