HIGHER TIER ONLY

THE BODY'S DEFENCE AGAINST BACTERIA AND VIRUSES

This slightly more detailed treatment builds upon the Foundation Tier material in the Core syllabus content, which introduces the ideas of phagocytosis and antibody production. Antibody-antigen interactions are introduced, in the context of artificial immunisation, in the Higher tier Core syllabus content. Here, production of antibodies by the body's own immune system (natural immunity) is emphasised. It is important to avoid burdening students with inappropriate detail.

Background

the immune system is a vital aspect of the body's ability to combat infection. It depends upon the recognition of molecules which enter the body tissues from outside as being 'non-self. such molecules include many of those present on the surface of 'foreign' cells, such as microorganisms. Molecules recognised in this way are called antigens. The molecules produced by the immune system capable of recognising antigens are called antibodies. They deal with antigens in a number of ways (see the Immune Response below).
phagocytes and lymphocytes are two types of white cell in blood; both play a part in the immune system Teachers may wish to refer to lymph nodes ('glands' which may swell after infection) but knowledge of the origin of the different types of lymphocyte, or of the lymphatic system itself, will not be tested in the examination.
lymphocytes are of two types: B lymphocytes and T lymphocytes.

B lymphocytes mature in the bone marrow. There are at least 100 million genetic types, each pre-programmed to recognise a particular one of any of the 'foreign' molecules, or antigens, which exist in nature. With so many different genetic types of B lymphocyte, only a few of each are routinely present in the body. B lymphocytes carry antibodies exposed on their cell surface. Importantly, they can rapidly divide to produce clones of plasma cells; these produce and secrete antibodies which circulate in the blood plasma.

T lymphocytes mature in the thymus gland. They do not produce antibodies, but, in association with phagocytes, they are essential to the success of the immune response.

Edexcel Microorganisms and disease booklet