Reticuloendothelial System Notes

Reticuloendothelial System And Tissue Macrophage Definition And Distribution

Reticuloendothelial System Or Macrophage System: The reticuloendothelial system or macrophage system is the system of primitive phagocytic cells that play important role in the defense mechanism of the body. The reticuloendothelial cells are found in the following structures

1. The endothelial lining of vascular and lymph channels
2. Connective tissue and some organs like spleen, liver, lungs, lymph nodes, bone marrow, etc. The reticular cells in these tissues form the tissue macrophage system.

Macrophage: Macrophage is a large cell derived from a monocyte. It has the property of phagocytosis. So, the macrophage is also defined as a large phagocytic cell.

Read And Learn More: Medical Physiology Notes

Classification Of Reticuloendothelial Cells

The reticuloendothelial cells are classified into two types:

1. Fixed reticuloendothelial cells or tissue macrophages
2. Wandering reticuloendothelial cells.

1. Fixed Reticuloendothelial Cells- Tissue Macrophages: The fixed reticuloendothelial cells are also called tissue macrophages or fixed histiocytes because these cells are usually located in the tissues. The tissue macrophages are present in the following

1. Connective Tissue: The reticuloendothelial cells in connective tissues and in serous membranes like pleura, omentum, and mesentery are called the fixed macrophages of connective tissue.
2. Endothelium of Blood Sinusoid: The endothelium of the blood sinusoid in bone marrow, liver, spleen, lymph nodes, adrenal glands, and pituitary glands also contain fixed cells. The Kupffer’s cells present in liver belong to this category.
3. Reticulum: The reticulum of spleen, lymph node, and bone marrow contain fixed reticuloendothelial cells.
4. Central Nervous System: Meningocytes of meninges and microglia form the tissue macrophages of brain.
5. Lungs: Tissue macrophages are present in the alveoli of lungs.
6. Subcutaneous Tissue: The fixed reticuloendothelial cells are present in subcutaneous tissue also.

2. Wandering Reticuloendothelial Cells And Tissue Macrophages

The wandering reticuloendothelial cells are also called free histiocytes. There are two types of wandering reticuloendothelial cells.

1. Free Histiocytes of Blood
   1. Neutrophils
   2. Monocytes, which become macrophages and migrate to the site of injury or infection.
2. Free Histiocytes of Solid Tissue: During an emergency, the fixed histiocytes from connective tissue and other organs become wandering cells and enter circulation.

Functions Of the Reticuloendothelial System

The reticuloendothelial system plays an important role in the defense mechanism of the body. Most of the functions of the reticuloendothelial system are carried out by the tissue macrophages. The functions of tissue macrophages are:

1. Phagocytic Function
   • Macrophages are the large phagocytic cells, which play an important role in defense of the body by phagocytosis.
   • When any foreign body invades, macrophages ingest them by phagocytosis and liberate the antigenic products of the organism.
   • The antigens activate the helper T lymphocytes and B lymphocytes.
   • The lysosomes of macrophages contain proteolytic enzymes and lipases which digest the bacteria and other foreign bodies.
2. Secretion of Bactericidal Agents: Tissue macrophages secrete many bactericidal agents which kill the bacteria.
   • The important bactericidal agents of macrophages are the oxidants. An oxidant is a substance that oxidizes another substance. The oxidants secreted by macrophages are:
     1. Superoxide (O₂^–)
     2. Hydrogen peroxide (H₂O₂)
     3. Hydroxyl ions (OH^–).
   • These oxidants are the most potent bactericidal agents that even the bacteria which can not be digested by lysosomal enzymes are degraded by these oxidants.
3. Secretion of Interleukins: Tissue macrophages secrete the following interleukins which help in immunity
   1. Interleukin-1 (IL-1): Accelerates the maturation and proliferation of specific B lymphocytes and T lymphocytes
   2. Interleukin-6 (IL-6): Causes growth of B lymphocytes and production of antibodies
   3. Interleukin-12 (IL-12): Influences the T-helper cells.
4. Secretion of Tumor Necrosis Factors: Two types of tumor necrosis factors (TNF) are secreted by tissue macrophages:
   1. TNF-α: Causes necrosis of tumor and activates the immune responses in the body
   2. TNF-β: Stimulates immune system and vascular response in addition to causing necrosis of tumor.
5. Secretion of Transforming Growth Factor: Tissue macrophages secrete transforming growth factor which plays an important role in preventing rejection of transplanted tissues or organs by immunosuppression.
6. Secretion of Colony Stimulation Factor: The colony stimulation factor secreted by macrophages is M-CSF. It accelerates growth of granulocytes, monocytes, and macrophages.
7. Secretion of Platelet-Derived Growth Factor: Tissue macrophages secrete the platelet-derived growth factor (PDGF), which accelerates repair of damaged blood vessel and wound healing.
8. Removal of Carbon Particles and Silicon: The macrophages ingest the substances like carbon dust particles and silicon which enter the body.
9. Destruction of Senile RBC: The reticuloendothelial cells, particularly those in spleen destroy the senile RBCs and release hemoglobin.
10. Destruction of Hemoglobin: The hemoglobin released from broken senile RBCs is degraded by the reticuloendothelial cells.
11. Hemopoietic Function: The reticuloendothelial cells also play an important role in the production of blood cells.