The Immune System: Overview

Introduction to the Immune System

The immune system is a functional system that provides resistance to disease and is essential for maintaining health. It is composed of two intrinsic systems: the innate (nonspecific) defense system and the adaptive (specific) defense system.

• Innate Defense System: Provides the first and second lines of defense against pathogens.

• Adaptive Defense System: Provides the third line of defense, targeting specific foreign substances.

Multifunctional Roles of Blood and Immune System

Blood and the immune system work together to protect the body from infection, dispose of debris, and facilitate repair.

• Blood: Transports immune cells and mediators throughout the body.

• Immune System: Detects and eliminates pathogens, and initiates healing processes.

Innate (Nonspecific) Defenses

First Line of Defense: Surface Barriers

The first line of defense consists of physical and chemical barriers that prevent the entry of pathogens.

• Physical Barriers: Skin and mucous membranes act as mechanical barriers.

• Chemical Barriers: Include acidity (acid mantle), enzymes (lysozyme in saliva, mucus, and lacrimal fluid), mucin, defensins, and other antimicrobial chemicals.

• Keratin: Provides resistance to acids, bases, bacterial enzymes, and toxins.

• Mucosae: Trap and remove microorganisms.

Example: Acid Mantle

The acidity of skin and vaginal secretions inhibits microbial growth, forming a protective acid mantle.

Example: Lysozyme

Lysozyme in saliva and tears destroys bacteria by breaking down their cell walls.

Example: Mucin

Sticky mucus traps pathogens in the digestive and respiratory tracts.

Example: Defensins

Antimicrobial peptides that disrupt microbial membranes.

Example: Sebum and Dermicidin

Lipids in sebum and dermicidin in sweat are toxic to some bacteria.

Respiratory System Modifications

Cilia and mucus in the respiratory tract help remove pathogens.

Second Line of Defense: Cells and Chemicals

When surface barriers are breached, the second line of defense is activated. This includes phagocytes, natural killer (NK) cells, inflammation, antimicrobial proteins, and fever.

• Phagocytes: Chief cells include macrophages and neutrophils.

• Pattern Recognition Receptors: Allow cells to recognize and bind to microbial structures.

Phagocytosis: Mechanism and Steps

Phagocytes and Their Types

• Macrophages: Develop from monocytes; chief phagocytic cells.

• Free Macrophages: Wander through tissues (e.g., alveolar macrophages).

• Fixed Macrophages: Permanent residents in organs (e.g., stellate macrophages in liver, microglia in brain).

Steps of Phagocytosis

Phagocytosis is the process by which phagocytes engulf and destroy pathogens. The steps are:

1. Adherence: Phagocyte recognizes and adheres to pathogen's carbohydrate signature.

2. Engulfment: Phagocyte forms pseudopods to engulf the pathogen, creating a phagosome.

3. Fusion: Lysosome fuses with the phagosome, forming a phagolysosome.

4. Destruction: Toxic compounds and lysosomal enzymes destroy the pathogen.

5. Exocytosis: Indigestible material is expelled from the cell.

Clinical Example: Tuberculosis

Some bacteria, such as tuberculosis bacilli, resist digestion by macrophages and can remain alive inside, forming granulomas. If immunity decreases, these bacteria may become active and cause disease.

Helper T Cells and Respiratory Burst

Helper T cells can trigger macrophages to produce a respiratory burst, killing pathogens resistant to lysosomal enzymes by:

• Releasing cell-killing free radicals

• Producing oxidizing chemicals (e.g., H2O2)

• Increasing pH and osmolarity of phagolysosome

Natural Killer (NK) Cells

Role and Mechanism

NK cells are nonphagocytic, large granular lymphocytes that police blood and lymph.

• Kill cancer and virus-infected cells before the adaptive immune system is activated

• Attack cells lacking "self" cell-surface receptors

• Induce apoptosis in target cells

• Secrete chemicals that enhance the inflammatory response

Inflammation: Tissue Response to Injury

Definition and Triggers

Inflammation is triggered whenever body tissues are injured by trauma, heat, chemicals, or infection.

Benefits of Inflammation

• Prevents spread of damaging agents

• Disposes of cell debris and pathogens

• Alerts the adaptive immune system

• Sets the stage for repair

Cardinal Signs of Acute Inflammation

• Redness

• Heat

• Swelling

• Pain

• Impairment of function (sometimes)

Inflammatory Chemical Release

Chemicals such as histamine, kinins, prostaglandins, and complement are released by injured tissues, immune cells, or blood proteins.

• Cause vasodilation of local arterioles

• Make capillaries leaky

• Attract leukocytes to the area

• Trigger pain receptors and prompt release of more inflammatory chemicals

Vasodilation and Increased Vascular Permeability

• Vasodilation: Causes hyperemia (increased blood flow), leading to redness and heat.

• Increased Permeability: Causes exudate (fluid with clotting factors and antibodies) to leak into tissue, resulting in swelling (edema) and pain.

• Benefits of Edema: Sweeps foreign material into lymphatic vessels, delivers clotting proteins and complement, forms fibrin mesh for repair, and isolates injured area.

Phagocyte Mobilization During Inflammation

Steps of Phagocyte Mobilization

1. Leukocytosis: Neutrophils enter blood from bone marrow.

2. Margination: Neutrophils cling to capillary walls.

3. Diapedesis: Neutrophils flatten and squeeze out of capillaries.

4. Chemotaxis: Neutrophils follow chemical trail to site of injury.

Summary Table: Inflammatory Response Steps

Conclusion

The innate immune system provides rapid, nonspecific defense against pathogens through surface barriers, phagocytes, NK cells, and inflammation. Understanding these mechanisms is essential for assessing patient health and recognizing signs of a healthy immune response. Additional info: Academic context was added to clarify the steps of phagocytosis, the role of chemical barriers, and the clinical relevance of inflammation and phagocyte mobilization.