White blood cells (lymphocytes) play an important part in the immune response. Production of lymphocytes starts in the bone marrow. Once mature, the cells circulate through the blood and lymphatic vessels. Lymphocytes fall into two groups—the B cells and the T cells (Alberts et al 1983 p.952).
B cells provide what is called humoral immunity. The B cells have glycoproteins, called immunoglobulins or antibodies, displayed on their surface. Each B cell displays many copies of just one antibody, which recognises a particular antigen and binds to it. The binding of an antibody to an antigen must happen first in order for the antigen to be recognised as a harmful organism. Different B cells carry different antibodies; antibodies have a special shaped receptor to combine with a specific antigen. The antigen-antibody complex that is formed when the antibody binds to the antigen makes the antigen harmless. B cells that remain in the lymphatic tissue are called memory B cells. “The memory B cells do not secrete antibodies but if they are later exposed to the antigen that triggered their formation, they become B cells that secrete antibodies. The memory B cells ensure immunity. If antigens are molecules in the surface membranes of invading cells, when antibodies combine with them, the resulting complexes may agglutinate the enemy cells by clumping them together. Then phagocytes may rapidly destroy the clumps of complexes by ingesting and digesting large numbers of them at one time.” (http://www.bhs.berkeley.k12.ca.us/departments/Science/anatomy/anatomy97/immune/html/antibodygen.html)
When a foreign organism invades through the bloodstream it meets many B cells, especially when it is trapped in the spleen or lymph nodes. Eventually it meets a B cell with antibodies that can recognise one of its antigens. There follows a mutual action involving T cells and cells called phagocytes where binding of the antigen to the B cell causes the B cell to divide quickly and repeatedly. The resulting collection of daughter cells is called a clone. Some of the cells in the clone mature into highly active but short term antibody factories, while others, as I mentioned earlier, are kept in reserve as memory cells (Alberts et al 1983 p.960).
T cells provide several activities collectively called cellular immunity, especially important in the body’s fight against viruses and in helping B cells make antibodies. T cells have protein receptors on their surface membrane. These recognise and bind to specific antigens, but only when the antigen is stuck on the surface of the body’s cells. Once they have bound to the antigen the T cells divide quickly and repeatedly to form an activated clone (Alberts et al 1983 p.958-959).
Because of mammal’s immune system, we are able to fight off most harmful organisms through the antigen-antibody complex. Antibodies work by binding to the antigen, therefore making it inactive. They also bind to antigens so the invading cells clump together to form insoluble immune complexes that are easily digested by phagocyte cells circulating in the blood and lymph.
Bibliography
Alberts, B.; Bray, D.; Lewis, J.; Raff, M.; Roberts, K.; Watson, J.D. Molecular Biology of the Cell. Garland Publishing, Inc: New York & London. 1983. 34-35, 951-1012.
“Antigens and Antibodies”. http://www.bhs.berkeley.k12.ca.us/departments/Science/anatomy/anatomy97/immune/html/antibodygen.html
Dowben, Robert M. Cell Biology. Harper & Row, Publishers: New York, Evanston, San Francisco, London. 1971.
Lackie, J.M. Cell Movement and Cell Behaviour. Allen & Unwin: Boston & Sydney. 1986.