Part 2: Immunity – Ability to Resist Infection and Disease

Overview of Immunity

Immunity is the body's ability to resist infection and disease. The immune system is divided into two main branches: the nonspecific (innate) defense system and the specific (adaptive) defense system. This section focuses on the innate immune system, which provides immediate, general protection against pathogens.

• Innate Immunity (Nonspecific Defense System): Present from birth, responds quickly to a wide range of pathogens without prior exposure.

• Adaptive Immunity (Specific Defense System): Develops after exposure to specific antigens, involves lymphocytes (B and T cells), and provides targeted responses.

Nonspecific Defense System: Innate Immunity

Physical Barriers

The first line of defense against pathogens consists of physical and chemical barriers that prevent entry of infectious agents.

• Skin (Epidermis): Acts as a tough, impermeable barrier to most microbes and particulates.

• Hair: Nasal hairs filter out microbes and particulates from inhaled air.

• Mucous Membranes: Line the respiratory, digestive, reproductive, and urinary tracts. Mucus traps microbes and particulates.

• Normal Flora: Nonpathogenic bacteria inhabit the body, occupying available nutrients and space, preventing pathogens from colonizing.

Examples of Physical Barriers

• Skin: Covers body surface, secretes sebum (oily substance) with antimicrobial properties.

• Mucous Membranes: Produce mucus to trap pathogens; cilia in respiratory tract move mucus out of the body.

• Other Mechanisms: Vomiting, defecation, and urination help eliminate microbes from the body.

Chemical Barriers

Chemical barriers enhance the effectiveness of physical barriers by creating hostile environments for pathogens.

• Acidic Secretions: Sweat and sebum (skin), gastric juice (stomach), and vaginal secretions lower pH, inhibiting microbial growth.

• Lysozyme: Enzyme found in saliva, tears, nasal secretions, and sweat; breaks down bacterial cell walls.

• Saliva: Continuously cleanses and lubricates the mouth; contains immunoglobulin A (IgA).

• Mucus: Traps microorganisms in the respiratory and digestive tracts.

Table: Examples of Physical and Chemical Barriers

Cellular Defenses

The second line of defense involves various white blood cells (leukocytes) that identify and eliminate pathogens that breach the first line of defense.

• Phagocytes: Cells that engulf and digest pathogens. Main types include:

  • Neutrophils: Most abundant phagocytes; die after ingesting pathogens.

  • Macrophages: Derived from monocytes; can engulf large numbers of microbes and debris.

  • Eosinophils: Attack parasites and participate in allergic responses.

• Natural Killer (NK) Cells: Lymphocytes that provide immune surveillance by detecting and destroying virus-infected and cancerous cells.

Phagocytosis Process

1. Phagocyte adheres to pathogen.

2. Engulfment: Pathogen is enclosed in a phagosome.

3. Lysosome fuses with phagosome, forming a phagolysosome.

4. Enzymes digest the pathogen.

5. Residual material is expelled from the cell.

Chemical and Protein Defenses

Certain chemicals and proteins in the body enhance innate immunity by attacking pathogens directly or by supporting cellular defenses.

• Inflammation: Local tissue response to injury or infection; characterized by redness, heat, swelling, and pain. Limits spread of infection and promotes healing.

• Antimicrobial Proteins:

  • Interferons (IFNs): Proteins released by virus-infected cells; help protect neighboring cells from viral infection.

  • Complement System: Group of plasma proteins that enhance phagocytosis, lyse pathogens, and promote inflammation.

  • Defensins: Small proteins that disrupt microbial membranes and provide a chemical barrier.

• Fever: Temporary elevation of body temperature (> 98.6°F or 37°C) that accelerates metabolism and enhances body defenses.

Table: Key Antimicrobial Proteins

Inflammatory Response

Inflammation is a complex response to tissue injury or infection, involving the release of chemical mediators and increased blood flow to the affected area.

• Key Steps:

  1. Release of inflammatory chemicals (e.g., histamine, prostaglandins, leukotrienes, complement).

  2. Vasodilation and increased vascular permeability (causing redness and swelling).

  3. Recruitment of phagocytes and other immune cells to the site (chemotaxis).

  4. Phagocytosis and removal of pathogens and debris.

  5. Tissue repair and resolution of inflammation.

• Purpose: To contain infection, remove pathogens, and initiate tissue repair.

Summary Table: Innate Immunity Components

Key Terms and Definitions

• Pathogen: A microorganism that causes disease.

• Phagocytosis: The process by which a cell engulfs and digests foreign particles.

• Antigen: Any substance that can trigger an immune response.

• Inflammation: A localized response to injury or infection, marked by redness, heat, swelling, and pain.

• Fever: An elevation in body temperature that enhances immune function.

Example: Application of Innate Immunity

When a person gets a small cut on the skin, the innate immune system responds immediately. The skin acts as a barrier, but once breached, phagocytes migrate to the site, engulf bacteria, and inflammatory mediators are released to contain the infection and promote healing. If a virus enters the respiratory tract, mucus and cilia help trap and remove it, while interferons are produced to protect neighboring cells.

Additional info: Adaptive immunity (not covered in detail here) involves specific recognition of antigens and memory responses, mediated by B and T lymphocytes.