Innate Immunity: Non-Specific Defenses of The Host

Immune System: Protects the human host against pathogens.
Immunity: Ability to fight off pathogens and prevent disease.
- Differentiates between:
- Host Resistance: The state of having immunity.
- Host Susceptibility: The state of lacking immunity.
Host Defenses: The ability of the host to remove pathogens to prevent disease.
- If host defenses are successful, immunity is established.
- If unsuccessful, disease manifests.
Conceptual metaphor: "War" between host and pathogen, affecting disease status.

Definition: Immunity or resistance to any pathogens, classified as non-specific immunity.
Characteristics:
- Present at birth
- No specific recognition of microbes
- Lacks immunological memory response
- Rapid immune response
Host Defenses Comprise:
- 1st and 2nd Lines of Defense

Definition: Immunity or resistance that develops to handle specific microbes, known as specific immunity.
Characteristics:
- Develops later in life
- Specific recognition of pathogens
- Possesses immunological memory
- Slower to respond compared to innate immunity
Host Defenses Comprise:
- 3rd Line of Defense

Innate Immunity:
1. 1st Line of Defense (Physical Barriers):
  - Skin
  - Mucous membranes
  - Antimicrobial substances
2. 2nd Line of Defense:
  - Formed elements in blood
  - Phagocytosis
  - Inflammation
  - Fever
  - Antimicrobial substances
Adaptive Immunity:
1. 3rd Line of Defense:
  - Humoral immunity
  - Cell-mediated immunity

Intact Skin: Closely packed epithelial cells that are continuous layers.
- Composed of keratin protein, which contributes to dryness and shedding, preventing pathogen entry.
Mucous Membranes: Line gastrointestinal, respiratory, and genitourinary tracts, secreting mucus to trap pathogens.
Ciliary Escalator: Epithelial cells with cilia sweep mucus (and trapped pathogens) out of the body.
Lacrimal Apparatus: Protects the eyes through continuous washing action with tears.
Saliva, Urine, and Vaginal Secretions: Serve as flushing mechanisms to wash away pathogens from different bodily systems.

Chemical Factors of Skin: Includes slightly acidic pH and salinity that inhibit pathogen growth.
Lysozymes: Enzymes in body secretions like sweat and tears that break down bacterial cell walls (specifically peptidoglycan).
Gastric Juices: Produced in the stomach and contain enzymes and acid that destroy most pathogens.
Blood Transferrins: Proteins that bind iron, rendering it unavailable for bacterial growth.

Normal Microbiota:
- Commensal Microbes: Beneficial to the microbe but not to the host.
- Beneficial Microbes: Provide benefits to the host (e.g., vitamin K-producing bacteria).
- Opportunistic Microbes: Can act as pathogens under certain conditions.
Competitive Exclusion: Normal microbiota compete with pathogens for nutrients and space, producing substances harmful to invaders.

Formed Elements in Blood: Components include erythrocytes, leukocytes, and thrombocytes, created through hematopoiesis in the red bone marrow.
Phagocytosis: The process where phagocytes ingest microbes or other substances.
Inflammation: The body’s response to infection characterized by pain, redness, immobility, swelling, and heat.
Fever: A systemic response to infection characterized by an increase in body temperature.
Antimicrobial Substances: Components such as the complement system and interferons that enhance the immune response.

Erythrocytes: Red blood cells that transport oxygen via hemoglobin.
Leukocytes: White blood cells involved in immune response, categorized into granulocytes and agranulocytes:
- Granulocytes: With visible granules, including basophils (in allergic responses), eosinophils (against parasites), and neutrophils (phagocytic responders).
- Agranulocytes: Without visible granules, including monocytes which mature into macrophages (phagocytic) and lymphocytes (T and B cells, involved in adaptive immunity).
Differential White Blood Cell Count: Abundance of each type of white blood cell indicated in a sample of 100.

Chemotaxis: Release of chemical signals (cytokines) by pathogens attracting phagocytes.
Adherence: Attachment of phagocytes to pathogen surfaces.
Ingestion: Phagocytosis occurs forming a phagosome that merges with a lysosome to create a phagolysosome.
Digestion: Pathogen is digested within the phagolysosome; discharge of waste materials occurs.

Pathogens can evade phagocytosis through mechanisms like capsule production, leukocidins that kill phagocytes, and mycolic acid that inhibits lysosomal enzymes.

Tissue damage leads to release of cytokines that promote chemotaxis.
Phagocytes migrate to the injury site, initiating phagocytosis.
Signs and Symptoms of inflammation include pain, redness (erythema), immobility, swelling (edema), and heat.

Release of toxins by bacteria induces cytokines that raise the body's temperature set point.
Fever enhances metabolic rate, boosts immune response, and increases production of antimicrobial substances.
- The decline in fever when pathogens are eliminated is termed "crisis".

Complement System: Proteins that enhance immune response through a cascade activation process leading to pathogen destruction via opsonization, inflammation, and cytolysis.
Interferons (IFNs): Small proteins produced by virus-infected cells to protect neighboring uninfected cells by enhancing their antiviral defenses.

Opsonization: Coating pathogens to promote phagocyte attraction.
Inflammation: Enhanced blood vessel permeability and phagocyte recruitment to injury sites.
Cytolysis: Formation of membrane attack complexes that leads to pathogen lysis.

Pathogens may evade the complement system through capsule production, inhibition of membrane attack complex (MAC) formation, and destruction of complement proteins by bacterial enzymes.

The innate immune system plays a critical role in the first lines of defense against pathogens, while the adaptive immune system provides a specialized and memory-driven response to infections. Understanding these mechanisms is essential for developing effective treatments and vaccines.