The Immune System: B and T Cells | A-level Biology | OCR, AQA, Edexcel

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The Role of T and B Lymphocytes

B lymphocytes are responsible for producing antibodies which are protein molecules that are specific to an antigen. Antibodies can bind to their specific antigen and neutralise it. Antibodies are present in the plasma and can bind to specific antigens free in bodily fluids or antigens on cells. We say B lymphocytes are involved in ‘humoral immunity’ -the antibodies are present in bodily fluids, which is also known as ‘humour’. T lymphocytes have specific receptors on their plasma membranes which are specific to antigens. T lymphocyte receptors can only bind directly to antigens present on a body cell. Therefore T lymphocytes are described to be involved in cell-mediated immunity.

The Cellular Response of Lymphocytes

There are millions of lymphocytes with different receptors which are specific for different pathogenic antigens. The lymphocyte with the correct receptor has to be found and activated when a specific pathogen invades the body. The receptor on a lymphocyte is complementary to the shape of the specific antigen. This contact and subsequent activation is called clonal selection. The contact between the lymphocyte receptor and antigen can be achieved either directly or indirectly. Direct contact happens when the lymphocyte comes across pathogenic cells in the lymph nodes. Indirect contact happens when the lymphocyte comes across an antigen presenting cell which phagocytosed the pathogen. Other immune cells like macrophages then secrete a type of cell signalling molecule called interleukins. These interleukins bind specifically to the selected lymphocyte and cause it to divide by mitosis to produce lots of clones. This increase in the number of cells by mitotic division is called clonal expansion. This whole process of selecting and expanding the correct lymphocyte takes a long time and is slow.

Summary

Neutrophils, monocytes and macrophages are non-specific immune cells
T and B lymphocytes are specific immune cells so they only produce a response against certain infections
Lymphocytes provide long term protection against specific pathogens
B lymphocytes make antibodies and are involved in the humoral response
T lymphocytes bind to antigens on antigen-presenting cells and are involved in the cell mediated response
To produce a response, lymphocytes must first undergo clonal selection where the correct specific lymphocyte is found
They then undergo clonal expansion where the chosen lymphocyte increases in number