In this essay we will discuss about the primary and secondary lymphoid organs.
Essay # 1. Primary Lymphoid Organs:
These are those lymphoid organs where immature lymphocytes differentiate to antigen-sensitive lymphocytes e.g:
1. Bone Marrow:
It is highly vascular fatty tissue present in the cavities of the long bones and is of two types: Red bone marrow in the epiphyses of the bone marrow and yellow bone marrow in the cavities of shaft (diaphysis) of long bones. It is the main lymphoid organ where all the blood corpuscles including lymphocytes are produced. In the human being, B-lymphocytes undergo processing in the liver during midfoetal life and in the bone marrow during the late foetal life and after birth.
2. Thymus:
It is soft, pinkish and bilobed mass of lymphoid tissue present just above the heart and beneath the sternum of the chest. It is large sized in the young child, grows to maximum size at puberty and then atrophies gradually. Internally it is formed of many lobules held together by connective tissue. Thymus secretes a polypeptide thymosine hormone which helps in development and maturation of T-lymphocytes of immune system.
Essay # 2. Secondary Lymphoid Organs:
These are those lymphoid organs where the fully differentiated lymphocytes migrate and interact with specific antigens and then proliferate to form effector cells which show immune response.
These include:
1. Lymph Nodes (Fig. 8.23):
These are small, oval or bean-like bodies placed in the course of the lymphatic vessels. These act as filters and are the sites of formation of the lymphocytes. These mainly lie in the neck, axilla, thorax, abdomen and groin. Lymph nodes are maximum in armpit and groin.
A lymph node has an inner border called hilum. It is composed of fibrous and muscular tissues externally covered by a fibrous capsule which is produced into a number of partitions called trabeculae. Inner matrix of lymph node is differentiated into two parts: Central medulla and outer cortex. Cortex has follicular aggregations of B-lymphocytes which form spherical masses of cells called primary nodules.
These transform into secondary nodules when stimulated by the antigens. Each secondary follicle has a germinal centre at the centre which is surrounded by B-cell areas. Germinal centre is the site of maturation of B-cells.
These contain the phagocytic WBCs, lymphocytes, plasma cells and macrophages which remove microorganisms and cellular debris. Lymph nodes also destroy cancer cells. These are also the sites of formation of lymphocytes which form antibodies to provide immunity. These also contain certain antigens which activate the lymphocytes present within and initiate the immune response.
2. Spleen (Fig. 8.24):
It is the largest lymphoid organ of the body and lies just behind the stomach. It is bean-shaped and is externally covered by a white fibrous capsule around a pulpy matrix of spleen. Capsule is produced inward as branched septa called trabeculae.
The pulpy matrix of spleen is with two types of pulp: Red pulp and white pulp. Red pulp forms bulk of matrix and is the site of formation of RBCs in the foetal life and also helps in storage of RBCs, while white pulp occurs in patches and stores the T- lymphocytes. So spleen acts as blood bank because it helps in storage of both RBCs and WBCs. It is also called graveyard of RBCs as it is site of haemolysis of dead RBCs. It also acts as a filter and traps the blood- born micro-organisms.
3. Mucosa-Associated Lymphoid Tissue (MALT):
It is formed of masses of lymphoid tissue present in the mucosa of respiratory, digestive and urinogenital tracts. The total volume of MALT is more or less equal to the lymphoid tissue present in lymph nodes and spleen. In the respiratory tract, these are present in the walls of trachea and large bronchi (so called BALT-Bronchial associated lymphoid tissue) while those associated with the wall of small intestine (especially of ileum) are called Peyer’s patches (or Gut-associated lymphoid tissue or GALT).
4. Tonsils:
These are masses of lymphoid tissues present at many sites inside the human body e.g:
(a) Palatine tonsils are largest sized tonsils present on either side of oropharynx.
(b) Pharyngeal tonsils are present on the posterior wall of the pharynx.
(c) Lingual tonsils are present on the dorsum of the posterior part of tongue.
