responses to pathogens

• The innate immunity system includes physical and chemical barriers against pathogens and is initiated by chemical signaling from damaged tissue, triggering the inflammatory response and phagocytosis by white blood cells (WBCs).
• The adaptive immunity system involves a specific defense by lymphocytes responding to chemical signals.

Antigens

• Antigens are molecules recognized by the host as foreign, triggering an immune response.
• Exogenous antigens are found on invading pathogens.
• Endogenous antigens originate from toxic chemicals produced by pathogens.

White Blood Cells (WBCs) and the Lymphatic System

• The lymphatic system filters and returns intercellular fluid to the blood using lymph nodes connected by lymph vessels.
• Lymph nodes filter and trap antigens and store B and T lymphocytes.

The Complement System

• The complement system comprises about twenty soluble proteins enhancing other defense mechanisms against extracellular pathogens.
• Complement proteins stimulate phagocyte activity, attract phagocytes to infection sites, and disrupt invading pathogen membranes.
• Liver cells and macrophages produce complement proteins.

Innate Immunity: First Line of Defense - Physical Barriers

Skin

• The skin protects cells from pathogens.
• The skin consists of the epidermis, dermis, and hypodermis.
• The skin is well-supplied with blood, allowing quick access for WBCs, red blood cells (RBCs), and platelets to wounds.
• The skin is waterproof, which inhibits pathogen growth.
• The skin's acidic environment results from bacteria breaking down sweat.
• Epidermis exfoliation removes pathogens.

Mucous Membranes

• Mucous membranes consist of specialized epithelial tissue.
• Epithelial cell junctions tightly restrict pathogen access.
• Cilia lining mucous membranes remove particles from the respiratory system.
• These membranes constantly grow and replace surface cells.
• Mucous membranes secrete mucus, lysozyme, and immunoglobulins (antibodies).

Mucus

• Goblet cells produce mucus, lining the respiratory tract.
• Mucus traps pathogens, dust, and pollen.

Peristalsis

• Peristalsis involves wave-like muscle contractions that move food through the digestive tract.

Sphincters

• Sphincters physically seal off body compartments reducing the likelihood of pathogen invasion.

Innate Immunity: First Line of Defense - Chemical Barriers

Urine

• Antimicrobial peptides (AMPs) are secreted by cells lining the urinary tract to prevent bacteria from binding to epithelial cells and breaking them down.
• The flushing action during urination helps keep pathogens away from the bladder.
• Acidic urine kills and hinders microbial growth.

Sebum and Sweat

• Sebum and sweat waterproof and lubricate the skin, hindering bacterial growth.
• Lysozyme in perspiration breaks down bacterial cell walls.

Saliva

• Saliva flushes away microbes and contains antimicrobial molecules like IgA and AMPs.

Tears

• The tear film contains antimicrobial substances such as lysozyme, lactoferrin, lipocalin, AMPs, complement, IgA, and mucins.

Gastric Secretions

• The stomach lining secretes hydrochloric acid, creating a highly acidic environment (pH 1–2) that discourages microbial growth and survival.

Innate Immunity: Second Line of Defense - Inflammation

• A cell invaded by pathogens releases chemical signals.
• Histamines trigger vasodilation and increase vascular permeability.
• Prostaglandins are associated with pain and fever.
• Endogenous pyrogens are released.
• Chemicals cause capillaries to dilate, increasing blood flow to the infection site, resulting in heat, swelling, pain, and reduced mobility.
• Increased blood vessel permeability allows WBCs to move from blood to tissue.
• Increased temperature slows pathogen reproduction.

Phagocytosis

• Phagocytes are specialized WBCs responsible for phagocytosis.
  • Neutrophils detect cell damage, move to infection sites, and block toxin spread, fighting acute infections.
  • Macrophages, differentiated from monocytes, engulf and destroy invaded particles
  • Dendritic cells are antigen-presenting cells bridging innate and adaptive immunity.

Phagocytosis Process

1. Phagocytes change shape to engulf non-specific antigens.
2. The phagocyte combines with a lysosome containing digestive enzymes to break down the antigen.
3. Decomposed matter is released, no longer toxic.

• Advantages include toxins being retained within the macrophage, distinguishing self from non-self markers, and destroying attempts at reproduction.

Apoptosis

• Infected cells trigger neighboring cells to die, forming a granuloma which is a wall of dead cells.
• The granuloma's three-layer structure includes phagocytes surrounding infected cells, lymphocytes surrounding phagocytes, and fiber cells enveloping the structure.
• Pathogens contained in the granuloma die due to deprivation of food supply, preventing disease transmission.

Pyrexia

• Exogenous pyrogens activate phagocytic cells, which then release endogenous pyrogens targeting the hypothalamus to elevate body temperature.
  • This kills or limits pathogen growth.
  • This enhances WBC activity, strengthening response to pathogen

Cytokines

• Cytokines are chemical messengers released by cells in response to pathogens.
  • Interleukin is a cytokine.
  • Interferon signals neighboring uninfected cells to destroy RNA, reduce protein synthesis, initiate apoptosis, and activate immune cells.