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Immune System Anatomy: Organs and Tissues

In June 1981, the Centers for Disease Control and Prevention (CDC), in Atlanta, Georgia, published a report of an unusual cluster of five patients in Los Angeles, California. All five were diagnosed with a rare pneumonia caused by a fungus called Pneumocystis jirovecii (formerly known as Pneumocystis carinii). Why was this unusual? Although commonly found in the lungs of healthy individuals, this fungus is an opportunistic pathogen that causes disease in individuals with suppressed or underdeveloped immune systems. The very young, whose immune systems have yet to mature, and the elderly, whose immune systems have declined with age, are particularly susceptible.

Figures (10)

A 3D medical animation still of spleen structure and exact location
Visceral surface of the spleen
Micrograph of splenic tissue showing the red pulp (red), white pulp (blue nuclei in lighter background) and a thickened inflamed capsule (mostly pink – top of image). H&E stain.
The spleen contains two different tissues, white pulp (A) and red pulp (B). The white pulp functions in producing and growing immune and blood cells. The red pulp functions in filtering blood of antigens, microorganisms, and defective or worn-out red blood cells.
Thalassemia-enlarged spleen taken after splenectomy
Laparoscopic view of a horse's spleen (the purple and grey mottled organ)
Scheme showing development of branchial epithelial bodies from the thoracic cavity of the fetus. I, II, III, IV. Branchial pouches.
Bone marrow aspirate showing normal "trilineage hematopoiesis": myelomonocytic cells (an eosinophil myelocyte marked), erythroid cells (an orthochromatic erythroblast marked), and megakaryocytic cells
Hematopoietic precursor cells: promyelocyte in the center, two metamyelocytes next to it and band cells from a bone marrow aspirate
A Wright's-stained bone marrow aspirate smear from a patient with leukemia

Key Points

  • Bone marrow is a semi-solid tissue found within the spongy (also known as cancellous) portions of bones.
  • The thymus is located in the upper front part of the chest, in the anterior superior mediastinum, behind the sternum, and in front of the heart.
  • The spleen is a center of activity of the mononuclear phagocyte system and is analogous to a large lymph node, as its absence causes a predisposition to certain infections.
  • Up to a quarter of lymphocytes are stored in the spleen at any one time.

Terms

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