Every day, your body encounters countless microorganisms: bacteria on your hands, viruses in the air, fungi on surfaces and parasites in some environments. Most are harmless, and many are even helpful, but some can cause disease. Your immune system is the body's defense network, a remarkable collection of barriers, cells and chemical signals that identifies threats, destroys them and remembers them for the future. It is so sophisticated that it can distinguish between millions of different invaders and the body's own cells. This explainer describes how the immune system works. It explains how the body works and is not medical advice.
The job of the immune system
The immune system has three main tasks:
- Keep invaders out with physical and chemical barriers.
- Detect and destroy germs that get inside, as well as damaged or abnormal cells.
- Remember past invaders so that it can respond faster and more strongly the next time.
Microorganisms that cause disease are called pathogens. They include bacteria, viruses, fungi and parasites. Molecules that the immune system recognizes as foreign are called antigens.
The first line of defense: barriers
Before germs can cause infection, they must get past the body's barriers:
- Skin: A tough, waterproof barrier with layers of dead cells on the surface. Its slightly acidic surface and natural oils discourage the growth of many microbes.
- Mucous membranes: The linings of the nose, mouth, airways, digestive tract and other openings produce sticky mucus that traps germs.
- Cilia: Tiny hair-like structures in the airways move mucus and trapped particles out of the lungs.
- Chemical defenses: Tears, saliva and mucus contain enzymes such as lysozyme that break down bacterial cell walls. Stomach acid kills many microbes in food.
- Friendly microbes: The trillions of beneficial bacteria living on the skin and in the gut, called the microbiome, compete with harmful microbes for space and nutrients.
The innate immune system: fast and general
If germs break through the barriers, for example through a cut, the innate immune system responds within minutes to hours. It reacts to general features shared by many germs rather than to specific pathogens.
Inflammation
When tissue is injured or infected, damaged cells and immune cells release chemical signals. Blood vessels widen and become leakier, bringing more blood, fluid and immune cells to the area. This causes the classic signs of inflammation: redness, heat, swelling and pain. Although uncomfortable, inflammation helps bring defenders to the site of infection and begins the healing process.
Phagocytes: the eaters
Phagocytes are white blood cells that engulf and digest invaders. The main types include:
- Neutrophils, the most common white blood cells, which rush to infections in large numbers. Pus consists largely of dead neutrophils.
- Macrophages, large cells that engulf germs and dead cells and also send signals to other immune cells.
- Dendritic cells, which capture pieces of germs and carry them to lymph nodes to alert the adaptive immune system.
Other innate defenders
- Natural killer (NK) cells recognize and destroy infected or abnormal cells, such as some virus-infected cells and cancer cells.
- The complement system is a group of proteins in the blood that can punch holes in bacteria, mark them for destruction and attract immune cells.
- Interferons are signaling proteins released by virus-infected cells that warn neighboring cells and help them resist infection.
Fever
When the body detects an infection, immune signals can reset the brain's thermostat in the hypothalamus to a higher temperature, causing fever. A higher temperature may slow the growth of some pathogens and enhance certain immune responses.
The adaptive immune system: specific and with memory
If the innate response does not eliminate an infection, the adaptive immune system takes over. It takes several days to respond fully the first time it meets a new pathogen, but it is highly specific and has memory.
The adaptive immune system relies on two main types of white blood cells called lymphocytes: B cells and T cells. Each B cell and T cell carries receptors that recognize one specific antigen. The body produces an enormous variety of these cells, capable together of recognizing millions of different antigens.
B cells and antibodies
B cells produce antibodies, Y-shaped proteins that bind to specific antigens. When a B cell recognizes its matching antigen, often with help from T cells, it multiplies and turns into plasma cells, which release large amounts of antibodies into the blood and body fluids.
Antibodies help fight infection in several ways:
- Neutralizing viruses and toxins by blocking the parts they use to attack cells.
- Tagging germs for destruction by phagocytes.
- Clumping germs together.
- Activating the complement system.
T cells
T cells come in several types:
- Helper T cells coordinate the immune response. They release signaling molecules called cytokines that activate B cells, killer T cells and macrophages.
- Cytotoxic (killer) T cells recognize and destroy cells infected with viruses or other intracellular pathogens, as well as some cancer cells.
- Regulatory T cells help calm the immune response once the threat is gone and prevent the immune system from attacking the body's own tissues.
How T cells recognize infected cells
Cells display small fragments of their proteins on their surface using molecules called MHC molecules, like showing an ID card. If a cell is infected with a virus, it displays fragments of viral proteins. Killer T cells inspect these fragments and destroy cells displaying foreign ones.
Immune memory
After an infection is cleared, most of the B cells and T cells that multiplied to fight it die off. But some remain as memory cells. These long-lived cells remember the specific antigen. If the same pathogen enters the body again, memory cells respond much faster and more strongly, often eliminating it before it causes symptoms. This is called immunity.
That is why many people get certain diseases, such as measles, only once.
How vaccines work
Vaccines use immune memory to protect people without them having to get sick first. A vaccine exposes the immune system to an antigen from a pathogen in a safe form, such as:
- A weakened or inactivated version of the pathogen.
- A harmless piece of the pathogen, such as a protein.
- Genetic instructions, such as mRNA, that tell the body's cells to produce a harmless piece of the pathogen.
The immune system responds by producing antibodies and memory cells. If the person later encounters the real pathogen, their immune system recognizes it and responds quickly. Vaccines have dramatically reduced diseases such as polio, measles and diphtheria, and helped eradicate smallpox.
When enough people in a community are immune, the spread of a disease is limited, protecting those who cannot be vaccinated. This is called community immunity or herd immunity.
The lymphatic system
The immune system works through the lymphatic system, a network of vessels and organs:
- Lymph nodes are small, bean-shaped structures where immune cells meet antigens. They may swell during infections as immune cells multiply.
- The spleen filters blood and helps fight certain infections.
- The thymus is where T cells mature.
- Bone marrow produces most blood cells, including B cells.
- Tonsils and patches of tissue in the gut also contain immune cells.
When the immune system goes wrong
The immune system must balance attacking invaders with leaving the body's own cells alone. Sometimes this balance fails:
- Allergies occur when the immune system overreacts to harmless substances, such as pollen, dust mites or certain foods.
- Autoimmune diseases, such as type 1 diabetes, rheumatoid arthritis and multiple sclerosis, occur when the immune system attacks the body's own tissues.
- Immunodeficiency occurs when parts of the immune system are weak or missing, due to genetic conditions, certain infections such as HIV, or medical treatments.
Supporting immune health
There is no single trick to "boost" the immune system. Health agencies emphasize a combination of good sleep, a balanced diet, regular physical activity, not smoking, managing stress, good hygiene such as handwashing, and recommended vaccinations.
The immune system through life
The immune system changes over a lifetime. Newborn babies receive antibodies from their mothers through the placenta before birth and through breast milk afterward, which helps protect them during their first months while their own immune systems mature. Childhood is a period of rapid immune learning, as the body encounters many new microbes for the first time and builds up a library of memory cells. In older age, the immune system gradually becomes less responsive, a process called immunosenescence, which is one reason older adults can be more vulnerable to some infections and why certain vaccines are recommended for them.
Questions readers ask
What are white blood cells? Immune cells that defend the body, including phagocytes like neutrophils and macrophages, and lymphocytes like B cells and T cells.
What do antibodies do? They bind to specific antigens, neutralizing germs and toxins and marking them for destruction.
Why do we get fevers? Immune signals raise the body's temperature set point, which may help fight infection.
How do vaccines protect us? They train the immune system with a safe antigen, creating memory cells that respond quickly if the real pathogen appears.
Why do lymph nodes swell? Immune cells multiply in lymph nodes when fighting infection, causing them to enlarge temporarily.
In short
The immune system defends the body in layers. Skin, mucus and other barriers keep most germs out. The innate immune system responds quickly with inflammation, phagocytes, natural killer cells and fever. The adaptive immune system, with B cells producing antibodies and T cells coordinating and killing infected cells, targets specific pathogens and creates long-lasting memory. Vaccines use this memory to prevent disease safely. Balanced and precise, the immune system works constantly, usually unnoticed, to keep us healthy.