Understanding the immune system: how the body defends itself

The body's layered defences

The immune system works in layers. The first line of defence is simply keeping invaders out: the skin acts as a physical barrier, while tears, saliva, mucus, and stomach acid trap or destroy germs before they take hold. If something breaks through, the body deploys its internal defences, carried through the blood and a network called the lymphatic system, which includes the lymph nodes you may feel as swollen glands when you are ill. Key organs include the bone marrow, where immune cells are made, and the thymus, where certain cells mature. White blood cells are the workforce of this system. Together these layers mean that most germs are stopped or dealt with before they ever cause noticeable illness.

Innate immunity: the rapid responders

The innate immune system is the body's fast, general-purpose defence, ready to act within minutes to hours of an invader appearing. It does not target one specific germ; instead it recognises common signs of danger and responds to almost any threat in a similar way. Specialised white blood cells, such as phagocytes, engulf and digest germs, while others release chemicals that trigger inflammation — the redness, heat, and swelling that bring more immune cells to the site of infection or injury. Fever is part of this response too, creating conditions less friendly to germs. Although innate immunity is quick and broad, it lacks precision and memory. It buys crucial time while the more targeted branch of the immune system gets organised.

Adaptive immunity and immune memory

The adaptive immune system is slower to start but highly precise, and it is what gives us lasting protection. It relies on two key players. B cells produce antibodies — Y-shaped proteins that lock onto a specific germ, marking it for destruction and neutralising it. T cells come in types that either coordinate the immune response or directly kill infected cells. Crucially, after an infection is cleared, the body keeps "memory" cells that remember that particular germ. If the same invader returns, the immune system recognises it instantly and responds far faster and more strongly, often stopping illness before it starts. This immune memory is why we usually catch certain illnesses, like chickenpox, only once, and it is the basis of vaccination.

How vaccines train the immune system

Vaccines work by safely teaching the adaptive immune system to recognise a germ without you having to catch the disease. A vaccine contains a harmless piece or weakened form of a germ — or, in newer vaccines, instructions for your cells to make a single germ protein. The immune system responds as if to a real infection, producing antibodies and, importantly, memory cells, but without the danger of the actual illness. If you later meet the real germ, your body is already primed to fight it off quickly. When enough people in a community are vaccinated, the germ struggles to spread, protecting even those who cannot be vaccinated — an effect known as herd immunity. This is how vaccination has controlled once-common serious diseases.

When the immune system goes wrong

A healthy immune system is carefully balanced, and problems arise when it does too little or too much. If it is weakened — through some inherited conditions, certain illnesses such as HIV, some cancer treatments, or medicines that suppress immunity — the body becomes more vulnerable to infections. At the other extreme, the immune system can overreact. In allergies, it responds aggressively to harmless things like pollen or foods. In autoimmune conditions, such as type 1 diabetes, rheumatoid arthritis, or multiple sclerosis, it mistakenly attacks the body's own tissues. Sometimes an overwhelming immune response to a severe infection, as can happen in sepsis, itself becomes dangerous. Understanding these malfunctions helps doctors treat conditions ranging from allergies to autoimmune disease, and to protect people whose defences are lowered.