Human Immune System

Lymphoid organs:

• The immune system of an organism consists of several structurally and functionally different organs and tissues that are widely dispersed in the body.
• The organs involved in the origin, maturation, and proliferation of lymphocytes are called lymphoid organs.
  

  Lymphoid organs in the human body

• Based on their functions, they are classified into primary or central lymphoid organs and secondary or peripheral lymphoid organs.
• The primary lymphoid organs provide an appropriate environment for lymphocytic maturation.
• The secondary lymphoid organs trap antigens and make it available for mature lymphocytes, which can effectively fight against these antigens.

1) Primary lymphoid organs:

• The Bursa of Fabricius of birds, bone marrow, and thymus gland of mammals constitute the primary lymphoid organs involved in the production and early selection of lymphocytes.
• These lymphocytes become dedicated to a particular antigenic specificity.
• Only when the lymphocytes mature in the primary lymphoidal organs, they become immunocompetent cells.
• In mammals, B cell maturation occurs in the bone marrow and T cell maturation occurs in the thymus.

i) Thymus: 

• The thymus is a flat and bilobed organ located behind the sternun, above the heart.
• Each lobe of the thymus contains numerous lobules, separated from each other by connective tissue called septa.
• Each lobule is differentiated into two compartments, the outer compartment or outer cortex is densely packed with immature T cells called thymocytes, whereas the inner compartment or medulla is sparsely populated with mature thymocytes.
• One of its main secretions is the hormone thymosin. It stimulates the T cell to become mature and immunocompetent.
• By the early teens, the thymus begins to atrophy and is replaced by adipose tissue. Thus thymus is most active during the neonatal and pre-adolescent periods.
  

  Primary lymphoid organ - Thymus A) Location B) Structure

ii) Bone marrow:

• Bone marrow is a lymphoid tissue found within the spongy portion of the bone.
• Bone marrow contains stem cells known as haematopoietic cells.
• These cells have the potential to multiply through cell division and either remain as stem cells or differentiate and mature into different kinds of blood cells.

2) Secondary or peripheral lymphoid organs:

• In secondary or peripheral lymphoid organs, antigen is localized so that it can be effectively exposed to mature lymphocytes.
• The best examples are lymph nodes, appendix, Peyer’s patches of gastrointestinal tract, tonsils, adenoids, spleen, MALT (Mucosal-Associated Lymphoid Tissue), GALT (Gut-Associated Lymphoid Tissue), BALT (Bronchial/Tracheal-Associated Lymphoid Tissue).

i) Lymph node:

• Lymph node is a small bean-shaped structure and is part of the body’s immune system.
• It is the first one to encounter the antigen that enters the tissue spaces.
• Lymph nodes filter and trap substances that travel through the lymphatic fluid.
• They are packed tightly with white blood cells, namely lymphocytes and macrophages.
• There are hundreds of lymph nodes found throughout the body. They are connected to one another by lymph vessels.
• Lymph is a clear, transparent, colourless, mobile, and extracellular fluid connective tissue.
• As the lymph percolates through the lymph node, the particulate antigen brought in by the lymph will be trapped by the phagocytic cells, follicular and interdigitating dendritic cells.
  

  Secondary lymphoid organ – Structure of lymph node

• The lymph node has three zones.
• They are the cortex, paracortex, and medulla.
• The outermost layer of the lymph node is called cortex, which consists of B-lymphocytes, macrophages, and follicular dendritic cells.
• The paracortex zone is beneath the cortex, which is richly populated by T lymphocytes and interdigitating dendritic cells.
• The innermost zone is called the medulla which is sparsely populated by lymphocytes, but many of them are plasma cells, which actively secrete antibody molecules.
• As the lymph enters, it slowly percolates through the cortex, paracortex, and medulla, giving sufficient chance for the phagocytic cells and dendritic cells to trap the antigen brought by the lymph.
• The lymph leaving a node carries enriched antibodies secreted by the medullary plasma cells against the antigens that enter the lymph node.
• Sometimes visible swelling of lymph nodes occurs due to active immune response and increased concentration of lymphocytes. Thus swollen lymph nodes may signal an infection. There are several groups of lymph nodes.
• The most frequently enlarged lymph nodes are found in the neck, under the chin, in the armpits, and in the groin.

ii) Peyer’s patches:

• Peyer’s patches are oval-shaped areas of thickened tissue that are embedded in the mucus-secreting lining of the small intestine of humans and other vertebrate animals.
• Peyer’s patches contain a variety of immune cells, including macrophages, dendritic cells, T cells, and B cells.

iii) Tonsils:

• The tonsils (palatine tonsils) are a pair of soft tissue masses located at the back of the throat (pharynx).
• The tonsils are part of the lymphatic system, which helps to fight infections. They stop invading germs including bacteria and viruses.

iv) Spleen:

• The spleen is a secondary lymphoid organ located in the upper part of the abdominal cavity close to the diaphragm.
• The spleen contains B and T cells. It brings humoral and cell mediated immunity.

v) Adenoids:

• The adenoids are glands located in the roof of the mouth, behind the soft palate where the nose connects to the throat.
• The adenoids produce antibodies that help to fight infections.
• Typically, the adenoids shrink during adolescence and may disappear by adulthood.

vi) Mucosa-associated lymphoid tissue (MALT):

• The mucosa-associated lymphoid tissue (MALT) is a diffuse system of small concentrations of lymphoid tissue in the alimentary, respiratory, and urino-genital tracts.
• MALT is populated by lymphocytes such as T and B cells, as well as plasma cells and macrophages, each of which is well situated to encounter antigens passing through the mucosal epithelium.
• It also possesses IgA antibodies.

vii) Gut-associated lymphoid tissue (GALT):

• Gut-associated lymphoid tissue (GALT) is a component of the mucosa-associated lymphoid tissue (MALT) which works in the immune system to protect the body from invasion in the gut.

viii) Bronchus Associated Lymphoid Tissues (BALT): 

• Bronchus Associated Lymphoid Tissues (BALT) also a component of MALT is made of lymphoid tissue (tonsils, lymph nodes, lymph follicles) is found in the respiratory mucosae from the nasal cavities to the lungs.