Antibodies

Antibodies

1. Antigens and antibodies

   • Antigens are molecules that can stimulate an immune response- they are usually glycoproteins or proteins. A foreign antigen will be detected by the immune system and will stimulate the production of antibodies. Antibodies are specific to the antigen
   • Antibodies are immunoglobulins (complex protein proteins produced by the plasma cells in the immune system. They are released in response to an infection. They have a region with a specific shape that is complementary to that of a particular antigen. Our immue system must manufacture one typ[e of antibody for every antigen detected. Antibodies attach to antigens and render them harmless.

2. Structure of an antibody

   • Antibody molecules are Y-shaped and have two distinct regions. They consist of 4 polypeptide chains
     • The variable region has a shape specific to the shape of the antigen. The constant region is the same in all antibodies. The hinge region allows flexibility so molecules can grip more than one antigen. Disulphide bridges hold polypeptides together

3. How antibodies work

   • Opsonins are a group of antibodies that bind to the antigens on a pathogen. They act as binding sites for phagocytic cells, so that these can more easily bind and destroy the pathogen. Some opsonins are not very specific and stick to types of molecules that are not found in the host cell. Other opsonins are produced as part of the immune response and bind to very specific antigens. The pathogen may have another use for this antigen molecule e.g. it may be a binding site used for attachment to the host cell
   • Anti-toxins- some antibodies bind to molecules that are released by pathogenic cells. These molecules may be toxic and the actions of anti-toxins renders them harmless
   • Agglutinins- because each antibody molecule has 2 identical binding sites it is able to 'cross link' pathogens by binding an antigen on one pathogen with one binding site and then an antigen on another pathogen with its other binding site. When many antibodies perform this crosslinking they clump together (agglutinate) patjogens. This has two advantages: the agglutinated pathogens are physically impeded from carrying out some functions, such as entering host cells, and the agglutinated pathogens are readily engulfed by phagocytes. This is particularly affective against viruses.

4. Primary and secondary responses

   • Antibodies are produced in response to an infection. When an infecting agent is first detected, the immune system starts to produce antibodies. But it takes a few days before the number of antibodies in the blood rises to a level that can combat the infection successfully. This is the primary immune response. Once the pathogens have been dealt with, the number of antibodies in the blood drops rapidly.
   • Antibodies don't stay in the blood. If the blood is infected a second time by the same pathogen, the antigens must be made again. However, as a result of the specific immune response there will be B and T memory cells circulating in the blood. These cells can recognise the specific antigens and the immune system can activate more quickly. This time the production of antibodies starts sooner and is much more rapid. So the concentration of antibodies rises sooner and reaches a higher concentration. This is the secondary immune response- it is usually quick enough to prevent any symptoms being detected by the host

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