Immunological Homeostasis & Lymphoid Organs
Homeostasis is the ability of an organism to regulate its internal environment and keep it stable. The immune system is a major part of this balance — it constantly distinguishes self from non-self, removes harmful agents, and preserves the body’s internal stability.
Immunological homeostasis has evolved over time and reflects the genetic uniqueness of each organism. It allows the immune system to recognise, bind, and eliminate foreign molecules, cells, bacteria, toxins, drugs, pollen, food antigens, and many other external substances.
Immunological Homeostasis
- Develops through evolution.
- Represents the genetic uniqueness of the organism.
- Recognises, binds, and removes foreign molecules, cells, bacteria, and their products.
- Responds to chemical substances, drugs, allergens, and food particles.
Lymphoid Organs
The organs of the immune system are grouped into primary (central) and secondary (peripheral) lymphoid organs.
Primary organs are where immune cells are produced and mature.
Secondary organs are where immune cells activate and respond to antigens.
Primary Lymphoid Organs
Thymus
The thymus is located in the anterior mediastinum above the heart. It is largest just before birth and gradually involutes after puberty. It plays a central role in T-cell development.
- Immature lymphocytes begin to enter the thymus around 90–100 days after fertilization.
- Early thymocytes originate from the yolk sac and foetal liver, later replaced by bone marrow–derived precursors.
- After crossing the blood-thymus barrier, the cells are called thymocytes.
- Mature T cells leave the medulla and migrate to lymph nodes, Peyer’s patches, and the spleen.
- The thymus becomes encapsulated and divided into cortex and medulla.
- Cortex → mostly immature thymocytes.
- Medulla → site where T cells learn to differentiate self vs non-self.
- About 98% of T cells die in the thymus after failing selection.
- T-cell production is highest before puberty and declines sharply afterward.
- Absence of the thymus (e.g., in DiGeorge syndrome) → severe T-cell deficiency but normal B-cell numbers.
Bone Marrow
Bone marrow is responsible for the generation of all immune cells through haematopoiesis.
It functions as both a primary and secondary lymphoid organ.
- Haematopoiesis begins in the yolk sac and para-aortic mesenchyme, later in the foetal liver and spleen, and finally shifts to bone marrow.
- Bone marrow–derived stem cells differentiate into:
- Mature immune cells
- Or precursors that migrate to the thymus (to become T cells)
- Produces B cells, natural killer (NK) cells, granulocytes, immature thymocytes, red blood cells, and platelets.
- Contains antibody-secreting plasma cells that migrated from peripheral tissues.
- Cytokines, especially colony stimulating factors (CSFs), regulate cell proliferation and maturation.