Computer graphics image of lymphocytes (white blood cells) & antibodies ('Y'-shaped objects) in an artery. Antibodies, special blood proteins, are synthesised in lymphoid tissue in response to the presence of a particular antigen (foreign sub- stance). A huge range of antibodies is produced, each with some slight structural difference enabling it to bind to the surface of a different antigen. They thus act as flexible adaptor mole- cules, so that a wide range of infectious agents may be recognised. Here, the arms of the 'Y' shape form the antigen binding portion (Fab), the rest is the Fc portion; this binds to Fc receptors on host tissue cells.

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