Pathology unit 4.1

• Explain the major causes of infection
• Describe the components of the inflammatory response

  1. Acute inflammation

  2. Chronic inflammation
• Infection


  Present when pathogenic micro-organisms have established themselves in the tissues of some part(s) of the body and are able to survive and reproduce themselves therein
• Micro-organisms responsible for causing infection

  • Bacteria
  • Viruses
  • Fungi
  • Prions
  • Parasites
• Bacteria


  • Small unicellular replicating organism that lacks an organized nucleus (prokaryotes), contains both DNA and RNA
  • Cell wall determines the shape (cocci, spirilla, bacilli)
  • Can be motile (flagella) and adhere (fimbriae/pili) to mucous membranes or other bacteria
• Viruses


  • Smallest intracellular pathogen, acellular (no cytoplasm or organelles), replicate using host cell's metabolic machinery, can transform normal cells into malignant cells
• Fungi


  • Free-living eukaryotic saprophytes, some present in normal human microflora, only a few capable of causing disease
  • Separated into yeasts (intra/extracellular) and molds (extracellular)
• Prions


  • Protein that lacks a demonstrable genome (misfolded proteins) which causes neurodegenerative diseases, fatal
• Parasites


  • Organisms that infect and cause disease in other animals
  • Protozoa (passed directly or indirectly)
  • Helminths (wormlike parasites, transmission through ingestion or penetration)
  • Arthropods (vectors of infectious diseases)
• Relationships between micro-organisms and their host


  • Commensal (micro-organism obtains nutritional support, host not adversely affected)
  • Mutualistic (micro-organism and host benefit)
  • Parasitic (infecting organism benefits at the expense of the host)
• Routes of entry of microbes

  • Direct - Inhalation, Ingestion, Direct contact (STDs, placental), Penetration (inoculation, urinary/genital tract)
• Body's mechanisms of defence


  • First line (physical and chemical barriers - skin, sweat, tears, mucous membranes, stomach acids, urine, neutrophils)
  • Second line (immune response, phagocytosis by macrophages and neutrophils)
  • Third line (adaptive/acquired immune response, B and T lymphocytes)
• Factors influencing the effect of a pathogen on the host

  • Nature of the organism (toxicity, quantity, portal of entry, virulence)
  • Nature of the host (general state of health, presence of pre-existing infections, health of the immune system)
• Spread of infection


  • Local - to adjacent structures
  • Lymphatic spread (lymphadenitis, lymphangitis)
  • Systemic - blood-spread to distant tissues (bacteraemia, septicaemia)