As briefed earlier, monocytes are agranulocytic lrukocytes, having a single large nucleus and absence of granules.

Monocytes are the largest of the leukocytes and perform very essential functions in both inflammation and immunity.

Monocytes however have two distinct types based largely on their location. When circulating in the bloodstream, they are considered monocytes. But some of these monocytes migrate into the tissues and develop into a type called macrophages.

Monocytes are actively motile and phagocytic with macrophages having two differing forms- fixed ones that are stationed at vantage locations and mobile ones that move around. Collections of fixed macrophages include:

• Synovial cells in joints
• Langerhans cells in the skin
• Microglia in the brain
• Hepatic macrophages (Kupffer cells) in the liver
• Alveolar macrophages in the lungs
• Sinus-lining macrophages (reticular cells) in the spleen, lymph nodes and thymus gland
• Mesangial cells in the glomerulus of nephrons in the kidney
• Osteoclasts in bone.

Monocytes/Macrophages have a diverse range of protective functions. Some of these functions are;

• They are actively phagocytic and are much more powerful and longer-lived than the smaller neutophils.
• They synthesise and release an array of biologically active chemicals, called cytokines, including interleukin 1.
• They also have a central role linking the non-specific and specific (immune) systems of body defence
• They produce factors important in inflammation and repair.
• They can ‘wall off’ indigestible pockets of material, isolating them from surrounding normal tissue.

References

Waugh, A., & Grant, A. (2014). Ross and Wilson: Anatomy and physiology in health and illness. (11th edition). International edition. Elsevier-Churchill Livingstone.