The key primary lymphoid organs of the immune system include the thymus and bone marrow, as well as secondary lymphatic tissues including spleen, tonsils, lymph vessels, lymph nodes, adenoids, skin, and liver.

Bone marrow is the flexible tissue found in the interior of bones. In humans, red blood cells are produced in the heads of long bones. The red bone marrow is a key element of the lymphatic system, being one of the primary lymphoid organs that generate lymphocytes from immature hematopoietic progenitor cells. Bone marrow and thymus constitute the primary lymphoid tissues involved in the production and early selection of lymphocytes.

The spleen is similar in structure to a large lymph node and acts primarily as a blood filter. The B lymphocytes in its white pulp secrete antibodies and the spleen also removes antibody-coated bacteria along with antibody-coated blood cells by way of blood and lymph node circulation.

The thymus “educates” T cells and provides an inductive environment for the development of T cells from hematopoietic progenitor cells. The thymus is the largest and most active during the neonatal and pre-adolescent periods of development. By the early teens, the thymus begins to atrophy is replaced by adipose tissue.

Lymph nodes are distributed widely throughout areas of the body, including the armpit and stomach, and linked by lymphatic vessels. Lymph nodes are garrisons of B lymphocytes, T lymphocytes and other immune cells. Lymph nodes act as filters or traps for foreign particles and are essential in the proper functioning of the immune system. They are packed tightly with the white blood cells, called lymphocytes and macrophages.

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Key Points

• The key primary lymphoid organs of the immune system are the thymus and bone marrow, and secondary lymphatic tissues such as the spleen, tonsils, lymph vessels, lymph nodes, adenoids, and skin and liver.

• Bone marrow produces lymphocytes that play a very essential role in the immunity of the body by fighting against the antigens.

• The spleen filters the foreign particles and also synthesizes antibodies thus help in the immune defense.

• The thymus is involved in the maturation of the T-lymphocytes that play a significant role in cell-mediated immunity.

• Lymph nodes act as filters or traps for foreign particles.

Key Terms

lymphocytes: A lymphocyte is a type of white blood cell in the vertebrate immune system. The three major types of lymphocyte are T cells, B cells and natural killer (NK) cells. T cells (thymus cells) and B cells (bursa-derived cells) are the major cellular components of the adaptive immune response

hematopoietic: related to blood cells.

hematopoietic progenitor cells: are the stem cells that give rise to other blood cells

bone marrow: is the spongy tissue inside bones that contains stem cells that develop into red and white blood cells

lymphatic system: the network of vessels through which lymph drains from the tissues into the blood

thymus: a specialized primary lymphoid organ of the immune system where T cells mature

antibody: protein produced in the blood to fight against an antigen

adipose tissue: fat tissue

B cell: a lymphocyte, developed in the bursa of birds and the bone marrow of other animals, that produces antibodies and is responsible for the immune system

T cell: a lymphocyte, from the thymus, that can recognize specific antigens and can activate or deactivate other immune cells

macrophage: is a type of phagocyte, which is a cell responsible for detecting, engulfing and destroying pathogens