Lymphoid System

The lymphoid system function is to protect the body from pathogens and diseased cells (e.g., virus-infected or tumour cells).

Immunity is manifested on molecular, cellular, and body levels. Body’s immune system is a total sum of lymphoid organs composed of central (thymus and bone marrow) and peripheral organs – lymph nodes, spleen, lymphocytes of peripheral blood,

  • Primary lymphoid organs – sites where lymphocytes mature and become immunocompetent – B cells in the bone marrow and T cells in the thymus.
  • Secondary lymphoid organs – mature lymphocytes are distributed via blood to secondary lymphoid organs like lymph nodes, spleen, and lymphoid tissues where they wait for activation.
  •  Mucosal-associated lymphoid tissue (MALT) comprises of gut-associated lymphoid tissue (GALT) with an appendix, Peyer’s patches and solitary lymphoid follicles; bronchial-associated lymphoid tissue (BALT) and nasal-associated lymphoid tissue (NALT) of the respiratory tract, conjunctival-associated lymphoid tissue or CALT, etc.
  • All of the blood cells are derived from hematopoietic stem cells.
  • Fetal hematopoiesis is performed in fetal liver and bone marrow, in adults – in bone marrow only.
  • Under the influence of different cytokines and growth factors, all of blood cell precursors arise. They further become differentiated into mature cell populations.
  • There are 3 main populations (subsets) of lymphocytes – T cells, B cells and NK cells (or natural killer cells).
  • They originate through the stage of common lymphoid progenitor cells from hematopoietic stem cells.
  • Stem cells growth factors, IL-3 and IL-7 are the major cytokines involved in their maturation.
  • T- and B cells undergo antigen-independent differentiation and maturation in central lymphoid organs followed by antigen-dependent differentiation after their migrations into specific zones of peripheral lymphoid tissues.

Lymphoid System

Lymph Nodes

  • Lymph nodes are small, round or bean-shaped which is spread throughout the lymphatic vessels connected by lymph vessels.
  • Lymph nodes are present in clusters mostly found in the neck, axilla, chest, abdomen, and groin.
  • Lymph nodes majorly work for the filtration of harmful substances as they majorly contain WBCs that helps in fighting against infection.


  • It is the largest lymphoid organ in the body
  • It functions in the filtration of blood and it recycles the old RBCs and it acts as the blood storage in the body and majorly helps in fighting against infection.


  • It is the major organ of the lymphoid system which consists of two lobes connected by an isthmus.
  • T cells mature in the thymus and it fights against many foreign antigens.