A-Level Biology (AQA) – T lymphocytes & Cell-mediated immunity

5.3 - T Lymphocytes & Cell-mediated Immunity

1The different types of lymphocytes
2The different types of T cells
3The role of T cells in the cellular response

9 min

Specific defence mechanisms

Specific defence mechanisms are slower than non-specific defences, but they produce a unique response for each type of pathogen. They also provide long-term immunity against specific pathogens.

Flowchart showing defence mechanisms including non-specific and specific responses with cellular and humoral responses.

The specific immune response depends on a type of white blood cell known as a lymphocyte. These cells are produced in the bone marrow.

There are two types of lymphocyte:

T lymphocytes or T cells - T cells mature in the thymus gland. They are involved in the cellular response where they respond to antigens presented on body cells.
B lymphocytes or B cells - B cells mature in the bone marrow. They are involved in the humoral response where they produce antibodies found in body fluids (or humors).

T cells

You need to know about the following types of T cell:

Helper T cells (T_H cells) These cells have receptors on their cell-surface that bind to complementary antigens on antigen-presenting cells. After binding, they can form memory cells, stimulate B cells or phagocytes, and activate cytotoxic T cells.
Cytotoxic T cells (T_C cells) These cells kill abnormal and foreign cells by producing a protein known as perforin. This protein makes holes in the cell-surface membrane, causing it to become freely permeable and causing cell death.
Memory T cells These cells provide long-term immunity against specific pathogens. They provide a rapid response if the body is re-infected by the same pathogen.

Stages of the cellular response

The cellular response is so-called because T cells will only respond to antigens presented on body cells rather than antigens within body fluid.

The stages of the cellular response are as follows:

Diagram showing stages of the cellular response including antigen-presenting cell and cloned T cells.

Phagocytes engulf pathogens and display their antigens on the cell-surface. They are now known as antigen-presenting cells.

The cloned T cells can carry out different functions:

Develop into memory cells - These circulate in the body to provide long-term immunity.
Stimulate phagocytosis - Cloned cells stimulate phagocytes to engulf pathogens.
Stimulate division of B cells - Cloned cells stimulate B cells to divide and produce antibodies.
Activate cytotoxic T cells - This allows T_C cells to kill infected cells.