Human defence systems — AQA GCSE Combined Science

The skin is the body's first line of defence against pathogens. Its outer layer of dead, keratinised cells forms a tough physical barrier that most bacteria, viruses and fungi cannot penetrate. Sebaceous glands secrete sebum, an oily substance that keeps skin supple and lowers surface pH, inhibiting microbial growth. When skin is cut, blood clots seal the wound, preventing entry of microorganisms. Sweat also contains lysozyme, an enzyme that breaks down bacterial cell walls. Intact skin therefore acts as a non-specific defence, meaning it works against all pathogens in the same way, unlike antibodies which target specific antigens. Students should link structure to function: the dry, slightly acidic, continuously shed surface makes colonisation difficult. Damage such as a burn or graze immediately increases infection risk, showing the barrier's importance.

The nose is part of the body's first line of defence against pathogens. As air is breathed in, it passes over hairs and a sticky mucous membrane. Hairs trap larger particles such as dust and some microorganisms, while mucus traps smaller particles and pathogens. Cilia, tiny hair-like structures on epithelial cells, beat rhythmically to sweep mucus containing trapped microbes towards the throat, where it is swallowed. Stomach acid then destroys many of the swallowed pathogens. This prevents most airborne microorganisms from reaching the lungs, where they could cause infection. The nose therefore provides a non-specific physical and chemical defence. Students should describe the sequence: inhalation, trapping by hairs and mucus, cilia movement, swallowing, and destruction by stomach acid.

The trachea and bronchi are airways that conduct air between the external environment and the lungs. Their walls contain C-shaped cartilage rings that hold the airway open, preventing collapse when pressure changes during breathing. The inner surface is lined with ciliated epithelial cells and mucus-secreting goblet cells. Mucus traps particles such as dust, pollen and microorganisms. Cilia beat in a coordinated way to move mucus upwards towards the throat, where it is swallowed. This is a non-specific defence because it acts against a wide range of potential pathogens before they reach the alveoli. In the context of human defence systems, these structures form part of the first line of defence, reducing the chance of infection. For example, inhaled smoke can paralyse cilia, allowing mucus and trapped pathogens to accumulate, which increases the risk of respiratory infections.

The stomach is a muscular organ that receives food from the oesophagus. It churns food and mixes it with gastric juice, which contains hydrochloric acid and enzymes such as protease. The hydrochloric acid creates a very acidic pH, typically around pH 2, which kills many microorganisms swallowed with food or drink. This is a non-specific defence because the acid destroys a wide range of pathogens, not just one type. The low pH also provides the optimum conditions for protease enzymes to digest proteins. The stomach wall is protected from self-digestion by a layer of mucus. In the context of human defence systems, the stomach is part of the first line of defence, reducing the number of viable pathogens that reach the intestines. For example, drinking contaminated water may introduce bacteria, but the acidic environment of the stomach often destroys them before they can cause disease.

The immune system is the body's specific defence against pathogens that have breached the non-specific barriers. White blood cells patrol the blood and lymph. Phagocytes engulf and digest pathogens by phagocytosis, while lymphocytes produce antibodies that bind to antigens on a pathogen's surface, clumping pathogens so phagocytes destroy them. Lymphocytes also produce antitoxins that neutralise toxins released by bacteria. Memory cells remain after infection, giving long-lasting immunity and a faster, larger response on re-exposure. For example, after measles infection, memory lymphocytes recognise measles antigens and destroy the virus before symptoms develop.

Once a pathogen gets past the skin, nose, trachea and stomach barriers, the immune system responds. White blood cells are central. Phagocytes engulf pathogens and digest them. Lymphocytes detect antigens on the pathogen surface and release antibodies that bind to those antigens, clumping the pathogens so phagocytes can destroy them. Lymphocytes also release antitoxins that neutralise bacterial toxins. This response is specific: antibodies for one antigen will not bind another. After the infection, memory cells remain, so a second infection by the same pathogen is destroyed quickly, often before symptoms appear.

White blood cells are part of the body's second line of defence, acting after physical and chemical barriers such as skin and mucus. They circulate in blood and tissue fluid and respond to pathogens that have entered the body. Defence occurs by two main mechanisms: phagocytosis, in which a white blood cell engulfs and digests a pathogen, and antibody production, in which lymphocytes release antibodies that bind to antigens on a pathogen, marking it for destruction or neutralising it. Some white blood cells also produce antitoxins that neutralise toxins released by bacteria. This statement introduces the general role of white blood cells; the named processes are covered in the following statements. A concrete example is a cut becoming infected: bacteria entering through the break are met by white blood cells that engulf them and by antibodies that clump them together.

Phagocytosis is the process by which a white blood cell engulfs and digests a pathogen. The white blood cell detects chemicals released by the pathogen and moves towards it. The cell membrane surrounds the pathogen, taking it into the cytoplasm inside a vacuole. Digestive enzymes inside the white blood cell break down the pathogen, destroying it. This is a non-specific defence because the same process can destroy many different types of pathogen. A concrete example is a phagocyte engulfing a bacterium at the site of an infected cut. Phagocytosis is one of the ways white blood cells help to defend against pathogens, alongside antibody production and antitoxin production.

Antibodies are Y-shaped proteins made by white blood cells called lymphocytes. Each antibody has binding sites whose shape is complementary to a specific antigen on a pathogen. When a lymphocyte meets its matching antigen, it divides and produces many copies of the same antibody, which bind to the antigens. This clumping immobilises pathogens and marks them for destruction by other white blood cells. Antibodies also give immunity: memory lymphocytes remain after an infection, so a later exposure triggers faster, larger antibody production. For example, after measles infection, measles-specific antibodies appear in the blood and prevent reinfection. Antibody production is a specific, adaptive response, unlike phagocytosis, which is non-specific.

Antitoxins are proteins produced by lymphocytes as part of the specific immune response. They bind to toxins released by pathogens, neutralising them so the toxin cannot damage cells. For example, in diphtheria the bacterium releases a toxin, and lymphocytes produce antitoxin molecules that lock onto it. Antitoxin production follows the same principle as antibody production: a lymphocyte recognises a specific toxin, divides, and releases matching antitoxin. Because the response is specific, antitoxins against one toxin do not neutralise another. Antitoxins do not kill the pathogen itself; they stop the harmful effect of its toxin while other defences destroy the pathogen.