Difference Between Monocyte and Macrophage

Main Difference –  Monocyte vs Macrophage

The main difference between monocyte and macrophage is that monocyte is the precursors of some of the macrophages whereas macrophages are the professional phagocytes, which engulf pathogens invading the body.

Monocyte and macrophage are two types of cells found in the immune system of organisms. They are considered as the front line of host defense. Monocytes are bean-shaped small cells whereas macrophages are irregular-shaped large cells. Both monocytes and macrophages are capable of secreting cytokines and chemokines.

This article explores,

1. What is a Monocyte
      – Characteristics, Structure, Function
2. What is a Macrophage
      – Characteristics, Structure, Function
3. What is the difference between Monocyte and Macrophage

What is a Monocyte

Monocytes are a type of immune cells found in blood; they are capable of migrating to tissues by differentiating into macrophages. Monocytes are a kind of white blood cells, produced in bone marrow. They are capable of being differentiated into dendritic cells as well. Monocytes are involved in the innate immunity of an organism, serving as the front line defense of the host. They also allow the adaptive immune system to be activated by triggering an inflammatory response. Monocytes secrete cytokines like IL-1, IL-2 and TNF and chemokines like monocyte chemotactic protein-1 and -3. Monocytes migrate into tissue within 8-12 hours, responding to inflammation.

Monoblasts in bone marrow are the precursors of monocytes. After releasing into the blood stream, monocytes circulate for three days along with the blood prior to differentiating into either macrophages or dendritic cells. Monocytes are the largest type of cells in the blood. Three types of monocytes are found in blood, depending on the receptors found on the surface of the cells. Classical monocytes contain the surface receptor, CD14. Non-classical monocytes contain CD16 along with the CD14. Intermediate monocytes contain CD14 and low levels of CD16 receptors on the cell surface. Half of the adult monocytes are stored in the spleen. Monocytes contain granules in the cytoplasm, which contain necessary enzymes for the digestion of engulfed pathogens. They contain bean-shaped, uni-lobed nucleus. Monocytes count for 2-10% of total white blood cell count in blood. 

Figure 1: A Monocyte

What is a Macrophage

Macrophages are a type of immune cells found in the extracellular fluid. They are differentiated from monocytes. Macrophages are large cells, capable of engulfing dead cells and ingested foreign material like bacteria and viruses by surrounding them by forming pseudopodia. Granules in the cytoplasm of macrophages contain enzymes for the digestion of engulfed material. Macrophages are considered to be professional phagocytes. Langerhans cells in the skin, Kupffer cells in the liver, the pigmented epithelium of the eye and the microglia in the brain contain macrophages as well.  Old and defective RBCs are removed from the circulation by macrophages in the spleen.

Monocytes in the blood migrate into the tissue in response to the inflammation, becoming macrophages. The diameter of a macrophage is 21 μm. Macrophages can survive for months, developing an innate immune response, which is non-specific. The main function of macrophages is phagocytosis. The engulfing particle is pinched off into the cytoplasm of the macrophage by forming a vesicle called phagosome. Phagosome is trafficked into a lysosome in order to fuse with it, forming the phagolysosome. Digestion of the particle occurs inside the phagolysosome. Macrophages are also known as inflammatory cells, which are capable of activating adaptive immunity by presenting antigens belonging to digested material on the surface of the cell. These antigens are identified by T helper cells, which stimulate B cells, secreting specific antibodies.

Figure 2: A Macrophage

Difference Between Monocyte and Macrophage

Presence

Monocyte: Monocytes are found in the blood.

Macrophage: Macrophages are found in the extracellular fluid.

Diameter

Monocyte: Diameter of a monocyte is around 7.72-9.99  μm.

Macrophage: Diameter of a macrophage is 21 μm.

Function

Monocyte: Monocytes are macrophages.

Macrophage: Macrophages phagocytize cell debris and foreign material like bacteria and virus.

Receptors

Monocyte: Monocytes contain CD14 and CD16 on the surface of the cell.

Macrophage: Macrophages contain CD14, Cd11b, CD68, MAC-1 and -3, EMR1 and Lysozyme M on the surface of the cells.

Function

Monocyte: Monocytes are involved in the innate immunity by differentiating into macrophages. They are involved in the adaptive immunity by secreting cytokines and chemokines.

Macrophage: Macrophages are involved in the innate immunity as well as the adaptive immunity by presenting the antigens of the foreign bodies on their MHC complex.

Conclusion

Monocytes and macrophages are two immune system cells, involved in the innate immunity as well as the adaptive immunity. Monocytes are found in blood. While responding to inflammation, monocytes migrate to the extracellular fluid, which surrounds the inflammatory tissue, differentiating into macrophages. Macrophages phagocytize pathogens and destroy them by phagocytosis during innate immunity. Innate immunity generates non-specific response while destroying pathogens. Macrophages present antigens of the destroyed pathogens in order to be identified by T helper cells. T helper cells then activate B lymphocytes, producing specific antigens for a particular pathogen. Specific antigens are involved in adaptive immunity. Macrophages in different organs are involved in clearing off defective cells in the body. However, the main difference between monocytes and macrophages is their location and function in immunity.

Reference:
1. Janeway, Charles A., and Jr. “The front line of host defense.” Immunobiology: The Immune System in Health and Disease. 5th edition. U.S. National Library of Medicine, 01 Jan. 1970. Web. 05 Apr. 2017.
2. Janeway, Charles A., and Jr. “Principles of innate and adaptive immunity.” Immunobiology: The Immune System in Health and Disease. 5th edition. U.S. National Library of Medicine, 01 Jan. 1970. Web. 05 Apr. 2017.
3. Janeway, Charles A., and Jr. “Induced innate responses to infection.” Immunobiology: The Immune System in Health and Disease. 5th edition. U.S. National Library of Medicine, 01 Jan. 1970. Web. 05 Apr. 2017.

Image Courtesy:
1. “Blausen 0649 Monocyte (crop)” By NIAID –  (CC BY 2.0) via Commons Wikimedia
2. “Macrophages 02” By Blausen.com staff (2014). “Medical gallery of Blausen Medical 2014”. WikiJournal of Medicine 1 (2). DOI:10.15347/wjm/2014.010. ISSN 2002-4436. (CC BY-SA 4.0) via Commons Wikimedia