Barriers to Infection – Immunity Ep 5

In order for a pathogen to enter the body and cause a disease, it must pass through the body’s non-specific defences. The barriers to infection are non-specific because they respond to all pathogens in the same way.

Infection routes

There are four main routes with a pathogen can take to enter the body:

  • The digestive system through swallowing a pathogen.
  • The respiratory system through inhalation.
  • Through broken skin (e.g. cuts).
  • Through mucosal surfaces (e.g. inside the nose, mouth, eyes, anus and genitals).

Barriers to infection

These are the physical and chemical barriers which are presented by the body. We will look at a couple in more detail further down the article.

  • Stomach acid – very acidic conditions in the stomach will kill most of the pathogens which are swallowed.
  • Mucous membranes – these cover mucosal surfaces. Many secrete mucus which contains lysozyme (an enzyme which kills bacteria by damaging their cell walls, also found in lysosomes). Mucus can also trap pathogens.
  • Inflammation – see more below.
  • Expulsive reflexes – these are automatic reflexes in response to irritation by foreign objects e.g. coughing and sneezing. Pathogens are expelled from the body.
  • Blood clotting – see more below.
  • Skin – a physical barrier covering the majority of the body. It also produces antimicrobial substances and maintains an acidic pH in order to kill pathogens. In addition, there are harmless microorganisms on the skin which compete with harmful microorganisms for resources such as nutrients. Hopefully the harmless ones win. Harmless microorganisms are also found in the intestines (sometimes called gut flora), and will hopefully outcompete any pathogens which have survived the stomach acid.
  • Wound repair – if the skin is broken, it is able to repair itself. Collagen fibres help to repair the wound (remember that collagen is a strong fibrous protein).


You probably all know what inflammation looks and feels like. The area becomes hot, red, swollen, and painful. The trigger could be tissue damage, or detection of a foreign antigen. Molecules called histamines are released by a specific type of cell called mast cells. Histamines trigger vasodilation (widening) of nearby blood vessels to increase blood flow to the area. That is why the area becomes red and hot. The blood vessels also become more permeable, meaning that fluid and white blood cells are able to leak into the surrounding tissue. The area becomes swollen and painful because of this. The white blood cells are able to respond to any pathogens in the area, and destroy them by phagocytosis.

Blood clotting

Blood clots not only provide a physical barrier to pathogens but also prevent blood loss from wounds. They are formed through a series of reactions:

  1. Thromboplastin (a protein) is released from damaged blood vessels.
  2. Calcium ions and thromboplastin trigger conversion the prothrombin protein to thrombin. Thrombin is a enzyme.
  3. Thrombin catalyses a reaction converting fibrinogen (a soluble protein) to fibrin (an insoluble fibrous protein).
  4. Fibrin forms a mesh with platelets and red blood cells to form a blood clot.
barriers to infection, blood clotting reaction cascade
The reactions leading to blood clotting

Barriers to infection in plants

So far we’ve been talking about humans and animals, but let’s not forget about plants! Physical barriers to infection in plants include the thick waxy cuticle, and a polysaccharide called callose which can build up between the cell wall and plasma membrane to block pathogens. Chemical barriers include antimicrobial chemicals called phytoalexins which can inhibit growth of pathogens such as fungi.


  • Barriers to infection can be physical or chemical.
  • Inflammation is triggered by tissue damage and results in white blood cells entering the tissue in the area.
  • Blood clotting is triggered by damaged blood vessels, which results in a cascade of reactions to produce fibrin.

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