International A Level Biology / Immunity

The specific response: T helper, T killer and T memory cells

Edexcel International A Level Biology (YBI11), chapter 18: Immunity, specification point 6.10. Free lesson.

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T helper cells coordinate the specific immune response by releasing cytokines, while T killer cells destroy infected body cells. T memory cells remain in the body, helping the response act faster if the same antigen returns.

How a macrophage activates a T helper cell

A macrophage engulfs and digests a pathogen, then displays some of its antigens on its surface. This makes it an antigen-presenting cell (APC). An MHC protein in the macrophage membrane holds an antigen in place for a T helper cell to bind. Each T helper cell has a receptor with a particular shape. Only a T helper cell whose receptor is complementary to the displayed antigen can bind. This makes the response specific to that antigen.

What cytokines from T helper cells switch on

Active T helper cells release cytokines, which are chemical signals. They activate other cells in the immune response, but active T helper cells do not attack pathogens themselves. Matching B cells divide to form plasma cells, which secrete antibodies, and B memory cells. Matching T killer cells are activated to divide and attack infected body cells. Macrophages are stimulated to carry out more phagocytosis, so they engulf and digest more pathogens. If T helper cells are lost, as in HIV, the whole immune response is weakened.

How T killer cells destroy infected cells

A T killer cell targets one of your own cells after a virus has entered it. The infected cell displays viral antigens on its surface, so a T killer cell with a complementary receptor can bind. Cytokines from active T helper cells activate the matching T killer cell. It divides by mitosis to form active T killer cells and T memory cells. An active T killer cell releases perforin, a protein that makes pores in the infected cell's membrane. The membrane becomes leaky and the cell is destroyed, stopping the virus from using it to replicate.

How B cells and T cells differ

Both B cells and T cells are lymphocytes, a type of white blood cell made in the bone marrow. They differ in where they mature and what they act against. B cells mature in the bone marrow. They act against pathogens and toxins in body fluids, in the humoral response. Their receptors bind free antigens or antigens on pathogens. T cells mature in the thymus, a gland in the chest. They act against infected body cells, in the cell-mediated response. Their receptors bind only antigens presented on a cell surface. B cell clones form plasma cells, which secrete antibodies. T cells do not…

Keep the order and cell names clear

The sequence matters: macrophages activate T helper cells, and T helper cells then activate B cells and T killer cells. An activated T helper cell does not turn into a T killer cell. They are different cells, with different receptors and roles. For an infected airway cell, describe matching T helper cells binding viral antigen presented by macrophages, then dividing and releasing cytokines. Matching T killer cells bind antigen on infected cells, are stimulated by cytokines to divide, and release perforin to make pores. T memory cells are also formed.

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