4.6.12 The major routes pathogens may take in entering the body and the role of barriers in protecting the body from infection

Barrier Mechanisms include;

Skin, Stomach Acid, Normal Flora, Epithelial cells.

Skin adaptions for defence

The skin is made from 2 layers

  • Outer epidermis layer
  • Inner dermis layer

The epidermis provides a physical barrier to invading pathogens. There are 2 layers in the epidermis;

  • Outer cornified layer, composed of compacted dead dry cells filled with indigestible keratin protein (which also forms nails and hair)


  • Inner Malpighian layer, site of rapid mitosis and


The skin also has chemical defence mechanisms;


  • sweat & sebaceous glands secrete sebum, which is an oil with pH 3 – 5. This makes the skin acidic


  • sebaceous glands also secrete the enzyme lysozyme, which is a natural antibiotic. Lysozyme destroys bacterial cell

Stomach Acid

Is made from HCl at pH 1 – 2. it is a very effective barrier.

Normal Flora

The skin, respiratory tract and gut are covered with commensual bacteria, which are part of the normal flora of the body. Commensual bacteria are adapted to live the environment of the skin and the gut and the and compete with invading pathogens for the limited supply of nutrients.

Epithelial call adaptions for defence

  1. Epithelial cells are closely packed & connected by tight junctions forming a continuous impermeable layer


  1. Epithelial cells have cilia, which form a direct physical barrier preventing pathogen attachment


  1. Cilia ‘beat’ in waves, which helps clear bacteria out of the lungs and into the throat, where they are swallowed. Ingested bacteria are quickly killed by the low stomach pH and digestive proteases. Cilia also beat in the GI


  1. Epithelial cells secrete mucus, which is trapped by cilia. Mucus also directly prevents pathogen attachment


  1. Mucus contains lysozyme