Immune System & Resistance to Disease: ANP Study Notes

Immune System & Resistance to Disease

Introduction

The immune system protects the human body from a vast array of pathogens, including viruses and bacteria encountered daily through air, food, and contact with surfaces. It maintains homeostasis by defending against disease, removing damaged cells, and preventing cancerous growths. The immune system is divided into innate (nonspecific) and adaptive (specific) defenses, each with distinct mechanisms and roles in disease resistance.

Types of Resistance

Innate Defenses

Innate defenses provide immediate, non-specific protection against pathogens. They act as the body's first and second lines of defense, preventing entry and removing foreign material.

• Physical Barriers: The skin and mucous membranes form a protective layer that blocks pathogen entry.

• Mechanical Barriers: Bodily fluids and movements help expel microbes:

  • Tears, sweat, mucus: Wash away pathogens.

  • Cilia beating: Moves mucus and trapped particles out of the respiratory tract.

  • Coughing: Expels irritants and microbes from airways.

• Chemical Barriers:

  • Gastric acid: Destroys ingested pathogens in the stomach.

  • Lysozyme: An enzyme in saliva and tears that breaks down bacterial cell walls.

  • Interferons: Proteins produced by virus-infected cells that trigger protective mechanisms in nearby uninfected cells, helping prevent viral spread.

• Normal Flora: Beneficial microorganisms that outcompete potential pathogens for resources and space.

• Phagocytes: White blood cells such as macrophages and neutrophils that engulf and digest microbes.

• Inflammation: A local response to tissue damage characterized by the release of chemicals (e.g., histamine from mast cells) that cause vasodilation, resulting in swelling, heat, redness, pain, and sometimes loss of function. Inflammation attracts neutrophils (within 1 hour) and then macrophages to the site of injury or infection.

• Fever: Immune cells and microbial products stimulate the hypothalamus to increase body temperature via prostaglandin release. Medications like aspirin, ibuprofen, and acetaminophen reduce fever by inhibiting prostaglandin synthesis.

• Natural Killer (NK) Cells: A type of lymphocyte that can non-selectively destroy cancer cells and virus-infected cells without prior sensitization.

Adaptive Defenses (Immunity)

Adaptive defenses provide specific, long-term protection by recognizing and targeting particular antigens. This system involves the production of specialized lymphocytes and antibodies.

• Antigen (Ag): A protein or polysaccharide recognized as foreign by the immune system (e.g., parts of bacteria, viruses, pollen, parasites, or transplanted tissues).

• Antibody (Ab): A plasma protein (γ globulin) produced by plasma cells that specifically binds to a matching antigen.

Immune Response Steps

1. Phagocytosis: A phagocyte (e.g., macrophage) engulfs an invader and displays fragments of its antigen on its surface.

2. Helper T-cell (TH) Activation: A helper T-cell binds to the antigen-presenting phagocyte and becomes activated, proliferating to produce more TH cells.

3. TH Cell Functions: Activated TH cells release chemicals that activate:

   • Cytotoxic T-Lymphocytes (CTLs): Proliferate and directly destroy virus-infected cells, cancer cells, or transplanted cells. (Cell-Mediated Immunity)

   • B Cells: Proliferate and differentiate into plasma cells that produce antibodies. (Humoral (Antibody-Mediated) Immunity)

4. Memory Cells: Both B and T memory cells are produced, enabling a rapid and robust response upon subsequent exposure to the same antigen.

Types of Humoral Immunity

• Active Immunity: Long-lasting (years). The body produces its own antibodies and memory B cells after:

  • Natural exposure: Infection by a pathogen.

  • Artificial exposure: Vaccination with killed or inactivated pathogens.

• Passive Immunity: Short-term (weeks), lasting as long as the antibody is present. The body receives antibodies produced elsewhere.

  • Natural passive immunity: Transfer of maternal antibodies to fetus via placenta or to infant via breast milk.

  • Artificial passive immunity: Injection of antibodies produced by another person, animal, or monoclonal source (e.g., antivenins for snake bites, rabies, tetanus, or Rh factor incompatibility).

Summary Table: Innate vs. Adaptive Defenses

Feature	Innate Defenses	Adaptive Defenses
Specificity	Non-specific	Specific to particular antigens
Response Time	Immediate	Slower (days to develop)
Memory	None	Memory cells provide faster secondary response
Main Components	Physical/chemical barriers, phagocytes, NK cells, inflammation, fever	B cells, T cells, antibodies
Examples	Skin, lysozyme, macrophages	Vaccination, antibody production

Example: Vaccination against measles exposes the immune system to an inactivated virus, prompting the production of memory B cells and antibodies. If the person encounters the live virus later, the adaptive immune system mounts a rapid, effective response, preventing illness.

Additional info: The immune system's ability to distinguish self from non-self is crucial for preventing autoimmune diseases, where the body attacks its own tissues. Immunology research continues to inform vaccine development, cancer therapies, and treatments for immune disorders.