Immune System and Infection

Right now, your immune system is quietly deciding, thousands of times a second, what belongs in your body and what does not, without you ever noticing the decision being made. This lesson looks at how that defence network works, and why sometimes it needs to be loud and inflamed, and sometimes it needs to stay quiet.

Right now, your immune system is quietly deciding, thousands of times a second, what belongs in your body and what does not, without you ever noticing the decision being made. This lesson looks at how that defence network works, and why sometimes it needs to be loud and inflamed, and sometimes it needs to stay quiet. Two layers of defence: fast and general, then slow and specific Before any germ even reaches an immune cell, the body's barriers do most of the work. Skin forms a physical wall. Mucus in the nose, throat and airways traps particles before they get further in. Stomach acid destroys many swallowed germs. Together, these barriers stop the vast majority of threats before the immune system proper needs to act at all. If something gets past them, a fast, general response called the innate immune system takes over. It does not need to recognise a specific germ, only that something foreign is present, and it can trigger inflammation and attract immune cells within minutes. A slower, more specific system, the adaptive immune system, follows behind. It learns to recognise a particular germ and produces antibodies, proteins built to identify and attach to that one target. This system is slower to start but far more precise, and it is the reason your body can remember a germ it has met before. Inflammation: the body's way of saying "something needs attention here" Inflammation, the redness, heat, swelling and pain around an infected or injured area, is not a malfunction. It is the visible sign of increased blood flow, immune cells and chemical signals arriving to deal with a problem. In the right amount, inflammation protects and heals. Too much, or inflammation that does not switch off when it should, is itself a source of harm, which is why doctors pay as much attention to controlling inflammation as to fighting the original threat. Immune memory: why vaccines work Once the adaptive immune system has met a germ, it keeps a memory of it, ready to respond much faster the second time. A vaccine works by giving the immune system a safe preview of a germ, or part of one, so that memory is already in place before a real infection ever happens. Infection: when germs get the upper hand Infection happens when a germ, bacteria, viruses, fungi or parasites, gets past the body's defences, multiplies, and causes a response or illness. Different types of germ need different treatments: antibiotics target bacteria specifically, and have no effect on viruses, which is why not every infection is treated the same way. Why this system needs balance An immune system that is too weak struggles to clear ordinary infections. One that overreacts can attack the body's own tissue, or cause allergic and inflammatory conditions covered in later lessons. Health here is not about maximum immune activity, it is about a system that responds proportionately and switches off when the job is done. What a doctor checks when this system is affected A doctor cannot see immune activity directly, but several signs and tests reveal a great deal. Looking. Fever, swollen lymph nodes, redness and swelling around a wound, or a rash can each point toward the immune system actively responding to something. Vital signs. Temperature, heart rate and blood pressure together help a doctor judge how significant an infection or inflammatory response is, and whether it is affecting the whole body rather than staying local. Special tests. A blood test can show a raised white cell count, a sign of active immune response, and can sometimes help identify which type of germ is most likely responsible. Specific tests can look for a particular infection directly. When this system is affected When the immune system is fighting an infection, people may notice fever, fatigue, swollen glands, or localised redness and swelling. When it is underactive, infections may recur more often or take longer to clear. When it overreacts, people may notice symptoms of allergy or ongoing inflammation without an obvious infection present. These patterns are explored further in the lessons "Fever: What It Means" and "Persistent Tiredness and Fatigue", and in the lessons on inflammation and allergic and autoimmune conditions.

The immune system defends the body in two stages: a fast, general innate response, and a slower, specific adaptive response that remembers germs it has met before, with inflammation as the visible sign of that defence at work.