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7. Proteins recognising other molecules Page 29
diagram of B cells Figure 1
Humoral immunity
7.1 Antigens and antibodies
When bacteria or viruses get through our body’s outer defences they may cause disease. To prevent this we have an immune system made up of cells and molecules that are constantly on patrol looking for and destroying foreign organisms.

For the system to be effective the cells and molecules that are involved must be able to recognise foreign material and distinguish it from similar chemicals that make up our tissues. This is a remarkable feat of molecular recognition.

Any material that can trigger the immune system into action is called an antigen. Antigens are usually the proteins, polysaccharides or glycoproteins that are on the surface of invading bacteria or viruses. Some of these invading pathogens produce harmful chemicals, called toxins. Toxins may also be antigens.

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.

B cells provide what is called humoral immunity (Figure 1).

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. Different B cells carry different antibodies.

diagram of T cells
Figure 2
Cellular immunity
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 concerted 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 are kept in reserve as long-lived memory cells.

T cells provide several activities collectively called cellular immunity, especially important in the body’s fight against viruses (Figure 2) and in helping B cells make antibodies (Figure 1).

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.

Antibodies work by:

  • binding to the antigen, making it inactive
  • binding to antigens in such a way that the molecules or invading cells clump together to form insoluble immune complexes that are easily digested by phagocyte cells circulating in the blood and lymph
  • binding to the antigens on the surface of an invading cell to form a complex that may attract phagocytes and killer cells or activate enzymes in the blood capable of destroying the cell.
Unilever Education Advanced Series: Proteins
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