The Immune System

Before the immune system even gets involved, your body has physical barriers that stop most pathogens from entering: - Skin — a tough, waterproof barrier. Slightly acidic (pH 5.5) which discourages bacterial growth. Sheds constantly, removing surface bacteria. - Mucus membranes — line the airways, gut, and urinary tract. Mucus traps pathogens. Tiny hairs called cilia in the airways sweep mucus and trapped particles up toward the throat. - Stomach acid — pH 1–2 kills most swallowed bacteria. - Saliva, tears, and nasal secretions — contain an enzyme called lysozyme that destroys bacterial cell walls. When a pathogen gets through: The innate immune response kicks in immediately: Inflammation is the first response. Damaged cells and immune cells release chemicals including histamine, which causes blood vessels to widen and become leaky. You see this as redness, swelling, and warmth at the site of infection. White blood cells can then escape the blood vessels and enter the infected tissue. Phagocytes (literally "cell eaters") are white blood cells that engulf and destroy pathogens. Neutrophils are the most common — they rush to infection sites and swallow bacteria whole. Macrophages are larger, longer-lived phagocytes that also clean up dead cells and debris. They also present bits of the pathogen to the adaptive immune system, starting the specific response. Natural killer cells patrol the body looking for cells that have been infected by viruses or have become cancerous. They destroy these cells before they can spread. Fever is part of the innate response. Chemicals called pyrogens raise the body's temperature set-point in the hypothalamus. A higher temperature makes the environment less friendly for bacteria and speeds up immune cell activity. This is why a fever, though uncomfortable, is actually helpful — but temperatures above 41°C can be dangerous.

The adaptive immune system is built around two types of white blood cells called lymphocytes: B cells and T cells. Both are produced in the bone marrow — B cells mature there, T cells mature in the thymus (a small gland in the chest, which is why they are called T cells). The key concept: antigens An antigen is any molecule that the immune system recognises as foreign — usually a protein on the surface of a pathogen. Every pathogen has unique antigens, like a molecular fingerprint. The adaptive immune system learns to recognise these specific antigens. T cells — cell-mediated immunity: - Helper T cells (CD4+) are the coordinators. When a macrophage presents a pathogen's antigen, helper T cells recognise it, activate, and multiply. They then send chemical signals (called cytokines) that activate B cells and killer T cells. - Cytotoxic T cells (CD8+) are the assassins. They kill cells that have been infected by viruses — by detecting viral proteins on the infected cell's surface and releasing toxic chemicals that destroy it. B cells — antibody-mediated immunity: When activated by helper T cells, B cells multiply and differentiate into plasma cells, which produce antibodies — Y-shaped proteins that are perfectly shaped to fit onto a specific antigen (like a lock and key). Antibodies work by: - Neutralisation — binding to a pathogen and blocking it from entering cells - Opsonisation — coating the pathogen so phagocytes recognise and eat it more easily - Complement activation — triggering a cascade of proteins that punch holes in bacterial membranes HIV and AIDS: HIV specifically targets and destroys helper T cells (CD4+ cells). As their numbers fall, the immune system loses its coordinator — both antibody production and cytotoxic responses fail. The patient becomes vulnerable to infections and cancers that a healthy immune system would easily defeat. This is AIDS (acquired immunodeficiency syndrome).

After an infection is cleared, most of the B and T cells that fought it die off. But a small number survive as memory cells — long-lived cells that remember the specific antigen. If the same pathogen ever enters the body again, memory cells recognise it immediately. Instead of taking days to mount a response (as the first time), the immune system responds within hours — so powerfully and quickly that you never even develop symptoms. This is immunological memory, and it is the reason you can only get some infections (like chickenpox) once. Vaccines exploit immunological memory: A vaccine exposes your immune system to a harmless form of a pathogen's antigen. Your immune system mounts a primary response, creates memory cells — and next time the real pathogen arrives, memory cells destroy it instantly. Different vaccines use different approaches: - Live attenuated vaccines — weakened live pathogen (MMR, chickenpox). Very effective, long-lasting memory. Cannot be given to immunocompromised patients. - Inactivated vaccines — killed pathogen (flu, some polio). Safer but may need boosters. - Subunit vaccines — just the antigen protein, not the whole pathogen (hepatitis B, HPV). Very safe. - mRNA vaccines — instruct your cells to make a specific antigen protein temporarily (COVID-19 Pfizer/Moderna). No live pathogen involved. Herd immunity: When enough people in a community are immune (from infection or vaccination), the pathogen cannot find enough hosts to spread — so even unvaccinated people are protected because the virus cannot reach them. The threshold varies: for measles (very contagious) about 95% of the population needs immunity; for COVID-19, around 70–80%.

The immune system is powerful — but it can also cause harm when it attacks the wrong target. Allergies: An allergy is the immune system overreacting to a harmless substance (an allergen) — pollen, peanuts, pet dander, insect venom. The first exposure sensitises the immune system; subsequent exposures trigger B cells to release large amounts of IgE antibodies, which bind to mast cells in the skin, airways, and gut. When the allergen binds to these IgE-coated mast cells, they release huge amounts of histamine — causing sneezing, itching, swelling, and in severe cases, anaphylaxis. Antihistamines work by blocking histamine receptors. Anaphylaxis is a severe, life-threatening allergic reaction where histamine release causes massive vasodilation, airway swelling, and a drop in blood pressure. Treatment is adrenaline (epinephrine) — which rapidly constricts blood vessels, opens airways, and reverses the dangerous effects. This is why people with severe allergies carry an EpiPen. Autoimmune disease: Sometimes the immune system attacks the body's own tissues, confusing "self" proteins with foreign antigens. Examples include: - Type 1 diabetes — immune system destroys the insulin-producing beta cells in the pancreas - Rheumatoid arthritis — attacks the lining of joints - Multiple sclerosis — attacks myelin sheaths around nerve fibres - Lupus — attacks multiple organs including skin, joints, and kidneys Treatment often involves suppressing the immune system (immunosuppressants) — which helps the autoimmune disease but increases infection risk. Immunodeficiency — when the immune system is too weak. Can be inherited (primary, e.g. SCID — severe combined immunodeficiency) or acquired (HIV/AIDS, chemotherapy, organ transplant immunosuppression). These patients are vulnerable to opportunistic infections — infections that a healthy immune system would easily defeat.