Barriers and non-specific defencesEdexcel International A Level Biology: Revision notes

A pathogen is an organism that causes disease. To cause an infection it must first get into the body. The main routes are:

• Gas exchange system – inhaled droplets, spores and dust (e.g. influenza virus, Mycobacterium tuberculosis)
• Digestive system – ingested food and water (e.g. Salmonella, Vibrio cholerae)
• Breaks in the skin – cuts, burns, insect bites and injection needles (e.g. tetanus bacteria, malaria parasite via a mosquito)
• Mucous membranes of the eyes, urinary tract and reproductive tract (e.g. HIV, chlamydia)

The skin is the first barrier. The outer layer, the epidermis, is made of keratinised, dead cells that form a tough, waterproof layer which most pathogens cannot penetrate.

Skin also has chemical defences: sebum and sweat make the surface slightly acidic and contain fatty acids that inhibit the growth of many microorganisms. The harmless skin flora (microorganisms that normally live on the skin) compete with pathogens for nutrients and space, so pathogens struggle to establish.

Pathogens that are swallowed meet hydrochloric acid in the stomach (about pH 2), which denatures the enzymes and other proteins of many pathogens and kills them.

The gut flora in the intestines compete with pathogens for nutrients and attachment sites and produce substances that inhibit them. This is why broad-spectrum antibiotics, which kill gut flora, can allow opportunistic pathogens to take hold.

Mucus is secreted by goblet cells lining the gas exchange and digestive systems. It traps pathogens, and in the airways cilia sweep the mucus up to be swallowed, where stomach acid destroys the pathogens.

Lysozyme is an enzyme found in tears, saliva, mucus and in the lysosomes of phagocytes. It hydrolyses peptidoglycan in bacterial cell walls. The weakened wall can no longer withstand the pressure inside the cell, so water enters by osmosis and the bacterium bursts (lysis).

When tissue is damaged or infected, mast cells release histamine. Histamine causes:

• Vasodilation of arterioles, so more blood flows to the area (redness and heat)
• Increased permeability of capillaries, so plasma leaks out as tissue fluid (swelling)

The extra blood brings phagocytes such as neutrophils, which move out of the capillaries towards the site of infection. Inflammation therefore delivers the cells needed to destroy pathogens and starts the repair of the tissue.

Interferons are signalling proteins released by virus-infected cells. They bind to receptors on neighbouring uninfected cells, which then make antiviral proteins. These proteins inhibit viral replication, for example by breaking down viral mRNA and stopping viral protein synthesis. The neighbouring cells are therefore protected before the virus reaches them.

Neutrophils and macrophages are phagocytes. They are attracted to the site of infection by chemicals released by the pathogen and damaged cells (chemotaxis).

1. Receptors on the phagocyte bind to the pathogen's surface.
2. The phagocyte engulfs the pathogen by endocytosis, forming a phagosome.
3. Lysosomes fuse with the phagosome and release hydrolytic enzymes (including lysozyme).
4. The enzymes digest the pathogen, and the products are absorbed or removed.

Phagocytosis is non-specific because the same process attacks many different pathogens.