The defense mechanisms of the adaptive immune system

The adaptive immune system reacts specifically to pathogens (germs). It is able to recognize attacking pathogens and fights against certain antigens or foreign substances. The next time a known antigen is encountered, the adaptive immune system can respond faster.

When the immune system is weakened by the virus HIV, it is mostly a specific group of T cells that is reduced in number, the so-called T helper cells. Then the body is especially susceptible to infections by what are usually harmless pathogens. Due to the lack of T cells, the immune system has difficulties properly coordinating the fight against the pathogens in the tissue. People who have the immune deficiency AIDS often develop a bacterial inflammation of the lungs with streptococci or get fungal or herpes virus infections, for example.

Specialists at work: T helper cells and T killer cells

The T cells have special building blocks on their surface, to which only one specific pathogen can attach – just like a key only fits into one certain lock. The T cells need help from other defense cells, because they can only identify a pathogen when it is bound to the surface of another defense cell.

During the course of an immune response T cells develop further to more specialized sub-forms that then each take on different roles.

T helper cells

T helper cells have two main jobs: The first is to release certain messenger substances called lymphokines. By doing this they help to destroy germs in cells that are already infected. Their second task: they stimulate the production of antibodies against the pathogen and their dispersal through the bloodstream. This means that an alarm is signaled when there is an infection and the information about the foreign pathogen is transmitted throughout the entire body. Then the immune system can fight against the infection on all fronts and eliminate the pathogen more quickly. T helper cells are also called T4 cells because they have the CD4 surface molecule.

The HI virus (HIV) also binds to this structure. This is why in AIDS, mostly the T helper cells are affected and reduced in number. This is why the body is prone to infections with normally harmless pathogens. Due to the reduced amount of T-cells, the immune system is not able to properly coordinate the fight against the pathogens in the tissue. People who have the immune deficiency AIDS often develop a bacterial inflammation of the lungs with streptococci or get fungal or herpes virus infections, for example.

Regulatory T cells

Regulatory T cells (also called T suppressor cells) have the opposite function: They are to slow down the workings of the body’s defense, so that the immune system “calms down” again after the infectious agent has been destroyed. The regulatory T cells have the receptor molecule CD8 on their surface. Together with the T killer cells, they are called the T8 cells.

T killer cells

T killer cells are capable of directly destroying bacteria or cells that have been infected by a virus or that have become cancerous. For this reason they are also called cytotoxic T cells (from the Ancient Greek words zyto, meaning “cell,” and toxikon, meaning “poison”). The T killer cells can also identify pathogens only when they are “presented” with the help of other defense cells in the body. Using this method, cells that have been infected by a virus, for example, can inform the immune system about their state very early from the place of infection in the tissue.

When T killer cells have been activated, they release a substance called perforin (from the Latin words per, meaning: “through,” and foramen, meaning: “hole”). This substance gets into the wall of the bacterium or of the infected cell and makes a hole in the wall. The bacterium or the infected cell together with the pathogen then dies of loss of fluid and electrolytes.

Memory T cells

Finally, memory T cells make sure that in case of another infection, measures are taken very quickly, so that most of the time the infection goes unnoticed. These cells remember information about the pathogen for a longer time and can pass on this information immediately if there is another infection.

B cells and antibody factories

Cells called B lymphocytes help the T cells in doing their job: B lymphocytes, or B cells, produce specific antibodies to fight off bacteria, bacterial toxins, viruses, fungi, parasites or other non-self substances that have entered the body. These antibodies travel around in the bloodstream and can quickly recognize possible pathogens and bind to them. From the beginning of an infection until enough of the right antibodies are produced, takes a week on average – this is the time when you feel “really ill.”

B cells are made in the bone marrow and this is also the place where they mature to become specialized defense cells. This is why they are called B cells – the B stands for bone marrow. Before a pathogen triggers the production of many antibodies, there are no free antibody proteins in the bloodstream, however, only antibodies that are attached to the surface of B cells. They are in the spaces between the cells of the organs and in the lymph fluid and wait until they are needed.

When the B cells recognize a non-self surface – a virus envelope, a bacterium or a changed body cell – the B cells turn into so-called plasma cells. These plasma cells are real “antibody factories.” They produce very large amounts of specialized antibodies in a short time and release them into the blood.

How do antibodies work?

Antibodies are proteins that contain a certain amount of sugars and are also called glycoproteins. They are made of four different chains, which combine to ball-shaped structures. This is where an antibody binds to the matching pathogen, if they fit together like a key to a lock.

Antibodies have three main functions:

  • They neutralize the pathogen,
  • activate other defense cells and 
  • activate the complement system.

Neutralization

Neutralizing pathogens or changed cells is one of the most important tasks of antibodies. To do this, they attach directly to the surface of a virus or bacterium and stop the pathogen from attaching itself to a normal body cell and infecting it. Or the antibody binds to toxins produced by bacteria. These substances can then no longer enter the body cells and damage them.

Activating many defense cells

Antibodies can activate many different defense cells, which are in the blood, the lymph fluid and the tissue. These include scavenger cells (phagocytes) in the tissue, blood platelets (thrombocytes) in the blood and mast cells, which are responsible for many allergic reactions. Scavenger cells, for example, can fight pathogens a lot better if they are packed with antibodies all around them. The scavenger cells can take in the germs better and faster, to digest them in the inside of the cell. When antibodies “mark” the germs this way, it is also called opsonization.

Activating the complement system

Antibodies are also capable of activating various other proteins that enhance the defense reaction. The proteins of the complement system have different effects: They can directly damage the cell walls of pathogens, widen blood vessels and support inflammatory reaction. Many proteins of the complement system can also attract defense cells to the site of the infection and activate scavenger cells, which then dissolve and destroy the germs.

Forms of antibodies

There are different forms of antibodies in the body. They can be grouped in five classes.

Blockbusters: IgG antibodies

Immunoglobulin G (IgG) constitutes the largest part of all antibodies (80%). These antibodies are produced mostly towards the end of a new infection. They are responsible for effectively fighting pathogens that are already known. They can both activate the complement system and mark pathogens by attaching all around the germ and thus making it palatable to the scavenger cells. In pregnant women, IgG antibodies can be passed on from the mother to the unborn child. This protects the child in the first time after birth from the most important pathogens, until its own immune system has become more mature.

In the first line: IgM antibodies

IgM antibodies are very big molecules and are capable of making infected cells lump together quickly. When a pathogen is identified, these are the first antibodies to be produced in order to quickly stop the pathogen from spreading. IgM antibodies are also responsible for incompatibility reactions between different blood groups. In people with the blood group A, for example, antibodies in the blood fluid would immediately attack blood cells from people with blood group B and make them lump together.

The bouncers: IgA antibodies

IgA antibodies are the specialists found in all body fluids like saliva, or mucus in the bronchi or the bowel. They help to defend against germs locally in the typical entry sites like the mouth, lungs or bowel. Some of them are also found in the blood.

Allergy and parasites: IgE antibodies

IgE antibodies play an important role in defending against infections of parasites like worms, for example. They are also responsible for many allergic reactions. They attach to mast cells, a special sub-form of white blood cells. Mast cells are responsible for typical allergic symptoms like reddening and hives on the skin, allergic rhinitis or asthma-like breathing difficulties.

The unknown: IgD antibodies

The function of this class of antibodies is not yet fully known. IgD antibodies are found on the surface of B cells and can also bind to pathogens, so that these can be destroyed by other defense cells.

Messenger substances of the immune system

Besides defense cells, many soluble substances also contribute to the elimination of pathogens. These substances are mainly proteins, for example inflammatory proteins or hormone-like messenger substances in the tissue – so-called cytokines.

The defense cells use cytokines to communicate with one another or with other body cells and thus make sure that infected or cancerous cells are recognized and eliminated. Cytokines are released by many cells of the immune system and they themselves activate many other cells or stimulate them to grow and to mature. Interleukins, interferons and tumor-necrosis factor are some of the best-known cytokines.

Scavenger cells, T and B cells and connective tissue cells of the skin are some of the cells that produce interleukins. By stimulating the multiplication of defense cells and connective tissue cells, interleukins support wound healing. They are also responsible for inducing fever and can attract other immune cells in the tissue. Other functions of interleukins are stimulating T cells to multiply and increasing antibody production in B cells.

Cells that have been infected by a virus release interferons in order to protect neighboring cells from the virus. The neighboring cells bind interferons to their surface, which makes them less susceptible to the virus.

One of the special functions of the so-called tumor necrosis factor is to destroy cancerous body cells.

Interferons are now used as medications, for example in the treatment of some rare forms of leukemia (cancer of the blood) and in different types of viral inflammation of the liver. If the immune system is weakened by another disease, they are also used for protection against infections.