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antibody

What are antibodies?

Antibodies - also known as immunoglobulins or Ak for short - are important components of the body's own defense system, which are formed by B cells or plasma cells, a subclass of lymphocytes.

It is a group of proteins formed by the human organism that serve to defend against alien material. Normally, this foreign material corresponds to pathogens such as bacteria, viruses or fungi. However, components of the red blood cells, the erythrocytes, can also be detected and eliminated. A pathological immune response can be found, for example, in an allergic reaction or in an autoimmune disease.

Depending on their function and place of production in the body they can be divided into five classes: IgA, IgG, IgM, IgE, IgD.Ig stands for immunoglobulin. This refers to a group of proteins in which the antibodies also fall. The antibodies belong to the specific immune system. This means that the antibodies are only responsible for a particular antigen. In contrast, the blood cells are part of the cellular immune system, the non-specific immune response. More specifically, the antibodies are produced by B lymphocytes, a subset of leukocytes. The antibodies are able to recognize and bind antigens. The antigens are located on the surface of the material to be eliminated. Each antibody has a specific binding site for a given antigen. This allows each antibody to recognize and eliminate a particular antigen, and the variety of antibodies is accordingly very large. Immunodeficiencies may result in decreased formation of one or more antibodies.

introduction

Antibodies are proteins that are composed of four different amino acid chains: two identical light and two identical heavy chains, but each antibody is different and individual and has a highly specific role in the immune system.

Each antibody formed can only recognize very specific structures as an antigen, bind (key lock principle) and fight, so that for each foreign substance and each pathogen that afflicts the body, specific antibodies are formed and present in the blood or in other body fluids are.

The antibodies acquire this specialization already during their formation by the B-cells / plasma cells: the latter comes in the context of the immune response in contact with an antigen (eg pathogens such as bacteria or viruses) or by other immune cells (T cells), the one Had antigen contact, so they immediately start to produce antibodies that have exactly the binding site necessary to catch the antigens from the blood.

When finished, they are released into the blood by the B-cells, where they then search for "their" antigen in order to bind it and thereby make it accessible to other immune cells, such as the phagocytes, for destruction.

The body's own antibodies of the immune system are subdivided into 5 subclasses, the immunoglobulins G, M, A, E, and D.

Artificially produced or derived from animals antibodies can be supplied to the body but also from the outside, for example as part of a therapy for disorders with disturbed or lacking immune system, as a passive vaccine against various pathogens or various cancers.

Structure of the antibodies

The structure of each antibody is usually the same and consists of four different amino acid chains (amino acids are the smallest building blocks of proteins), two of which are called heavy and two light chains. The two light chains and the two heavy chains are completely identical and are interconnected by molecular bridges (disulfide bridges) and brought into the characteristic Ypsilonform an antibody.

The light and heavy chains consist of constant amino acid sections that are the same for all different antibody classes and variable sections that differ from antibody to antibody (IgG therefore has a variable section other than IgE).

The variable domains of the light and heavy chains together form the respective specific binding site for the antibodies matching the antibodies (any structures or substances in the body).

In the region of the constant part, there is a second binding site (Fc part) in each individual antibody, which, however, is not intended for an antigen, but rather a binding site with which it binds to specific cells of the immune system and activates their function can.

Tasks of antibodies

Antibodies are proteins, ie proteins, built-up structures that are formed by the immune system. They serve the recognition and binding of foreign cell structures.

They look like a "Y". With the two short, upper arms they can bind the foreign cells. Either they use both or only one arm. If you use only one arm, you can use the other arm to bind to another antibody. If this happens with several antibodies, they clump into a pile and can be eaten by macrophages. The macrophages then break down these clusters and destroy the foreign cells.

If they use both upper arms, they can bind with their lower arm directly to other cells of the immune system, such as T helper cells. The T helper cells then take the antibodies in, degrade them and incorporate the foreign cell components into their own membrane. In this way they mediate as information cells for other immune cells. Roughly help antibodies thereby to recognize foreign cells and allow to destroy by other cells. So they serve as a kind of link between the immune cells.

Antibodies in the blood

If a pathogen or other foreign substance (antigen) enters the human body (via, for example, the skin or the mucous membranes), it is first recognized and bound by the "superficial" defense cells of the immune system (so-called dendritic cells) in order to subsequently enter the human body to wander the lower-lying lymph nodes. There, the dendritic cells show the antigen the so-called T lymphocytes, a class of white blood cells. These are thereby awakened to "helper cells" and in turn activate the B lymphocytes, which begin immediately with the production of antibodies that are suitable for the respective antigen to be rendered harmless. These antibodies are released into the circulating blood after complete formation, so that they can reach the physiological blood circulation to all parts of the body.

Another possibility of B-cell activation is the direct contact of a B-cell floating in the blood with the pathogen or foreign substance, without prior activation by a T-cell. The antibodies released into the blood (also called immunoglobulins) can generally be divided into different classes (IgG, IgM, IgA, IgD and IgE) and can be determined by blood sampling and subsequent laboratory medical examinations.

What are antigens?

Antigens are structures or substances on the surface of cells in the human body. They are mostly proteins, but can also be fats, carbohydrates or even completely different compositions.

Either these are the body's own structures, which under normal circumstances are always present in the human body, or foreign structures or substances that have entered the body but do not actually belong there.

These foreign antigens are usually recognized by the B- or T-lymphocytes of the immune system and made by specific antibodies that have been previously formed by the B-lymphocytes bound and rendered harmless. From the beginning, the immune system learns to differentiate the body's own structures from those outside the body, so that under healthy circumstances only foreign antigens are combated. However, if the immune system incorrectly identifies and combats the body's own innocuous structures as foreign antigens, this pathological process is called an autoimmune reaction from which autoimmune diseases can arise.

Function of antibodies

The main purpose of the antibodies is to detect, bind and destroy pathogens or foreign substances or substances that have entered the body.

The protein molecules produced by the B lymphocytes (a specific sub-type of white blood cells) can be subdivided into different antibody classes, each of which has different tasks and properties and sometimes also has its main site of action in different parts of the body.

When the pathogen or foreign molecule (antigen) in the body is recognized by the defense system, the B cells immediately begin to produce the appropriate antibody, which subsequently docks to the target structure with its one junction and to other defense cells at the other junction of the body (eg macrophages = phagocytes).

These are then activated and ingest the antibody-antigen complexes, thereby rendering the foreign substances or the pathogens harmless.

Antibody screening

The Antibody Screening Test (AKS) is a test in laboratory medicine in which the patient's blood serum is screened for certain antibodies directed against specific structures (antigens) on the red blood cell membrane (erythrocytes). One distinguishes between regular and irregular antibodies against the red blood cells: the regular ones are the so-called anti-A and anti-B antibodies, the anti-A antibody is present in patients who have blood group B, the anti-B antibody accordingly in patients with blood group A. Among the irregular antibodies is, inter alia, the anti-D antibody, which is directed against Rhesus factor-D.

To find the regular and irregular antibodies in the blood serum of the patient, the patient serum is added to the blood after taking the corresponding antigens, so that it comes in the case of existing antibodies to a clumping reaction of the blood: the test is then rated as positive. The antibody screening test is primarily used to prepare for upcoming blood transfusions and screening for pregnancy. In everyday clinical practice, the term "antibody screening test" is also generally used for the determination of antibodies in the context of, for example, infectious or autoimmune diseases, but should not be confused with the actual meaning as described above.

Antibody treatment

As described above, antibodies actually serve to protect against diseases, so they are part of the immune system. However, some diseases, such as cancer, can not fight our immune system alone because it's not quick and effective enough.

Through many years of research, antibodies have been found for some of these diseases that can be produced biotechnologically and then given to the patient as a drug, for example cancer patients. This brings huge benefits. While chemotherapy or radiation therapy attacks the entire body and destroys all cells, including healthy cells, antibodies only have a specific effect on the cancer cells.

This specificity is in the nature of the antibodies. Antibodies are proteins that are normally produced by cells of the immune system. But before these cells of the immune system, the plasma cells, can do this, they must have come into contact with the foreign cells once. Instead, they absorb the foreign cells, degrade them, and recognize superficial structures that "identify" the cells as if they were an ID card. Against these superficial structures, also called surface markers, the antibodies are then formed.

This principle has been exploited in research. Cancer cells have been screened for surface markers found only on the cancerous cells but not on the body's own cells. Antibodies were then formed against these markers which can be given to the patient in the form of antibody treatment. The antibodies then bind in the body to the cancer cells and thus help the body's immune system to recognize the malignant cells and kill them.

Thus, the antibody rituximab in certain types of leukemia and non-Hodgkin's lymphoma and the antibody trastuzumab against breast cancer cells and some gastric cancer cells. In addition to these relatively "disease-specific antibodies, " there are also those that, for example, inhibit the growth of new blood vessels and thus prevent the cancer from being supplied with nutrients from the blood. One such antibody would be bevacizumab. It can be used on many different cancers.

Immunoglobulins IgG, IgM, IgA, IgE

The antibodies produced by B lymphocytes, also called immunoglobulins, can generally be classified into 5 subclasses: immunoglobulin M (IgM), immunoglobulin G (IgG), immunoglobulin A (IgA), immunoglobulin E (IgE) and immunoglobulin D (IgD ).

The various antibody subclasses have different tasks in the immune system and also differ in the main abode (free, dissolved in the blood or in other body fluids and on the membrane of defense cells).

Type A

IgA is found mainly in body fluids and on mucous membranes. Important to mention here are the oral mucosa and saliva, mucous membrane of the respiratory tract, mucous membrane of the gastrointestinal tract and gastric juice and the vaginal mucosa. The IgA prevents pathogens from entering the organism through intact mucous membranes. This function is particularly important in the non-sterile areas of the body as well as the body openings, which are in constant contact with the environment, eg mouth and nose. In addition, the IgA is involved in the pathogens that we take daily with the food, liquid or breathing air eliminate. IgA is also found in breast milk. By breastfeeding so the mother's antibodies are transmitted to the child and thus ensure the immunity of the child to pathogens, without the infant has come into contact with the pathogen. This mechanism is known as nest protection.

Type D

Immunoglobulins of type D are also almost not free in the blood plasma. Rather, they occur bound to the membrane of B lymphocytes, where they form a type of receptor for certain antigens that stimulate the B cells to further produce antibodies.

Type E

IgE is of particular importance in the development of allergies. IgE is produced by B lymphocytes on first contact with an allergen, such as pollen in hay fever. Once the IgE is formed, renewed contact with the inhaled pollen leads to an allergic reaction. The IgE stimulates histamine-containing mast cells, releasing the histamine.

Depending on the severity of the reaction and the location of the allergen, histamine symptoms may be present. The symptoms of hay fever can be burning, itchy eyes, a runny, itchy nose or shortness of breath. In the worst case, the allergic reaction will result in anaphylactic shock characterized by respiratory distress, respiratory tract swelling, hypotension as a sign of shock and unconsciousness. This is a medical emergency and requires immediate medical attention. The allergic symptoms can be alleviated by histamine blockers. These block the receptors for histamine, so that the effect of histamine after release fails. One of the major side effects of histamine blockers is fatigue.

Another task of IgE antibodies is the elimination of parasites.

Type G

The amount of IgG is the largest among antibodies. The IgG is formed during the course of the infection, so it is part of the late immune response. In the presence of IgG in the blood can be concluded that an expired or just decaying infection, the full immunity is ensured by the IgG. The fact that the immune system "remembers" its antibodies produced in the case of reinfection with the same pathogen, the antibody can be quickly reproduced and it does not come to the onset of infection with disease signs.

The special thing about the IgG is that this antibody is placental. Thus, the unborn child can receive IgG antibodies from the mother and is immune to pathogens without being in contact with them. This is called nest protection. However, rhesus antibodies are also IgG antibodies and therefore day-to-day. Thus, if a rhesus-negative mother has antibodies against the Rhesus factor by Rhesus-positive erythrocytes of the child, these antibodies can be transmitted to the child in the subsequent pregnancy and destroy the erythrocytes of the child. This leads to the decay of the erythrocytes, also known as hemolysis, which leads to anemia (anemia) in the child. The disease in the infant is called haemolytic neonatorum. In rhesus-negative mothers with a rhesus-positive father, passive immunization with anti-D antibodies (rhesus prophylaxis) may be performed during pregnancy.

Type M

The IgM (immunoglobulin M) is the structurally largest antibody. It is formed in new infections and is involved in rapidly eliminating pathogens and preventing their spread. IgM antibodies in the blood indicate a recent, fresh infection.

The IgM antibody also has a binding site for other systems of the immune system. Thus, a part of the complement system, which consists of about twenty proteins and also serves to defend against infection, can bind to the antibody-antigen complex. This activates the complement system. The antibodies against a foreign blood group, which are formed for example in a blood transfusion with the wrong blood group, are also IgM antibodies. These lead to a reaction to the foreign blood and cause a thickening of the blood (coagulation). This can have serious consequences for the person affected and can even end in a very short time. Therefore, before blood transfusion care must always be taken to ensure that the donor and recipient's blood types match up. This is guaranteed by the so-called "bedside test", in which the blood of the donor is mixed with that of the recipient immediately before the transfusion and is observed. If no reaction occurs, the blood can be transfused.

Autoantibodies

Auto-antibodies are antibodies that the body produces to recognize and bind endogenous cells to tissues, hormones or other antibodies. By binding the auto-antibodies to these structures, the immune system is activated and fights these structures.

Auto-antibodies are formed in the context of autoimmune diseases. Auto-antibodies therefore do not help our immune system to remove foreign bacteria or viruses from our body, as normal antibodies do, but attack our own body. Whenever the immune system forms auto-antibodies against one's own body, it is highly pathological and leads to the destruction of actually healthy tissue.
This destruction in turn entails the loss of tasks that the tissue should actually take over. So the immune system makes the body sick instead of keeping it healthy and functional. There are many different auto-antibodies known that trigger different diseases depending on the structure they are attacking. Examples of such diseases include diabetes mellitus type I, which may be caused by four different auto-antibodies. But also lupus erythematosus or rheumatoid arthritis are caused by autoantibodies.

Hashimotoerkrankung

Since Hashimoto's thyroiditis is one of the autoimmune diseases, the blood serum of the affected patient usually has antibodies specific for the disease, which can be determined by blood sampling and laboratory testing and measured in their present amount. On the one hand, this serves to diagnose the Hashimoto's disease, if at first only suspicion exists. On the other hand, this also serves to monitor the progress and to monitor an already diagnosed existing Hashimoto thyroiditis.

The characteristic antibodies in this disease are the so-called thyroglobulin antibodies (Tg-Ab) and the thyroid peroxidase antibodies (TPO-Ab). The Tg antibodies are directed against the Thyroglobulin of the thyroid, a protein that is formed by the thyroid cells and with the help of which the thyroid hormones are stored before being released into the blood.

The TPO antibodies, however, are directed against the thyroid enzyme thyroid peroxidase, which is involved in the formation of thyroid hormones. In about 10-20% of Hashimoto-affected patients, these antibodies are not found in the blood, although there is a Hashimoto's disease.

Unlike Basedow's thyroid disease, it is not believed that these autoantibodies to the thyroid tissue in Hashimoto's disease are responsible for the damage or destruction of the thyroid gland, as these are often only increased in phases and the level of antibody levels also not correlated with the disease intensity.

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