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

Pathogens must get past the body's surfaces. The main routes are:

• Skin: cuts, wounds, bites and injections
• Gas exchange system: droplets and spores inhaled into the airways and alveoli
• Digestive system: contaminated food and water
• Mucous membranes of the eyes, nose, mouth, and urinary and reproductive tracts

Healthy intact skin is a very effective barrier, so most infections enter through a break or a moist, thin surface.

Skin: a tough layer of keratinised, dead cells forms a physical barrier. Sebum keeps the surface slightly acidic, which inhibits bacterial growth.

Stomach acid: at about pH 2 it denatures the enzymes of many ingested pathogens and kills them.

Gut and skin flora: harmless bacteria already living on the skin and in the gut compete with pathogens for nutrients and space, and some produce chemicals that inhibit pathogens. Antibiotics that kill the flora can allow a pathogen such as Clostridium difficile to overgrow.

Mucus and cilia in the airways trap pathogens and sweep them out. Lysozyme in tears, saliva and mucus breaks down bacterial walls.

Inflammation is a non-specific response to damage or infection.

1. Damaged cells and mast cells release histamine (and other chemicals).
2. Vasodilation of arterioles increases blood flow, causing heat and redness.
3. Capillaries become more permeable, so plasma leaks into the tissue, causing swelling and carrying defence proteins.
4. Phagocytes are attracted and move into the tissue to engulf pathogens.
5. Pus is a mixture of dead phagocytes, dead pathogens and tissue fluid.

Pain results from pressure and chemicals acting on nerve endings.

Lysozyme is an enzyme in tears, saliva, mucus and phagocytes. It hydrolyses peptidoglycan in bacterial cell walls, so the weakened walls cannot withstand osmotic pressure and the bacterium lyses.

Interferon is a cytokine (signalling protein) released by virus-infected cells. It binds to receptors on neighbouring uninfected cells and stimulates them to make enzymes that degrade viral mRNA and block viral protein synthesis, so the virus cannot replicate. It also helps to activate other immune cells. Interferon protects cells that have not yet been infected; it does not destroy viruses directly.

Phagocytes (neutrophils and macrophages) engulf and destroy pathogens.

1. Chemicals from pathogens or damaged cells attract the phagocyte.
2. Receptors on the phagocyte bind to the pathogen.
3. The pathogen is engulfed by endocytosis into a vesicle, the phagosome.
4. Lysosomes fuse with the phagosome and release hydrolytic enzymes that digest the pathogen.

Phagocytosis is non-specific, because phagocytes attack many types of pathogen. A macrophage can also display pathogen antigens on its surface, which links the non-specific response to the specific immune response.