The innate and adaptive immune systems

The immune system fights germs on the skin, in the tissues of the body, and in bodily fluids such as blood. It is made up of the innate (general) and the adaptive (specialized) . These two systems work closely together and take on different tasks.

Illustration: The innate and adaptive immune systems

The innate immune system: Fast and general effectiveness

The innate is the body's first line of defense against intruders. It responds in the same way to all germs and foreign substances, which is why it is sometimes referred to as the "non-specific" . It acts very quickly – for instance, it makes sure that that have entered the skin through a small wound are detected and destroyed on the spot within a few hours. But the innate can’t always stop germs from spreading.

The innate provides

  • protection offered by the skin and mucous membranes
  • protection offered by cells and proteins

Protection offered by the skin and mucous membranes

All outer and inner surfaces of the human body are a key part of the innate . The closed surface of the skin and mucous membranes already forms a physical barrier that stops germs from entering. On top of that, substances like acid, enzymes and mucus prevent bacteria and viruses from growing. Certain movements in the body also stop germs from settling – for example, movements of hair-like structures (cilia) in the lungs or movements of the bowel muscles. Some fluids in the body have a similar effect – including tear fluid, sweat and urine (which flushes the organs of the urinary system).

Protection offered by immune system cells and proteins

If germs get past the skin or mucous membranes and enter the body, the innate fights them using special cells and proteins.

What happens during an inflammation?

If, for example, an area of skin becomes infected, cells spring into action – either by moving to the area or by being activated locally. Certain cells of the release substances to make the blood vessels wider and more “leaky.” This causes the area around the to swell, become warm and turn red – visible signs of the inflammation that has developed. A fever may develop too. Then the blood vessels get wider and even more cells arrive to fight the .

Certain proteins known as enzymes are also activated to help defend the body (see below).

Phagocytes: Making germs harmless

Bacteria or viruses that enter the body can be stopped right away by phagocytes, also known as scavenger cells. These special white blood cells (leukocytes) enclose germs and "digest" them, making them harmless. The remains of the germs move to the surface of the phagocytes, where they can be detected by the adaptive .

There are also other types of cells that release substances to kill and other germs. But it’s not only the germs that die – tissue cells and cells die and break down too. Together, their remains form a yellowish fluid called pus.

The role of proteins

Several proteins (enzymes) help the cells of the innate . A total of nine different enzymes activate each other in a kind of chain reaction: One enzyme in the first stage alerts several enzymes in a second stage, each of which activates several enzymes in a third stage, and so on. This allows the immune response to grow stronger very quickly.

The tasks of these enzymes include:

  • marking germs as targets for phagocytes,
  • attracting other cells from the bloodstream,
  • destroying cell walls to kill the , and
  • fighting viruses by destroying the viral envelope (the outermost layer of a ) or cells that have been infected with viruses.

Natural killer cells: Searching for body cells that have changed

The natural killer cells are the third major part of the innate . Their main job is to identify cells that have been infected by a , as well as abnormal cells that may turn into (or have turned into) tumor cells. To do this, they search for cells with an abnormal surface, and then destroy the cell surface using substances called cytotoxins.

The adaptive immune system: Fighting the germs directly

If the innate (general) fails to destroy the germs, the adaptive (specialized) takes over. The adaptive specifically targets the type of germ that is causing the . But to do that, it first needs to recognize the germ as such. This means that it’s slower to respond than the innate , but it’s more accurate when it does respond. It also has the advantage of being able to "remember" germs. So the next time the adaptive faces a germ it has already met, it can start fighting the germ faster.

This memory is also the reason why there are some illnesses you can only get once in your life, because afterwards your body becomes “immune” to them. It may take a few days for the adaptive to respond the first time it comes into contact with the germ, but the next time the body can react immediately. The second then usually goes unnoticed, or is at least milder.

The adaptive is made up of:

  • T cells in the tissue between the body's cells
  • B cells (also in the tissue between the body's cells)
  • Antibodies in the blood and other bodily fluids

T cells

T cells (also called T lymphocytes) are made in bone marrow. They travel in the bloodstream to the thymus, where they mature. The "T" in their name comes from "thymus."

T cells have three main jobs:

  • They use chemical messengers to activate other cells of the , starting the adaptive response (T helper cells).
  • They detect tumor cells or cells that have been infected by viruses and destroy them (cytotoxic T cells).
  • Some T helper cells become memory T cells after the has cleared up. They "remember" the germ that was fought off, and are then ready to activate the adaptive quickly if the body is infected by the same germ again.

T cells have specific features (receptors) on their surfaces that germs can attach to – like a lock that one particular key will fit. The can make a matching T cell type for each germ within a few days of .

Then if a germ attaches to a matching T cell, the T cell starts to multiply – making more T cells that can specifically fight that germ. Because only the cells that match the germ multiply, the immune response is “tailor-made.”

B cells

B cells (B lymphocytes) are made in the bone marrow, where they mature into specialized cells. They take their name from the "B" in "bone marrow." Like the T cells, there are many different types of B cells that match particular germs.

B cells are activated by T helper cells: T helper cells send signals to B cells that match the same germs as they do. This stimulates the B cells to make copies of themselves and turn into plasma cells. The plasma cells quickly make very large amounts of antibodies and release them into the blood. Because the T helper cells only activate the B cells that match the attacking germs, the body only makes the exact antibodies that are needed.

Some of the activated B cells turn into memory cells and become part of the "memory" of the adaptive .

The different cells of the adaptive communicate either directly or through soluble chemical messengers such as cytokines (usually proteins). These chemical messengers are made by various cells in the body.


Antibodies (proteins with sugar groups attached to them) travel around the body in the bloodstream. They are made by the to fight germs and foreign substances. Antibodies can quickly recognize germs and other potentially harmful substances, and then attach to them. This makes the "intruders" harmless and attracts other cells to help. Antibodies are made by B cells. Germs and substances that can trigger the production of antibodies are called "antigens."

An antibody only attaches to an antigen if it matches exactly, like a key in the lock of the antibody. In this way, antibodies recognize matching germs and trigger the fast response of the adaptive .

Antibodies have three main functions:

  • They make germs harmless – for example, by directly attaching to the cell surface of viruses or , or by binding harmful substances made by these germs. This prevents the germs from latching onto normal body cells and infecting them.
  • They activate other cells by attaching to their surface. Also, it’s much easier for phagocytes to fight off germs that have a lot of antibodies attached to them.
  • They activate proteins that help in the response.

So the antibodies of the adaptive also help the innate to do its job.

Brandes R, Lang F, Schmidt R. Physiologie des Menschen: mit Pathophysiologie. Berlin: Springer; 2019.

Menche N. Biologie Anatomie Physiologie. München: Urban und Fischer; 2023.

Pschyrembel Online. 2023.

IQWiG health information is written with the aim of helping people understand the advantages and disadvantages of the main treatment options and health care services.

Because IQWiG is a German institute, some of the information provided here is specific to the German health care system. The suitability of any of the described options in an individual case can be determined by talking to a doctor. can provide support for talks with doctors and other medical professionals, but cannot replace them. We do not offer individual consultations.

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Updated on August 14, 2023

Next planned update: 2026


Institute for Quality and Efficiency in Health Care (IQWiG, Germany)

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