From the 1984 lecture programme:
The body protects itself against infection by bacteria, viruses, molds and parasites by having a system for recognising these invading organisms as foreign.
A subset of the blood's white cells, the lymphocytes, manufactures a very complex set of proteins, the antibodies, which have exquisite sensitivity for foreignness.
Literally millions of different types of antibodies can be made to recognise any sort of infection. Vaccination is a form of priming the body, for example, by killed virus, so that when infection comes the body can respond quickly enough to stop the infection from gaining a hold.
New types of vaccines are now being produced by genetic engineering techniques. The body's ability to recognise foreignness can be exploited experimentally to make antibodies to almost any substance. These antibodies can then be used to identify and measure specifically the presence of a substance, for example, of the hormone which only appears very early in pregnancy — hence the pregnancy test.
The body's recognition system also recognises the foreignness of tissues from another person. Hence the need to match for blood transfusion and kidney grafting.
The tissue types which are recognised in graft rejection are also involved in controlling the recognition system itself. Sometimes when the system does not work properly, the body's own tissues are attacked causing diseases such as rheumatoid arthritis and juvenile diabetes, but only in people with certain tissue types.
Using genetic engineering techniques the antibody and tissue type genes have been characterised leading to new possibilities for controlling the body's recognition system, and so the diseases it sometimes causes, as well as graft rejection.