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|What is an antigen?|
| ||Antigens are proteins, glycoproteins or polysaccharides. They may be free molecules, or they may be present on the surface of an invading microbe, or on the cell membranes of a transplanted organ, such as a replacement kidney. Antigenic molecules are present on all cells, but they usually provoke an immune response only if they are foreign.|
|Taking a look at antibodies|
| ||Antibodies are glycoproteins. They all have a similar basic structure (see Figure 2) and are known collectively as immunoglobulins (often written as 'Ig' for short).
Antibodies are synthesised by a type of white blood cell called a B cell and are displayed on the surface of the cell. Each B cell is genetically programmed to display only one specific antibody.
B cells arise through the differentiation of stem cells in the bone marrow. During this process, extensive rearrangements of the antibody genes occur to produce several million different variants. Each unique genetic sequence then codes for the production of a specific antibody. The number of different antibodies is so large that at least one will recognise a foreign antigen, even if the body has never previously been exposed to it
The B cell may be described as a 'genetic display package', with the antibody on the outside where it can meet and bind with the antigen, and with the genes which code for the antibody within.