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ethiology genetic and environmental factors pathogenesis reactivity and resistance

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disease etiology pathogenesis

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etiology pathogenesis genetic environmental factors host reactivity resistance definitions pathology

These terms describe how a disease develops and why it affects some people more than others:
  1. Etiology
    • The cause or origin of a disease.
    • It may be a single cause, such as Mycobacterium tuberculosis causing tuberculosis, or multifactorial, involving genes, environment, and lifestyle.
  2. Genetic factors
    • Inherited or acquired genetic variations that alter disease susceptibility, severity, or response to exposures.
    • Examples: family history, gene mutations, polygenic predisposition, immune-response genes.
  3. Environmental factors
    • External exposures that cause, trigger, or modify disease.
    • Examples: infections, diet, smoking, alcohol, pollutants, occupation, medications, stress, physical activity, and socioeconomic conditions.
  4. Pathogenesis
    • The sequence of biological events through which the cause produces disease.
    • In simple form:
      etiologic factor → molecular/cellular injury or dysfunction → tissue/organ changes → symptoms and complications
  5. Reactivity
    • The way an individual’s body, especially the immune, inflammatory, hormonal, or vascular system, responds to a stimulus.
    • Excessive reactivity can cause tissue damage, such as allergy or autoimmune inflammation.
    • Reduced reactivity can impair defense against infection.
  6. Resistance
    • The ability to withstand, limit, or recover from a harmful agent or disease.
    • It may depend on intact barriers, innate immunity, adaptive immunity, nutrition, vaccination, genetics, and prior exposure.
    • Low resistance increases susceptibility to disease.
Overall relationship
Disease occurrence and severity result from interaction among the cause (etiology), genetic susceptibility, environmental exposures, and the host’s reactivity and resistance.
Pathogenesis explains the mechanism linking these factors to the clinical disease.

Explain these concepts using one disease example

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tuberculosis host susceptibility

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CDC tuberculosis risk factors immune system pathogenesis latent tuberculosis

Example: Tuberculosis (TB)

Tuberculosis shows clearly how etiology, genetic and environmental factors, pathogenesis, reactivity, and resistance work together.
ConceptTB example
EtiologyTB is caused by the bacterium Mycobacterium tuberculosis. A person acquires it by inhaling tiny airborne droplets released when someone with infectious pulmonary TB coughs, speaks, or sings.
Genetic factorsSome people have inherited variations in immune-response pathways that make them more susceptible or more resistant to TB. Genetic differences can influence how effectively macrophages and T lymphocytes control the bacteria.
Environmental factorsCrowded housing, poor ventilation, close contact with a person who has TB, poverty, malnutrition, smoking, silica exposure, and limited access to health care increase exposure or disease risk. HIV, diabetes, and immunosuppressive medicines also increase the likelihood of active TB after infection.
PathogenesisInhaled bacilli reach the lung alveoli and are taken up by macrophages. The bacilli can survive inside these cells. After a few weeks, a cell-mediated immune response develops. T cells activate macrophages and form a granuloma that walls off the organisms. This produces latent TB infection. If immune control fails, the organisms multiply, damage tissue, form caseating lesions and cavities, and cause active TB.
ReactivityReactivity means the intensity and nature of the host immune response. In TB, a strong delayed, T-cell-mediated response helps form granulomas and restrict infection. However, excessive or poorly controlled immune inflammation also contributes to tissue destruction, caseous necrosis, cavitation, fever, and weight loss.
ResistanceHost resistance is the capacity to prevent infection from progressing to disease. Effective macrophage activation and CD4 T-cell responses keep bacilli contained in latent TB. Resistance is reduced in HIV infection, diabetes, malnutrition, young children, older age, and immunosuppressive therapy. The CDC notes that macrophages and other immune cells can encapsulate bacilli in granulomas, producing latent infection; untreated latent infection progresses to active TB in roughly 5% to 10% of people over their lifetime. CDC TB overview

Flow of events

Exposure to infected droplets
↓
Inhalation of M. tuberculosis
↓
Bacilli enter alveoli and survive within macrophages
↓
Host genetic makeup and environmental conditions influence susceptibility
↓
T-cell and macrophage response forms granulomas
↓
  • Adequate resistance: latent TB infection, no symptoms
  • Reduced resistance or unfavorable reactivity: active TB disease with lung destruction and possible transmission

Key distinction

  • Etiology: What causes TB?
    M. tuberculosis.
  • Pathogenesis: How does it produce disease?
    Survival in macrophages, granuloma formation, and possible failure of immune containment.
  • Reactivity: How does the body respond?
    Mainly a T-cell-mediated inflammatory response.
  • Resistance: How effectively can the body control it?
    Effective cellular immunity contains the bacilli; impaired immunity permits active disease.
Recent genetic evidence also supports an association between host immune-genetic variation and TB susceptibility, though individual genes do not fully determine who develops disease. Recent systematic review
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