The lymph nodes of the lymphatic system play an important role in protecting the body against the invasion of potentially harmful micro-organisms. However the lymph nodes are not the first barrier.

The defence mechanisms of the body against infection are many. First we have as a natural barrier the skin, where sweat glands excrete products that are toxic for the micro-organisms. If the micro-organisms penetrate this barrier (for example through an abrasion or a cut), they encounter neutrophilic granulocytes and macrophages that hasten to the place of intrusion. They ingest foreign material, no matter what kind, and because of their activity the place of infection becomes inflamed. This is termed aspecific immunity. When bacteria continue to live, despite these defence mechanisms, they are drained by the lymph and conveyed to the lymph nodes. In the cortex of the lymph node are lymph follicles, nests of lymphocytes, that play an active role in immunity. In contrast to the neutrophilic leucocytes, the lymphocyte immunity is specific. Specifity means that the lymphocytes form antibodies that react only with ‘their’ antigen.

An antibody against polio virus, for example, shows no reaction when encountering a typhoid bacillus. Lymphocytes are divided into two major classes: B-cells that are derived from the bone marrow and gut-related tissue, and T-cells that depend on the thymus for their development. They have different roles in immunity. The B-lymphocytes in the lymph nodes are responsible for humoral immunity. When foreign material (an antigen) comes into contact with a B-lymphocyte, this cell is stimulated to divide itself. That is why the lymph glands in your neck become swollen when you have a severe sore throat. Lymph nodes can swell in response to certain infectious agents, mostly in the region of infection. In glandular fever, however, lymph nodes throughout the body may swell. They can also enlarge because of infiltration by malignant cells. This occurs in Hodg-kin’s disease and in some types of leukemia. If a lymph node is suspected to contain malignant cells, a biopsy is undertaken. A biopsy involves the removal of a small sample of living tissue in order to look at it under a microscope as an aid to diagnosis.

The number of B-cells is increased and some are changed into plasma cells. These plasma cells rapidly form antibodies (a kind of proteins called immunoglobulins) that are specifically directed against the invading antigen. Before a sufficient concentration of antibodies is formed you become ill; when the number of antibodies can cope with the infection, you get better. After the concentration of antibodies reaches its peak, it gradually decreases until after two months practically no more antibodies are left. Doctors then consider that the body is immunized. But when a second contact with the same antigen takes place, antibodies are formed much faster (so you are hardly ill at all or only for a very short time) and their concentration decreases slower, thus the period of immunity is longer. Probably there is a kind of memory cell that circulates in the bloodstream and on second contact produces antibodies immediately.

The T-lymphocytes give us cellular immunity. They are formed in the thymus and originate other lymphoid structures. Coming into contact with an antigen the T-cells can react directly with it, again specifically, thus rendering it harmless. This cellular immunity also functions faster upon second contact with the antigen. The T-lymphocytes also play a role in humoral immunity. They help the B-lymphocytes to react better to smaller amounts of antigens. The T-lymphocytes probably form a chemical substance that signals the B-lymphocytes to come into action. They can also activate macrophages to react better. This latter type of immunity is called natural active immunity.