Barriers preventing pathogen infection
The first line of defense in an immune system is to prevent pathogens from gaining entry into the host organism.
Use this page to revise the following concepts within barriers preventing pathogen infection:
Cellular and non-cellular pathogens
A pathogen is an agent that causes disease, or harmful malfunctioning of a structure or function, in a host organism. A host is an organism, such as an animal or plant, that has other organisms living on or inside it. Most pathogens are microbes, though not all microbes cause disease in animals and/or plants. For example, some bacteria on human skin and in our digestive system are harmless or beneficial to humans. Those that do cause harm are called pathogens.
Pathogens can be cellular organisms (bacteria, fungi, parasites or protists) or non-cellular agents (prions or viruses). A pathogen is cellular if it is made up of cells, and can reproduce and grow independently. Viruses and prions are non-cellular as they are not made of cells. Viruses require a host cell to reproduce, and prions require host proteins to multiply. Below are some examples of cellular and non-cellular pathogens that cause disease in humans.
Barriers to pathogen entry
In order to prevent infection by a pathogen , an animal or a plant is best served by preventing that pathogen from gaining access to their cells and internal transport systems. Several strategies are used, often in combination. These fall into three main types: physical barriers, chemical barriers and microbiota.
Physical barriers
Physical barriers are structures that prevent the pathogen gaining access to internal areas of the organism or hold a pathogen ‘stuck’ in place for immune system cells to come and destroy it more easily.
Chemical barriers
Chemical barriers are chemical environments that can damage the pathogen’s outer layers. Pathogens are destroyed if they enter these environments. Typical examples are high or low pH outside of the normal range for that pathogen or specialised chemicals such as lysozymes that open holes in the outer layers of viruses and bacteria.
Microbiota
Microbiota refers to the bacteria or fungi that normally coexist on or in the host organism. These provide a benefit to the host by outcompeting the invading pathogen, thereby limiting the invading bacteria’s growth and damage to the plant or animal host’s body. As such, a healthy microbiota acts like a barrier to pathogens.
Click on the hotspots in the following image to see some more specific examples of barriers to pathogen entry to animals.
Click on the hotspots in the following image to see some examples of defensive barriers in plants.
