How does the immune system work? Innate and adaptive, explained

The immune system is the body's network of barriers, cells and proteins that finds and removes germs and damaged cells. How it does that work comes down to two layers that cooperate. The innate system responds within minutes to hours to anything that looks foreign, with inflammation and cells that swallow invaders. The adaptive system takes days to get going, but it learns the exact shape of one germ, attacks it precisely and remembers it for years.

In short

• The innate system is fast and general; the adaptive system is slow, specific and has memory.
• The two depend on each other: innate cells carry samples of the germ to the adaptive system and tell it how serious the threat is.
• Many symptoms of an infection, including fever, a runny nose and aches, are the immune response itself, not damage done by the germ.

What happens when a cold virus gets in?

The clearest way to see the system is to follow one infection. Take a common cold, most often caused by a rhinovirus, a small virus that infects the cells lining the nose and throat. The timings below are typical, not exact; they vary with the virus, the person and whether they have met that strain before.

When	What is happening
Minute zero	Virus lands in the nose, on a hand or in droplets. Most is caught in mucus and swept away.
First hours	Some virus gets into lining cells and starts copying itself. Infected cells release interferons, warning signals.
Hours to day 1	Innate cells arrive; inflammation begins. A scratchy throat is often the first sign.
Days 2 to 3	Symptoms build: congestion, sneezing, sometimes a mild fever. The NHS says cold symptoms come on gradually over 2 to 3 days.
Days 4 to 7	T cells and B cells that recognize this virus have multiplied and join in; antibodies rise.
1 to 2 weeks	The virus is cleared and most people feel better. Memory cells remain.

The sections below walk through each layer in that order.

What are the first lines of defense?

The first defenses are barriers. Skin is a tough, dry, acidic wall that few germs can cross unbroken. The airways are lined with sticky mucus and tiny beating hairs that carry trapped particles up to the throat to be swallowed. Tears, saliva and sweat contain enzymes that break down bacterial walls, and stomach acid kills much of what is swallowed. Most germs never get further than this.

When a virus does get into a cell, the cell raises the alarm. OpenStax's biology textbook describes how infected cells release interferons, signaling proteins that tell neighboring cells to slow their own protein-making and break down viral RNA, so the virus has a harder time spreading. Interferons also call in immune cells.

How does the innate immune system fight?

Innate immune cells recognize broad patterns that mark something as foreign or damaged: molecules found on many bacteria, or double-stranded RNA made by many viruses. They do not need to have met that germ before. This layer is ancient: every animal has one and plants have a version, which suggests that telling self from foreign is nearly as old as life made of cells.

• Macrophages ("big eaters") sit in tissues, swallow germs and debris, and release chemical signals called cytokines.
• Neutrophils are the most common white blood cells; they rush in from the blood within hours, swallow bacteria and fungi, and die in large numbers doing it. Pus is mostly dead neutrophils.
• Natural killer cells check other cells' surfaces and kill those that look infected or cancerous.
• Complement is a set of about 20 proteins in the blood that coat germs, mark them for swallowing and punch holes in some bacteria.

Together these produce inflammation. Blood vessels near the infection widen and leak, so fluid and white cells pour into the tissue. That causes the four classic signs, redness, heat, swelling and pain, and in a cold it is the source of the stuffy, runny nose. Inflammation is the body doing its job, not the virus doing damage.

The most important innate cell for what comes next is the dendritic cell. It swallows bits of the germ, travels to the nearest lymph node, and displays pieces of them on its surface, together with signals that say how dangerous the threat seems. The innate system doesn't just fight; it decides whether and how the adaptive system wakes up.

How does the adaptive immune system learn a germ?

The adaptive system is built from two kinds of white blood cell, T cells and B cells, and its trick is variety. While each cell is being made, its receptor gene is shuffled at random, so each one recognizes a different molecular shape, an antigen. The body carries many millions of different T and B cells, each matching a different possible antigen, almost all of which it will never need.

When a dendritic cell shows a piece of the cold virus in a lymph node, the rare T cells that happen to fit it are switched on and multiply into an army of identical copies. This is clonal selection: the germ picks its own enemies from the crowd. Because it starts from a handful of cells, it takes time, which is why OpenStax notes that the adaptive response takes days or even weeks to become established.

The activated cells split the work:

1. Helper T cells coordinate. They release cytokines that tell B cells, killer T cells and macrophages what to do. They are the cells HIV destroys, which is why losing them cripples the whole system.
2. Cytotoxic, or killer, T cells find body cells that are displaying pieces of the virus on their surface and make them self-destruct, removing the virus's factories.
3. B cells, once they meet their antigen and get the go-ahead from a helper T cell, turn into plasma cells that pour out antibodies: Y-shaped proteins that latch onto the virus, block it from entering cells, and tag it for destruction.

How does the body remember?

After the infection is cleared, most of the army dies off, but some T and B cells stay on as memory cells. If the same germ returns, they respond in a day or two instead of a week, and OpenStax notes that memory B cells can produce tens to hundreds of times more antibody than in the first response. Often the germ is cleared before you notice anything. This is the principle behind how vaccines work: they give the adaptive system its first lesson without the illness.

Colds keep coming back for a different reason. There are more than a hundred types of rhinovirus, plus other cold viruses, and memory of one type does not protect well against the next.

	Innate immunity	Adaptive immunity
Speed	Minutes to hours	Days to weeks the first time
What it recognizes	Broad patterns shared by many germs	One exact antigen
Main parts	Barriers, macrophages, neutrophils, natural killer cells, complement	T cells, B cells, antibodies
Memory	Little or none	Long-lasting
Found in	All animals, and plants have a version	Vertebrates

Why do we get a fever?

A fever is a deliberate change, not a breakdown. Immune cells release signaling molecules that cause a chemical called prostaglandin E2 to act on the hypothalamus, the brain's thermostat, and raise its set point. The body then works to reach the new target, by shivering and narrowing skin blood vessels, which is why a rising fever feels like being cold. It is a reset of homeostasis, not a failure of it. Higher temperatures slow the growth of many germs and speed some immune responses, although research summarized there suggests treating an ordinary fever neither improves nor worsens outcomes.

Can you boost your immune system?

Not in the way products promise. Harvard Health Publishing puts it plainly in its overview of immune health: "the concept of boosting immunity actually makes little sense scientifically." More immune activity is not better. An immune system that attacks too much causes allergies, and one that attacks the body's own tissues causes autoimmune diseases such as type 1 diabetes and rheumatoid arthritis. The goal is a response that is balanced.

What is well supported is avoiding the things that weaken it: severe malnutrition and real vitamin deficiencies, chronic sleep loss, smoking, heavy drinking and long-term stress all impair immune function. Vaccines are the one intervention that reliably trains it against specific germs. Antibiotics help only against bacteria, not cold viruses, which is one of the differences between viruses and bacteria. This note explains how the system works; it isn't medical advice, and a doctor can advise on any individual case.

Check yourself

• Without looking back, explain why the adaptive response takes days the first time and only a day or two the second.
• What does a dendritic cell do that links the two layers of the immune system?
• Why are a runny nose and a fever signs of the immune system working rather than failing?

Sources

• 42.1 Innate Immune Response — OpenStax, Biology 2e, 2018
• 42.2 Adaptive Immune Response — OpenStax, Biology 2e, 2018
• Overview of the Immune System — US National Institute of Allergy and Infectious Diseases
• How to boost your immune system — Harvard Health Publishing
• Common cold — NHS
• Fever — Wikipedia