Lymph nodules are small masses of lymph tissue (up to a millimetre or so in diameter) in which lymphocytes are produced. Lymph nodules are scattered throughout loose connective tissue, especially beneath moist epithelial membranes such as those that line the upper respiratory tract, intestine, and urinary tract. Lymph nodules appear to be strategically distributed to defend the body against disease organisms that penetrate the lining of passageways that communicate with the outside of the body. A lymph nodule consists mainly of large numbers of lymphocytes enmeshed within reticular fibres. Lymph nodules do not have vessels bringing lymph to them. The periphery of the nodule is not sharply defined. Some lymph nodules develop germinal centres, central areas filled with immature lymphocytes. Here new lymphocytes proliferate from stem cells that originate in the bone marrow. The lighter-staining germinal centre is surrounded by a darker-staining region called the cortex.

Most lymphatic nodules are small and solitary. However, Some are found in large clusters. For example large aggregates of lymph nodules occur in the wall of the lower portion (ileum) of the small intestine. These large masses of lymph nodules are known as Peyer's patches. Tonsils are also aggregates of lymph nodules. They are located strategically to defend against invading bacteria. The tonsils produce lymphocytes. They are located under the epithelial lining of the oral cavity and pharynx. The lingual tonsils are located at the base of the tongue. The single pharyngeal tonsil is located in the posterior wall of the nasal portion of the pharynx above the soft palate and is often referred to as the adenoids.

So to summarise lymph nodules comprise:

	palatine and lingual tonsils--between the mouth and the oral part of the pharynx.
	pharyngeal tonsil--on the wall of the nasal part of the pharynx
	solitary lymphatic follicles
	aggregated lymphatic follicles (Peyer's patches)--in the wall of the small intestine
	vermiform appendix--an outgrowth from the caecum (first part of the large intestine).

Collectively this tissue is referred to as mucosa associated lymph tissue (MALT) and along with the spleen and thymus it is involved in the development of immunity. However, unlike lymph nodes, MALT has no afferent lymph vessels and therefore does not filter lymph. MALT is strategically positioned to protect the respiratory and gastrointestinal tracts from microbes and other foreign material which has entered.

Lymph Nodes

All the small and medium-sized lymph vessels open into lymph nodes which are situated in strategic positions throughout the body. The lymph drains through a number of nodes, usually 8 to 10, before returning to the blood. These nodes vary considerably in size: some are as small as a pin head and the largest are about size of an almond.

Lymph nodes have a surrounding capsule of fibrous tissue which dips down into the node substance forming partitions, or trabeculae. The main substance of the node consists of reticular and lymphatic tissue containing many lymphocytes and macrophages. As many as four or five afferent Iymph vessels may enter a lymph node while only one efferent vessel carries lymph away from the node. Each node has a concave surface called the hilum where an artery enters and vein and the efferent lymph vessel leave. The large numbers of lymph nodes situated in strategic positions throughout the body deep and superficial groups.

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[1] Mastoid and Sub occipital nodes of the head

[2] Cervical lymph nodes of the neck

[3] Axillary lymph nodes under the arms

[4] Inguinal lymph nodes of the groin area

[5] Popliteal noded behind the knee

An enlargement of these nodes is common in inflammation and malignant disease. As a result palpation (feeling) of the neck, armpits and the groin area is an important part of clinical investigation

Lymph from the head and neck passes through deep and superficial cervical nodes. Lymph from the upper limbs passes through nodes situated in the elbow region then through the deep and superficial axillary nodes. Lymph from organs and tissues in the thoracic cavity drains through groups of nodes, including: parasternal, intercostal, brachiocephalic, mediastinal, tracheobronchial, bronchopulmonary and oesophageal nodes.

Most of the lymph from the breast passes through the axillary nodes. Lymph from the pelvic and abdominal cavities passes through many lymph nodes before entering the cisterna chyli. The abdominal and pelvic nodes are situated mainly in association with the blood vessels supplying the organs and close to the main arteries, i.e. the aorta and the external and internal iliac arteries. The lymph from the lower limbs drains through deep and superficial nodes including popliteal nodes and inguinal nodes.

Functions of lymph nodes

Filtering and phagocytosis

Lymph is filtered by the reticular and lymphoid tissue as it passes through lymph nodes. Particulate matter may include microbes, dead and live phagocytes containing ingested microbes, cells from malignant tumours, worn out and damaged tissue cells, and inhaled particles. Organic material is destroyed in the lymph nodes by macrophages and antibodies. Some inorganic inhaled particles cannot be destroyed by phagocytosis. These remain inside the macrophage either causing no damage or destroying it. Material not filtered off and dealt with in one lymph node passes on to the next and so on. Thus by the time the lymph reaches the blood it has usually been cleaned of all impurities such as cell debris and foreign bodies. In some instances where phagocytation is incomplete the node may swell. Swelling of lymph nodes is often an indication of an infection. You may well have experienced swollen cervical lymph nodes. These often accompany a sore throat due to streptoccocal infection. Infections in almost any part of thre body may result in swelling and tenderness of the lymph nodes associated with that part of the body.

Proliferation of lymphocytes

Activated T- and B- lymphocytes multiply in the lymph nodes. T- and B- Lymphocytes are added to the lymph as it flows through the sinuses of a lymph node. Thus the lymph leaving the node is richer in lymphocytes.  Antibodies produced by the B- lymphocytes enter the lymph and the blood draining the node.