Immunity: How the Body Prevents and Fights Foreign Invaders

Overview of Body Defenses

The body employs two defense systems that work together to resist disease:

1. Innate (Nonspecific) Defense System: Present at birth, it responds immediately to protect against all foreign substances.
2. Adaptive (Specific) Defense System: This system requires activation and is not present at birth. It uses lymphocytes to target specific foreign substances.

Pathogens and Immune Response

Pathogens are foreign substances that can cause disease. Key types include:

• Viruses: Very small pathogens that invade cells and use the cellular machinery to replicate. Examples: SARS-COV2, Influenza.
• Parasites: Single or multicellular organisms that extract nutrients from a host. Examples: Trypanosomes, tapeworms, hookworms.
• Fungi: Organisms that break down a host’s cells and absorb the nutrients. Examples: Valley fever, athlete’s foot.
• Bacteria: Single-cell organisms that release toxins, damaging the host’s cells. Examples: E. coli, anthrax, pneumonia.

Innate Defense System

The innate defense system consists of two lines of defense:

First Line of Defense: Barriers

• Physical Barriers:
  • Skin: A physical barrier with acidic secretions and sphincters.
  • Mucus Membranes: Found in the respiratory and digestive tracts.
• Chemical Barriers:
  • Lysozyme: Present in saliva.
  • HCl: Found in the stomach.
  • Acid: Present in the urinary tract (and vagina in females).
  • Fatty acids: Secreted by sebaceous glands.
  • Mucus: Traps pathogens in the respiratory tract.
  • Ciliated Cells: In the respiratory tract, they sweep mucus and trapped pathogens away.

Second Line of Defense: Internal Defenses

• Phagocytes: Cells that ingest and break down antigens or pathogens (phagocytosis).
  • Neutrophils
  • Eosinophils
  • Macrophages
• Natural Killer (NK) Cells: Non-specific lymphocytes that attach to infected or tumor cells, causing apoptosis (programmed cell death).
• Antimicrobial Proteins: Include interferons and complement proteins.
• Fever: An increase in body temperature.
• Inflammation: A series of events initiated by injury or infection.

Internal Defenses - Antimicrobial Chemicals

• Interferons: Proteins released by infected cells, inducing viral resistance in healthy cells.
• Complement Proteins: Plasma proteins that create holes in cell membranes, causing the cell to burst.

Internal Defenses - Fever

• Leukocytes secrete pyrogens, which increase body temperature.
  • Creates a favorable environment for lymphocyte and monocyte migration and proliferation.
  • Increases metabolic rate for tissue repair.
  • Creates an unfavorable environment for pathogens by limiting access to essential ions like iron (Fe^{2+}).

Internal Defenses - The Inflammatory Response

A series of events initiated by injury, chemical irritation, intense heat, or pathogen infection.

1. Prevents the spread of pathogens.
2. Disposes of cell debris and pathogens.
3. Alerts the adaptive immune system.
4. Helps with tissue repair.

Stages of the Inflammatory Response

1. Tissue Injury: Injury occurs, damaging body cells.
2. Local Macrophage Activation: Macrophages ingest foreign matter and cellular debris, releasing cytokines to attract more leukocytes and stop pathogens from spreading.
3. Vasodilation and Increased Capillary Permeability: Histamine is released, causing vasodilation and attracting leukocytes, defensive proteins, and fluids to the area, resulting in pain, swelling (edema), redness, and heat (local fever).
4. Containment: Defensive proteins surround injured cells and foreign material. Heparin prevents blood clotting, ensuring continued blood flow. Clotting proteins wall off the damaged area, creating a scab.
5. Leukocyte Proliferation and Migration: More leukocytes migrate to the area, performing phagocytosis. Dead leukocytes, microbes, and damaged cells create “pus.”
6. Continued Recruitment and Tissue Repair: Phagocytes clear the infection, and tissue healing begins.

Acquired (Specific) Defense System

The adaptive defense system is not present at birth and requires lymphocyte activation.

• Humoral Immunity: B-lymphocytes create antibodies that circulate in fluid (blood or lymph) to neutralize foreign substances.
• Cellular Immunity: T-lymphocytes seek and destroy infected body cells or tumor cells.

How the Specific Immune Response Works

• The body recognizes “self” and “non-self” for defense.
• Antigens: Substances not normally present in the body that initiate an immune response. They can be on the surface of a foreign invader, free-floating in fluids, and usually have many different determinants or epitopes.

Antibodies (Immunoglobulins)

• Antibodies (or immunoglobulins, Igs) are “Y” shaped proteins produced by activated B-lymphocytes (plasma cells) in response to an antigen.
• The variable region has an antigen-binding site with single specificity, meaning once an antigen binds to an antibody, the antibody can ONLY react to that antigen and no other.

Types of Immunoglobulins (IG) or Antibodies

• IgM: Most common in the primary response; neutralizes, agglutinates, and activates complement.
• IgD: An antigen receptor on B cells.
• IgG: Most common in blood, major class in secondary responses, crosses the placenta; neutralizes, agglutinates, activates complement, opsonizes, and enhances NK cell activity.
• IgE: Involved in allergies; binds to mast cells and basophils, causing histamine release.
• IgA: Crosses epithelial cells, present on mucosal surfaces and in breast milk; neutralizes and agglutinates.

Antibody Structure & Variability

• Antibodies can move freely in plasma and lymph.
• Antibodies can be embedded in the lymphocyte’s cell membrane.
  • B-cells have IgD antibodies for antigen-binding.
  • T-cells have cell membrane receptors that bind antigens but are structurally different.
• Both allow for antigen binding but result in different responses.

Effects of Immunoglobulins (IG) or Antibodies

• Neutralization: Masks dangerous parts of bacterial exotoxins and viruses.
• Agglutination: Clumping of cell-bound antigens.
• Precipitation: Clumping of soluble antigens.
• Complement Activation: Leads to cell lysis.
• Enhancement of Phagocytosis: Makes pathogens easier to engulf.
• Inflammation: Attracts immune cells and promotes inflammation.