Immune System and Pathogens

The Immune System and Infectious Diseases

• The primary function of the immune system is to protect the host from death and disability caused by infectious diseases.
• A "host" refers to a human or animal infected by a pathogenic organism.

Types of Pathogens

• Pathogens can be viruses, bacteria, fungi, protozoa, or multicellular parasites.
• Examples of pathogens include:
  • Measles virus
  • Cholera bacterium (Vibrio cholerae)
  • Yeast Candida albicans
  • Malaria protozoa (Plasmodium falciparum and others)
  • Hookworm nematodes (Necator americanus and Ancylostoma duodenale)
  • Liver fluke (Schistosoma mansoni)

Pathogen Strategies

• Many pathogens have evolved methods to evade the innate immune response and require adaptive immunity to be cleared.
• Some pathogens, such as malaria protozoa and HIV, can evade adaptive immunity as well.

Opportunistic Pathogens and Commensal Organisms

• Opportunistic pathogens can cause disease when the immune system is compromised by malnutrition, other infections, or advancing age.
• Commensal organisms, such as those colonizing the skin, intestine, and urogenital tracts, are generally benign or beneficial to the host.
• However, these organisms can also be harmful under certain circumstances and are subject to control by the immune system.