9.3.1 Role of Monocytes in Third Line of Defence

Monocytes are a kind of WBCs, which are produced by lymphoid tissues. The monocytes circulate in blood for one to three days and ultimately they leave the blood and come into the intercellular space of tissues. In the tissues, they swell and attain a larger size to become tissue macrophages or dendritic cells which are irregular in shape with many projections. When these cells perform phagocytosis of invaded microorganisms, after digesting them they not only display microbial antigens on their surfaces. Macrophages begin to secrete about 100 different compounds including various enzymes, interferons and a protein called interleukin-1. The interleukin-1 secreted by macrophages activates the T cells that in turn begin to secrete interleukin-2, which then activates the B cells. Interleukin-1 also promotes a general response to injury, causing fever and activating other mechanisms that defend the body against invasion. While dendritic cells after phagocytosis become antigen presenting cells for T cell activation.

Definition

Monocytes: Monocytes are a kind of WBCs, which are produced by lymphoid tissues. The monocytes circulate in blood for one to three days and ultimately they leave the blood and come into the intercellular space of tissues.

Macrophages: In the tissues, monocytes swell and attain a larger size to become tissue macrophages which are irregular in shape with many projections.

Dendritic Cells: Monocytes can also become dendritic cells which are irregular in shape with many projections.

Main Idea

• Monocytes are a kind of WBCs.
• They are produced by lymphoid tissues.
• They circulate in blood for one to three days.
• They leave the blood and come into the intercellular space of tissues.
• In the tissues, they swell and become:
• Tissue macrophages
• Dendritic cells
• Both are irregular in shape with many projections.
• When these cells perform phagocytosis:
• They digest microorganisms.
• They display microbial antigens on their surfaces.
• Macrophages secrete about 100 different compounds:
• Various enzymes
• Interferons
• Interleukin-1
• Interleukin-1 activates T cells.
• T cells secrete interleukin-2.
• Interleukin-2 activates B cells.
• Interleukin-1 also promotes a general response to injury:
• Causes fever
• Activates other defence mechanisms
• Dendritic cells become antigen-presenting cells for T cell activation.

Core Concept

• Monocytes = WBCs produced by lymphoid tissues
• Circulation = blood for 1-3 days
• Migration = into tissues
• Differentiation = macrophages or dendritic cells
• Macrophages = irregular in shape; many projections
• Dendritic cells = irregular in shape; many projections
• Phagocytosis = digest microorganisms; display antigens
• Macrophages secrete = about 100 different compounds
• Interleukin-1 = activates T cells; causes fever
• Interleukin-2 = activates B cells
• Dendritic cells = antigen-presenting cells for T cell activation

Detailed Explanation

Role of Monocytes in Third Line of Defence

• As already described in second line of defence that monocytes are kind of WBCs, which are produced by lymphoid tissues.
• The monocytes circulate in blood for one to three days and ultimately they leave the blood and come into the intercellular space of tissues.
• In the tissues, they swell and attain a larger size to become tissue macrophages or dendritic cells which are irregular in shape with many projections.
• When these cells perform phagocytosis of invaded microorganisms, after digesting them they not only display microbial antigens on their surfaces.
• Macrophages begin to secrete about 100 different compounds including various enzymes, interferons and a protein called interleukin-1.
• The interleukin-1 secreted by macrophages activates the T cells that in turn begin to secrete interleukin-2, which then activates the B cells.
• Interleukin-1 also promotes a general response to injury, causing fever and activating other mechanisms that defend the body against invasion.
• While dendritic cells after phagocytosis become antigen presenting cells for T cell activation.

Summary Table of Role of Monocytes

Summary of Interleukins

Intermediate Level

Role of Monocytes in Third Line of Defence

Interleukins

Summary of the Process

Formula / Mathematical Form

Not applicable to this topic.

Solved Example

Not applicable to this topic (descriptive topic).

Important Points

• 1. Monocytes are a kind of WBCs.
• 2. They are produced by lymphoid tissues.
• 3. They circulate in blood for 1-3 days.
• 4. They leave the blood and come into the intercellular space of tissues.
• 5. In the tissues, they swell and become larger.
• 6. They become tissue macrophages or dendritic cells.
• 7. Both are irregular in shape with many projections.
• 8. They perform phagocytosis of invaded microorganisms.
• 9. They digest microorganisms.
• 10. They display microbial antigens on their surfaces.
• 11. Macrophages secrete about 100 different compounds.
• 12. Compounds include enzymes, interferons, and interleukin-1.
• 13. Interleukin-1 activates T cells.
• 14. T cells secrete interleukin-2.
• 15. Interleukin-2 activates B cells.
• 16. Interleukin-1 causes fever.
• 17. Interleukin-1 activates other defence mechanisms.
• 18. Dendritic cells become antigen-presenting cells.
• 19. Dendritic cells activate T cells.
• 20. Monocytes link nonspecific and specific defences.

Remember

• - Monocytes = WBCs produced by lymphoid tissues
• - Circulation = blood for 1-3 days
• - Migration = into tissues
• - Differentiation = macrophages or dendritic cells
• - Macrophages = irregular in shape; many projections
• - Dendritic cells = irregular in shape; many projections
• - Phagocytosis = digest microorganisms; display antigens
• - Macrophages secrete = about 100 different compounds
• - Interleukin-1 = activates T cells; causes fever
• - Interleukin-2 = activates B cells
• - Dendritic cells = antigen-presenting cells for T cell activation

Common Mistakes

• 1. Confusing monocytes and macrophages — Monocytes become macrophages in tissues.
• 2. Mixing up macrophages and dendritic cells — Both from monocytes; different functions.
• 3. Forgetting the role of interleukin-1 — Activates T cells; causes fever.
• 4. Mixing up interleukin-1 and interleukin-2 — IL-1 from macrophages; IL-2 from T cells.
• 5. Forgetting the role of dendritic cells — Antigen-presenting cells.
• 6. Believing monocytes only phagocytose — They also activate lymphocytes.
• 7. Mixing up the location — Monocytes in blood; macrophages and dendritic cells in tissues.
• 8. Forgetting the number of compounds — About 100 different compounds.
• 9. Thinking monocytes are not important — They link nonspecific and specific defences.
• 10. Mixing up the functions — Macrophages: phagocytosis, antigen presentation, secretion; dendritic cells: antigen presentation.

Quick Revision

As already described in second line of defence that monocytes are kind of WBCs, which are produced by lymphoid tissues. The monocytes circulate in blood for one to three days and ultimately they leave the blood and come into the intercellular space of tissues. In the tissues, they swell and attain a larger size to become tissue macrophages or dendritic cells which are irregular in shape with many projections. When these cells perform phagocytosis of invaded microorganisms, after digesting them they not only display microbial antigens on their surfaces. Macrophages begin to secrete about 100 different compounds including various enzymes, interferons and a protein called interleukin-1. The interleukin-1 secreted by macrophages activates the T cells that in turn begin to secrete interleukin-2, which then activates the B cells. Interleukin-1 also promotes a general response to injury, causing fever and activating other mechanisms that defend the body against invasion. While dendritic cells after phagocytosis become antigen presenting cells for T cell activation.

60-Second Revision

• Monocytes = WBCs produced by lymphoid tissues
• Circulation = blood for 1-3 days
• Migration = into tissues
• Differentiation = macrophages or dendritic cells
• Macrophages = irregular in shape; many projections
• Dendritic cells = irregular in shape; many projections
• Phagocytosis = digest microorganisms; display antigens
• Macrophages secrete = about 100 different compounds
• Interleukin-1 = activates T cells; causes fever
• Interleukin-2 = activates B cells
• Dendritic cells = antigen-presenting cells for T cell activation
• Warning: Monocytes become macrophages or dendritic cells in tissues

NEXT TOPIC

• Chapter:
• 9. Immunity

• Topic:
• 9.3.2 ROLE OF T CELLS IN THIRD LINE OF DEFENCE (CELL MEDIATED IMMUNE RESPONSE)

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