Part of the TeachMe Series

Barriers to Infection

star star star star star_half
based on 9 ratings

Original Author(s): Jess Speller
Last updated: 5th May 2020
Revisions: 10

Original Author(s): Jess Speller
Last updated: 5th May 2020
Revisions: 10

format_list_bulletedContents add remove

The innate immune system has a number of first line barriers to infection that are in place to prevent the entry and growth of pathogens.

This article shall consider the physical, physiological, chemical and biological barriers that perform this role, as well as clinical conditions that may result from their failure.

Physical Barriers

These barriers provide a physical block against pathogens from entering the immune system. The largest barrier comprises of the skin, which has a large surface area and covers the majority of the external surfaces of the body.

The mucous membranes of the mouth, respiratory tract, GI tract and urinary tract also line the the areas of the body that are in contact with the external environment, preventing pathogens from crossing.

The final physical barrier of the innate system is the bronchial cilia. These cells form a mucociliary escalator which allows gradual removal of pathogens from the respiratory system.

Fig 1 – Electron micrograph of the cilia within the lungs.

Physiological Barriers

The physiological barriers of the body are processes that occur in response to pathogens in order to remove them from the system and vary depending on the location and situation. The main physiological barriers are:

  • Diarrhoea
  • Vomiting
  • Coughing
  • Sneezing

Chemical Barriers

There are two main chemical barriers to infection, the relatively low pH of parts of the body and antimicrobial molecules. The following areas of the body have an acidic pH:

  • Skin – pH 5.5
  • Gastric acid – pH 1-3
  • Vagina – pH 4.4

These pHs create a hostile environment that pathogens struggle to thrive in and certainly in the case of gastric acid, can denature the proteins of pathogens.

There are also a variety of innate antimicrobial molecules throughout the body which can aid in the destruction of pathogens:

  • IgA – present in tears, saliva and mucous membranes
  • Lysozyme – present in sebum, perspiration and urine
  • Mucus – present in mucous membranes
  • Beta-defensins – present in epithelia
  • Pepsin – present in the gastrointestinal tract

Biological Barriers

The main biological barrier to pathogens is the normal flora found within the body. These are non-pathogenic microbes found within strategic locations of the body – those in contact with the external environment. They are typically found in the:

  • Nasopharynx
  • Mouth and throat
  • Skin
  • GI tract
  • Vagina

These compete with pathogens for attachment sites and resources, and may even produce antimicrobial chemicals. In addition to this, a number of them also produce essential vitamins, such as Vitamin K and B12.

Fig 2 – Mechanisms by which normal flora can prevent infection.

Examples of Normal Flora

Location Examples of Normal Flora
Conjunctiva Coagulase-negative Staphylococci

Haemophilus spp.

Staphylococcus aureus

Nose Coagulase-negative staphylococci

Viridans streptococci

Staphylococcus aureus

Neisseria spp.

Haemophilus spp.

Steptococcus pneumoniae

Outer ear Coagulase-negative staphylococci

Pseudomonas spp.

Mouth and oropharynx Viridans streptococci

Coagulase-negative staphylococci

Neisseria spp.

Streptococcus pneumoniae

Beta-haemolytic streptococci (not group A)

Candida spp.

Haemophilus spp.

Skin Coagulase-negative staphylococci

Staphylococcus aureus

Streptococci (various species)

Bacillum spp.

Candida spp.

Stomach Streptococcus



Small intestine Lactobacillus spp.

Bacteriodes spp.

Clostridium spp.



Large intestine Bacteriodes spp.

Clostridium spp.

Escherichia coli

Klebsiella spp.

Lactobacillus spp.


Acinetobacter spp.

Proteus spp.

Urethra Cogulase-negative staphylococci


Streptococci (various species)

Mycobacterium spp.

Bacteriodes spp.

Vagina Lactobacillus spp.


Clostridium spp.

Bacteriodes spp.

Candida spp.


Clinical Relevance


Meningitis is an acute infection of the protective membranes covering the brain and spinal cord – the meninges. It can the result of either a bacterial or viral infection, although the majority are viral cases. The main pathogen responsible for bacterial infections is Neisseria meningitidis. The most common symptoms are:

  • Fever
  • Headache
  • Neck stiffness
  • Confusion
  • Altered consciousness
  • Vomiting
  • Photophobia

Cases caused by meningococcal bacteria may also present with a characteristic, non-blanching rash. Due to the proximity of the infection to the central nervous system it is classified as a medical emergency, and it can be diagnosed with a lumbar puncture.

Treatment varies depending on the cause, for example, in suspected bacterial meningitis empirical antibiotics (ceflosporins) should be immediately commenced. In addition to this, supportive therapy such as fluids and potentially corticosteroids are given. In severe cases, admission to an intensive care unit may be required, especially in cases with altered consciousness and respiratory failure.

Fig 3 – Typical symptoms of meningitis.