RK Goit, Lecturer

Department of Physiology

Nepalgunj Medical College, Nepal

Reticuloendothelial system

• phagocytosis – ingestion of microbes or foreign cells or solid materials by a phagocyte

• phagocytes are neutrophils, monocytes & macrophages

• monocytes transform themselves into macrophages in tissue

• these macrophages are mononuclear cells, & this system of phagocytes is called as mononuclear phagocyte system (MPS)

• this system of cells was known as reticuloendothelial system– neither they are reticular in appearance

– nor they have endothelial origin

• Therefore, the term reticuloendothelial system is obsolete.

Macrophages (Kupffer cells) in the liver sinusoids

bacteria from ingested food pass through GIT into the portal blood

↓

it passes through the sinusoids of the liver

↓

these sinusoids are lined with Kupffer cells

↓

these cells form an effective particulate filtration system

Alveolar macrophages in the lungs

• organisms enter the body through the lungs

• macrophages can phagocytize particles

1. if the particles are digestible, the macrophages can digest them & release the digestive products into the lymph

2. if the particle is not digestible, the macrophages often form a “giant cell” capsule around the particle→ slowly dissolve

Microglia of brain

• microglia cells in brain & spinal cord appear to be inactive

• in inflammatory disease of the CNS, they become the immune effector cells

• they proliferate & become antigen presenting cells

Tissue macrophages in the lymph nodes

• if the particles are not destroyed locally in the tissues, they enter the lymph & flow to the lymph nodes

• foreign particles are then trapped in these nodes in a meshwork of sinuses lined by tissue macrophages

Tissue macrophages in the spleen & bone

• if an invading organism succeeds in entering the general circulation, there are other lines of defense

Mesangial cells in kidney

• are an unusual example of phagocytic cells derived from smooth muscle & not monocytes

• aid neutrophils in removing other mesangial cells undergoing apoptosis & also other debris in glomerulus

• when infection begins in a subcutaneous tissue & local inflammation ensues, local tissue macrophages can divide in situ & form still more macrophages

• then they perform the usual functions of attacking & destroying the infectious agents

Tissue macrophages in the skin & subcutaneous tissues (Histiocytes)

1. Phagocytic function– when any foreign body invades, macrophages ingest them by

phagocytosis & liberate the antigenic products of the organism

– antigens activate the helper T lymphocytes & B lymphocytes

2. Secretion of bactericidal agents– secrete many bactericidal agents which kill the bacteria

I. Superoxide (O-2)

II. Hydrogen peroxide (H2O2)

III. Hydroxyl ions (-OH-)

3. Secretion of interleukins– IL-1: accelerate maturation & proliferation of specific B

lymphocytes & T lymphocytes

– IL-6: cause growth of B lymphocytes

– IL-12: influence the T-helper cells

4. Secretion of tumor necrosis factors

– TNF-α: causes necrosis of tumor & activates the immune responses in the body

– TNF-β: stimulates immune system

5. Secretion of platelet derived growth factor

– accelerates repair of damaged blood vessel & wound healing

6. Secretion of colony stimulation factor

– M-CSF accelerates growth of granulocytes, monocytes & macrophages

7. Removal of carbon particles & silicon

– ingest the substances like carbon dust particles & silicon which enter the body

8. Destruction of RBC

– remove aged RBC

9. Giant cell

– 20 or more macrophages can fuse to form a multinucleate ‘giant cell’ that engulfs a bacillus

10. Foam cells

– macrophages store excess lipids & mucoprotein & become swollen to form ‘foam cells’

Functions of spleen

• Formation of blood cells

– play in important role in the hemopoietic function in embryo

– during the hepatic stage, spleen produces the blood cells along with liver

• Destruction of blood cells

– the older RBCs, lymphocytes & thrombocytes are destroyed in the spleen

• Reservoir function

– a large number of RBCs are stored in spleen

– RBCs are released form spleen into circulation during the emergency conditions like hypoxia & hemorrhage

• Role in defense of body

– spleen filters the blood by removing the microorganism

– macrophages in splenic pulp phagocytose the microorganism & other foreign bodies

– spleen contains about 25% of T lymphocytes & 15% of B lymphocytes & form the site of antibody production

Functions of lymph nodes

Lymph nodes serve as filters which filter bacteria & toxic substances form the lymph.

• when lymph passes through the lymph nodes, it is filtered i.e. the water & electrolytes are removed

– but the proteins & lipids are retained in the lymph

• bacteria & other toxic substances are destroyed by macrophages of lymph nodes

Thank You