Learning Goals: First Line of Defence – Preventing Entry
- Describe physical, chemical, and microbiota barriers in animals and plants.
- Explain how these barriers reduce the likelihood of infection.
The Immune System
Organisms have evolved a range of defences as part of the immune system to stop pathogens before they enter the body. These first-line defences are always present and act as a crucial preventative shield against infection. They are non-specific, meaning they respond the same way regardless of the pathogen. If pathogens breach this barrier, the second line of defence responds with rapid, non-specific actions. The third line of defence provides a targeted, specific response and creates long-term memory.
Key Concepts
- Physical barriers block pathogens from entering the body.
- Animals: intact skin (tough outer layer), mucous membranes trap pathogens, cilia sweep out foreign material.
- Plants: thick cell walls (cellulose), waxy cuticle on leaves, bark covering stems and trunks.
- Chemical barriers destroy or inhibit pathogens on surfaces.
- Animals: stomach acid (low pH kills microbes), lysozymes in tears and saliva (digest bacterial walls), antimicrobial peptides.
- Plants: antimicrobial compounds (e.g. saponins, defensins), enzymes toxic to pathogens.
- Microbiota barriers: naturally occurring non-pathogenic microbes that outcompete pathogens for space and resources, especially in the gut and on skin.