The human body

The innate and adaptive immune systems

The immune system rests on two major pillars: the innate, general immune system and the adaptive, specialized immune system. Both systems work closely together and take on different tasks.

Defense cells in the tissue

Both parts of the immune system work on different levels: Firstly, there are special defense cells in the different tissues that are directly involved in eliminating pathogens (germs). These cells belong to the so-called cellular defense.

Defense cells in the blood and other fluids

Secondly, both the innate and the adaptive immune system also need several soluble substances found in blood and other body fluids. These are mainly proteins like enzymes, antibodies and short amino acid chains. These substances belong to the humoral defense (from the Latin humor meaning: “fluid”). So both parts of the immune system – the innate and the adaptive – use cellular and humoral defense strategies.

Illustration: The innate and the adaptive immune systems

Innate immune system: Fast and broadly effective

The strength of the innate, general defense is to be able to take action very quickly. It makes sure, for example that bacteria that have entered the skin through a small wound are detected and partly destroyed on the spot within a few hours. As the innate immune response is not specialized for specific pathogens, it does not need a long start-up phase. Because of this broad effect, it is only capable to a certain degree of stopping germs from entering and spreading in the body.

The innate defense consists of several elements:

  • The skin and all mucous membranes in the body openings, which form external barriers
  • Different defense cells from the white blood cell group (leukocytes)
  • Various substances in the blood and in body fluids

Protection from the outside: Skin and mucous membranes

All external and internal surfaces of the human body are a key element of the innate immune system. The closed surface of the skin and of all mucous membranes already forms a mechanical barrier for pathogens, which prevents them from entering. Additionally, chemical substances like acid, enzymes or mucus prevent the bacteria or viruses from gaining a foothold. Movements created, for example, by hair-like structures in the bronchi (cilia) or by bowel muscles stop germs from settling in the body. Tear fluid, sweat, or urine rinsing the urinary organs all have a similar effect.

Protection from the inside: Defense cells and proteins

If, despite all obstacles, pathogens make it past the skin or mucous membranes and enter the body, the innate system’s second line of defense comes into action. Inflammatory cells move to the site of infection, or defense cells that are already there are activated. Soluble protein substances of the complement system (see below) are activated, too, and help to defend the body. This leads to an inflammatory reaction where blood circulation is increased and the affected area becomes swollen and hot. Sometimes there is also a fever.

If bacteria or viruses manage to enter the body they can be eliminated directly on the spot by scavenger cells or phagocytes (from the Greek phagein, meaning: “to eat”). Two types of defense cells are the most effective ones: macrophages, which are found in the tissue, and neutrophil granulocytes, which are in the blood and tissue. These cells enclose the pathogens and digest them in their interior. Scavenger cells can work best if the pathogen has already been marked by antibodies or proteins of the complement system. This makes the pathogen more “palatable” for the scavenger cells.

So at this point, antibodies of the adaptive immune system support the innate defense. Vice versa, the scavenger cells can help the adaptive immune system by taking up and digesting the marked pathogens very quickly.

Complement system: Proteins in a chain reaction

Soluble substances support the defense cells of the innate immune system. A total of nine different enzymes activate one another in a process similar to a chain reaction: one enzyme of the first stage alerts several enzymes of the second stage, each of which again activates several enzymes of the third stage, and so on. This process quickly makes the defense reaction a lot stronger, because the production of these protein substances increases in such large jumps (exponentially).

The tasks of these enzymes:

  • They mark pathogens, making them more attractive for scavenger cells.
  • They attract other immune cells from the blood.
  • They dissolve the cell walls of bacteria, so that they lose fluid and minerals and die.
  • They fight viruses directly by destroying the virus envelopes, or indirectly by destroying cells infected by viruses.

Natural killer cells: Searching for changed body cells

The natural killer cells are the third important part of the innate immune system. They specialize in identifying cells that are infected by a virus or that have become tumorous. They do this by looking for changes in cell surfaces. If natural killer cells find cells with a changed surface, they dissolve them using cell poisons, also called cytotoxins.

The adaptive immune system: Precision and a long memory

If the body’s first line of defense – the innate immune system – is unsuccessful in destroying the pathogens, after about four to seven days the specific adaptive immune response sets in. This means that the adaptive defense takes longer, but it targets the pathogen more accurately. Another advantage: It can remember the aggressor and acts specifically against certain antigens. If there is new contact with an antigen that is already known, the defense response can then be quicker. This way the defense responses of the adaptive immune system are more efficient and faster than those of the innate defense, if the antigen is already known.

The adaptive immune system can remember the antigens because it produces memory cells. This is also the reason why there are some illnesses you can only get once in your life, because afterwards your body becomes “immune.” While after first contact with the pathogen it takes several days for the immune system to respond, a second infection often has no consequences, or at least the symptoms are weaker.

The adaptive immune system has several parts that react in different ways, depending on the place in the body where the pathogen is. Antibodies are made available for germs outside the cells (in the blood and in body fluids). To eliminate pathogens that are inside the tissue, a cell-mediated immune response is necessary.

These parts of the adaptive defense include:

  • T lymphocytes
  • B lymphocytes
  • antibodies as soluble proteins in the blood
  • cytokines in the blood and tissue as hormone-like messenger substances

T lymphocytes

In the adaptive immune system, T lymphocytes (T cells) are responsible for the special defense in the tissue, which is carried out by cells. They recognize infected cells and are responsible for their destruction and elimination from the body.

T lymphocytes belong to the group of white blood cells and, in adults, are produced in the bone marrow. In the thymus gland, they mature into cells that are capable of recognizing self from non-self cells. T cells have characteristic structures on their surfaces that pathogens can bind to – similar to a lock that a specific key fits to.

A pathogen that exactly fits a T cell stimulates this T cell to multiply quickly and to develop into specialized T cells. At the same time, the great number of newly produced T cells triggers other defense reactions. This leads to the pathogens being destroyed and eliminated from the body.

During the course of a defense reaction, T lymphocytes develop into specialized cells. These include:

  • T helper cells
  • T killer cells or cytotoxic T cells
  • memory T cells
  • regulatory T cells

B lymphocytes

B lymphocytes are an important pillar of the adaptive defense: They produce antibodies, which are in the blood as soluble proteins and are specialized for exactly one pathogen.

The cells of the adaptive immune systems interact either directly by binding to the surface of different defense cells or they use soluble messenger substances like the cytokines. These messenger substances are mostly proteins and are produced by different cells in the organism.