International A Level Biology / Immunity

Non-specific responses: inflammation, lysozyme and interferon revision notes

Edexcel International A Level Biology (YBI11), chapter 18: Immunity, specification point 6.8. Free lesson.

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Inflammation, lysozyme and interferon are fast, non-specific defences against pathogens. They help phagocytes reach infection, destroy bacteria and slow the spread of viruses.

How histamine starts inflammation

Inflammation begins when tissue is damaged or infected, such as when a splinter breaks the skin and lets bacteria in. Mast cells in the tissue release histamine, which changes nearby blood vessels. Histamine does not attack pathogens itself. Instead, it helps phagocytes reach them so they can engulf them. Histamine makes nearby arterioles widen, or dilate. More blood then flows into the capillaries. It also makes capillary walls more permeable, so fluid leaks into the tissue and phagocytes can squeeze through the wall.

Why inflammation causes redness, heat and swelling

Each sign of inflammation is linked to changes in blood flow or capillary permeability. These changes also help phagocytes get to pathogens and destroy them. Redness: dilated arterioles bring more blood near the surface. More blood also means more phagocytes arrive. Heat: extra warm blood flows into the area. The warmth speeds up enzyme-controlled reactions, including digestion inside phagocytes, so bacteria are destroyed faster. It can also push a pathogen's enzymes past their optimum, slowing its reproduction. Swelling: fluid leaks from permeable capillaries and builds up in the…

How lysozyme bursts bacteria

Lysozyme is an enzyme found in tears, saliva and the mucus lining the airways. It hydrolyses the peptidoglycan in bacterial cell walls. Peptidoglycan consists of sugar chains linked by peptides; lysozyme breaks bonds between the sugars. The weakened wall cannot resist water pressure, so water enters by osmosis. The bacterium swells, bursts and dies. This bursting is called lysis. Lysozyme is an enzyme that works outside cells in secretions such as tears. A lysosome is different: it is an organelle containing enzymes. The correct term for the enzyme that breaks down bacterial walls…

How interferon slows a virus

Viruses have no cell wall, so lysozyme cannot destroy them. Instead, an infected body cell releases interferon, a protein signal. Interferon binds to receptors on nearby, uninfected cells, making them produce antiviral proteins. These proteins inhibit viral replication, for example by stopping viral proteins being made. The virus then spreads to new cells more slowly, buying time for the specific response. Interferon acts on cells, not directly on the virus. It is not specific to one virus, so it can act against many different viruses.

How these defences fit together

These are non-specific responses, meaning they work in the same way against different pathogens. They act quickly, before the specific response has had time to develop. Histamine changes blood vessels so phagocytes can reach pathogens. Lysozyme damages bacterial cell walls, while interferon helps neighbouring cells inhibit viral replication.