THE IMMUNE RESPONSE

An incoming antigen is recognised by the corresponding antibody present on the cell surface of a lymphocyte by the same kind of 'lock and key' mechanism that may be familiar to students from studies of enzymes, or of artificial immunisation (Core syllabus content). (However, there is no syllabus requirement to study enzyme specificity and so the idea of recognition of some molecules by others may need explanation at this point.)

Students should be aware of these events following antibody/antigen recognition:

• rapid multiplication of the lymphocytes concerned, producing a clone of identical plasma cells. This is the primary immune response; this term should be known. It takes some time for plasma cells to build up to very large numbers and begin manufacturing and secreting antibodies. This gives time for pathogens to increase in number. therefore disease symptoms are often produced.
  

• certain cells from a clone of lymphocytes become memory cells. Unlike plasma cells, which usually die in a few days, these have a life of several years. Clones of memory cells are stored in the lymph nodes. If the body is again exposed to the same antigen, memory cells divide straight away and extremely rapidly. This swift and vigorous response means that the pathogen is often destroyed before its numbers can build up to levels which produce disease symptoms. This response by the memory cells is the secondary immune response, The effect is reinforced at any subsequent exposure to the antigen and is the basis for acquired active immunity.
  

• antibodies deal with antigens in several ways. These include clumping them together ready for phagocytosis or further attack by other antibodies, binding to them, making them harmless, and 'labelling' them for attack by enzymes in the blood plasma. Further details of these actions are not required.

Students should study quantitative data showing variation in blood antibody levels with time following first exposure to antigen, such as inoculation with inactIvated pathogenic virus (primary immune response) and a subsequent exposure (secondary Immune response). Most textbooks provide a suitable graphical illustration, similar to the one overleaf.

Students may be asked to interpret such information and use it to describe differences between the primary and secondary responses or to suggest, for example, optimal timing for second vaccination.
Students should be aware of the role of T lymphocytes along the following lines:

• like B lymphocytes, they are each highly specific for a particuLar antigen.
  

• T-killer cells recognise and kill body cells which have been invaded by viruses, thus depriving the virus of its means of reproduction. (T lymphocytes cannot deal with viruses direct: they can only do so when the virus is inside host cells). These cells also leave memory cells, enabling rapid response to subsequent infection.
  

• T-helper cells are crucial to the success of the immune system, as B lymphocytes cannot act without them. Teachers could link this fad to the devastating effects of the HIV virus.