Lymph nodes or glands are fleshy pea sized structures found in groups or chains throughout the body. The nodes in the neck, armpits and the groins are easily felt; the diagram below shows their distribution. They are linked by special lymph channels that ultimately drain into the blood going to the right side of the heart.

Lymph nodes serve largely as a barrier to the spread of infection. Each lymph node has a capsule and an internal mass of lymphoid tissue. Several lymph vessels carry lymphatic fluid into the node (afferent lymphatics), with a single large vessel (the efferent lymphatic) carrying the lymphatic fluid back out.

The main function of the lymph glands is to trap infection and foreign material by acting like a sieve. The lymphocytes in the nodes produce particular antibodies that help to fight infection and provide immunity (resistance) against infection.

There are mainly two sub-types of lymphocytes - 'B' lymphocytes (B cells) which are produced in the bone marrow and 'T' lymphocytes (T cells) which are produced in a specific lymph gland called the 'thymus'.

B cells are responsible for making antibodies. Antibodies are special molecules, tailor made to react with foreign material, bacteria, and viruses that enter the body. Once formed, the antibodies help the body to eliminate these foreign materials. A proportion of these cells become 'memory' cells and produce the antibody again when rechallenged with infection - thus giving resistance or immunity.

The T lymphocytes have two functions - first they assist the B cells in producing antibodies, and secondly they recognise and eliminate cells that seem foreign to the body. Hence T cells are responsible when an organ transplant is rejected. T cells are also responsible for eliminating otherwise normal cells that have been infected with a virus.

The spleen and thymus are special lymph nodes that have more specific function than other lymph nodes. The lymphocyte aggregates in the bone marrow are also more specialised.

What are the causes of enlarged lymph nodes?

Bacterial infection such as tonsillitis, infected cuts and wounds and abscesses are the commonest causes of enlarged lymph glands. Another common cause is glandular fever. In these conditions the nodes enlarge rapidly, are painful and usually disappear when the infection resolves.

The other common cause of enlarged lymph nodes is cancer. Cancers of the breast, lung, stomach, throat and melanoma (a type of skin cancer) commonly seed into nearby lymph glands. These types of cancer cells may be trapped by the sieve like action of the lymph nodes - resulting in secondary cancers occurring in the nodes. Lymphoma cells are cancer cells produced within the lymphatic system.