Respiratory Immunity

1. Introduction

The respiratory system is one of the organs most vulnerable to attack by pathogens, including bacteria, viruses, fungi, parasites, and allergens. To protect the body from these agents, the immune system plays a crucial role.

2. Non-specific Immune System

The non-specific immune system (also known as innate immunity) is the body’s first line of defense, acting quickly and not targeting specific agents. The non-specific immune system consists of the following components:

• Physical barriers:
  • Skin: the outer epidermis protects the body from the invasion of pathogens.
  • Mucous membrane: the mucous membrane lining the respiratory tract produces mucus that traps and eliminates pathogens.
  • Cilia: help to push pathogens out of the body.
• Chemical barriers:
  • Lactic acid on the skin: creates an acidic environment, inhibiting bacterial growth.
  • Lysozyme in mucus: an enzyme that breaks down bacterial cell walls.
  • Interferon (IFN): a protein produced by virus-infected cells that inhibits viral replication.
  • Complement system: a system of plasma proteins that can destroy bacteria, viruses, fungi, and parasites.
• Cellular barriers:
  • Neutrophils: a type of white blood cell that can phagocytize and destroy pathogens.
  • Macrophages: larger white blood cells than neutrophils, also capable of phagocytizing and destroying pathogens.
  • NK (Natural Killer) cells: a type of lymphocyte that can destroy virus-infected cells and cancer cells.

3. Mechanism of Microbe Destruction

The non-specific immune system destroys microbes through two main mechanisms:

• Oxygen-independent mechanism:
  • Use lysosomal enzymes present in phagocytes to break down the cell walls of microbes.
• Oxygen-dependent mechanism:
  • Using 3 oxygen-dependent enzyme systems:
    • Oxidase: produces oxygen free radicals that kill microbes.
    • Myeloperoxidase: combines with oxygen free radicals to form toxic compounds that kill microbes.
    • NO synthetase: produces nitric oxide (NO) that kills microbes.

4. Results of Non-specific Immunity

The non-specific immune system leads to non-specific inflammation, which has the following functions:

• Eliminate microbes from the body.
• Prevent the spread of microbes.
• Activate the specific immune system.

5. Non-specific Immunity Against Extracellular Bacteria

The non-specific immune system fights extracellular bacteria through the following mechanisms:

• Phagocytosis:
  • Neutrophils, monocytes, and macrophages phagocytize and destroy bacteria.
• Complement activation:
  • Complement C3b binds to the surface of bacteria, enhancing phagocytosis.
  • Complement C5a attracts white blood cells to the site of inflammation.
• LPS endotoxin:
  • Bacterial LPS endotoxin stimulates macrophages to produce TNF, IL6 cytokines.
  • TNF and IL6 increase vascular permeability, leading to leukocyte transmigration, increased inflammation, and activation of the specific immune system.

6. Non-specific Immunity Against Intracellular Bacteria

The non-specific immune system is less effective against intracellular bacteria than extracellular bacteria.

7. Antiviral

The non-specific immune system fights viruses through the following mechanisms:

• Increased IFN production:
  • IFN is produced by virus-infected cells, inhibiting viral replication.
• Increased activity of NK, C, TB cells:
  • NK cells destroy virus-infected cells.
  • Complement lyses viruses.
  • Other immune cells play a role in destroying viruses.

8. Anti-parasite

The non-specific immune system fights parasites by:

• Complement activation:
  • Complement lyses parasites.
• Enhanced phagocytosis:
  • White blood cells phagocytize parasites.

9. Specific Immune System

The specific immune system (also known as adaptive immunity) is the body’s second line of defense, acting slower but more precisely than the non-specific immune system. The specific immune system can recognize and destroy specific pathogens.

10. Steps of the Specific Immune System

The specific immune system operates in 3 main steps:

• Recognition: Immune cells recognize the antigen of the pathogen.
• Activation: Immune cells are activated and produce specific antibodies or T cells.
• Effect: Antibodies or T cells destroy the pathogen.

11. Modes of Response of the Specific Immune System

The specific immune system has two main response modes:

• Humoral:
  • B lymphocytes are responsible for producing antibodies that dissolve in the blood.
  • Antibodies bind to the antigen of the pathogen, helping to destroy or inactivate the pathogen.
• Cell-mediated:
  • CD4+ and CD8+ T cells play a major role in cell-mediated specific immune responses.
  • CD4+ T cells recognize the pathogen’s antigen on the surface of antigen-presenting cells (APCs).
  • CD4+ T cells activate other immune cells such as B cells, CD8+ T cells, and macrophages.
  • CD8+ T cells directly destroy virus-infected cells or cancer cells.

12. Specific Immunity Against Extracellular Bacteria

Specific immunity against extracellular bacteria mainly relies on humoral responses, creating antibodies capable of:

• Precipitating bacteria.
• Neutralizing bacterial toxins.
• Activating complement to lyse bacteria.

13. Specific Immunity Against Intracellular Bacteria

Specific immunity against intracellular bacteria mainly relies on cell-mediated responses, with the role of CD4+ and CD8+ T cells:

• CD4+ T cells recognize the antigen of intracellular bacteria on the surface of APCs.
• CD8+ T cells destroy cells infected with intracellular bacteria.

14. Specific Immunity Against Viruses

Specific immunity against viruses relies on both humoral and cell-mediated responses:

• Humoral:
  • IgM and IgG antibodies prevent viruses from attaching to cells.
  • IgA antibodies secreted into the mucosa prevent viruses from entering the body.
• Cell-mediated:
  • CD8+ T cells recognize viral antigens on the surface of virus-infected cells.
  • CD8+ T cells destroy virus-infected cells.
  • NK cells kill virus-infected cells by ADCC (Antibody-Dependent Cell-Mediated Cytotoxicity).

15. Specific Immunity Against Parasites

Specific immunity against parasites is more complex, and may include:

• CD4+ T cells: release cytokines to activate other immune cells.
• CD8+ T cells: destroy cells infected with parasites.
• NK cells: kill cells infected with parasites by ADCC.

16. Notes:

• The immune system is a complex system involving many different factors.
• The immune system can be weakened by various causes such as age, stress, unhealthy diet, illness…
• Strengthening the immune system is essential to protect the body from disease.

17. Conclusion

The immune system, including both the non-specific immune system and the specific immune system, plays a vital role in protecting the body from the invasion of pathogens. Understanding the mechanisms of the immune system helps us to effectively prevent and treat diseases.