Human and plant defences against pathogens

In a nutshell

Animals have many physical and chemical barriers that prevent infection from occurring before the immune system detects a pathogen. These barriers are primary, non-specific defences. Plants also have defences against pathogens which can be either chemical or physical.

Animal barriers

These barriers are the initial defences used by animals when they encounter a pathogen or the pathogen has managed to enter the body.

Defence	Description
Skin	The skin is a physical barrier that blocks pathogens from entering the body. The skin can also be a chemical barrier by producing antimicrobial chemicals that lower pH.
Mucous membranes	Mucous membranes protect body openings that are exposed to the environment, such as the mouth and the nostrils. These membranes secrete mucus that traps pathogens and has antimicrobial enzymes.
Blood clotting	Blood clots are made up of a protein called fibrin: these are fibres that have a mesh formation. They are formed by a series of chemical reactions when platelets in the blood are exposed to damaged blood vessels.
Inflammation	Inflammation causes swelling, pain, redness and heat. When tissues get damaged, they release molecules that increase the permeability of blood vessels, causing them to leak fluid. This is what causes swelling and allows pathogens that may have entered the damaged tissue to be isolated. These molecules also cause vasodilation, which increases blood flow and explains why the area is hot.
Wound repair	The skin can repair itself if it gets injured: this protects it from pathogens. The surface is repaired when the outer layer of skin cells divide and migrate to the edge of a wound. The tissue underneath the wound then contracts which brings the edges of the wound closer. Collagen fibres then repair the wound. Curiosity: When too many collagen fibres repair a wound it forms a scar.
Expulsive reflexes	Expulsive forces include coughing and sneezing. A cough occurs when the respiratory tract gets irritated, equally, a sneeze occurs when the mucous membranes in the nostrils get irritated by dust or dirt. Both coughing and sneezing are your body's way of automatically expelling foreign objects such as pathogens.

Plant defences

Physical defences

Plants also have physical defences that protect them from infection by pathogens.

Feature	Description
Waxy cuticle	The waxy cuticle provides a physical barrier against pathogen entry. It also stops water from collecting on the leaf, which prevents infections that are transmitted in water from infecting the plant.
Cell walls	Plant cell walls form a physical barrier against pathogens that make it past the waxy cuticle.
Callose	Callose is a polysaccharide that gets deposited between plant cell walls and plasma membranes during periods of stress such as infection by pathogens. The deposition of callose makes it harder for pathogens to enter cells. When callose gets deposited at the plasmodesmata, viruses are less likely to infect the plant.

Chemical defences

Plants also produce antimicrobial compounds that can kill pathogens or inhibit their growth, such as antibiotics. Plants can also secrete chemicals that are toxic to plants: this prevents insects from feeding on them and transmitting infections carried by insect vectors.

Examples

Antimicrobial compound	Description
*Saponins*	Saponins destroy the cell membranes of fungi.
*Phytoalexins*	Phytoalexins inhibit the growth of fungi and other pathogens.