Host invasion by microorganisms
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Introduction
One of the main causes of disease is infection by microorganisms, with four types so far identified:
- Viruses
- Bacteria
- Fungi
- Parasites- comprising protozoa and helminths
All exist as either:
- Primary pathogens (obligate): presence always corresponds with disease
- Secondary (opportunistic): prerequisites must be fulfilled before disease occurs
Microorganisms can enter the body by three routes:
- Contact: typically seen in dermatological infections
- Aerosol: typically respiratory infections
- Orofecal:typically enteric infections
Viruses
Bacteria
Parasites
Types
Parasites broadly include:
- Helminths (worms)
- Arthropods (insects)
- Protozoa (single-celled eukaryotes)
Immunity to Parasites
The immune system relies on granulocytes (mast cells and eosinophils) and the production of IgE antibody to combat parasitic infection. Once produced in response to danger signals, IgE attaches to circulating granulocytes. IgE then binds to the parasite, signals mast cell and eosinophil degranulation, and causes enzymatic digestion of the intruder.
References
- Murphy, K., Travers, P. and Walport, M. (2008) Janeway's Immunobiology. 7th ed. New York: Garland Science Publishing.