The blood and the immune system

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Last Medical Review: March 28, 2020
Medically Reviewed by Dr. Elisabeth Vincent Hamelin


The blood has many tasks

The blood has many important functions in the body, including that

  • transport oxygen from the lungs to the body’s cells, and carbon dioxide from the cells to the lungs
  • bring nutrients and residues to and from the cells
  • participate in the immune system
  • transporting hormones and other neurotransmitters
  • regulate the acidity and content of salts and fluids in the body
  • participate in the regulation of body temperature
  • Make sure the blood is alive if you have a blood vessel damaged.

The heart pumps around the blood in the body’s blood vessels.

By the immune system is meant the body’s defense against bacteria, viruses, cells that have been transformed into cancer cells, foreign cells or particles. They can cause infections or cancer if the immune system does not work properly.

Three different types of blood cells

The blood is called a liquid tissue because the cells of the blood are not joined together but move freely in a liquid.

Almost a tenth of the body weight consists of blood. A person weighing 70 kilos has about five to six liters of blood. The blood consists of blood cells and blood plasma. Almost half of the blood volume is blood cells, or blood cells as they are also called.

There are three different types of blood cells, which have different functions. They are red blood cells, white blood cells and platelets. The various blood cells are formed in the red bone marrow which in adults is found in the sternum, hip bones, skull bone, vertebrae and tubular ends. Examples of tube legs are the femur and the legs of the arms. In children, red bone marrow is also found in the marrow holes of the tubular bone. In the red bone marrow, origin cells or so-called stem cells are first formed, which then mature into different types of blood cells. During growing up, a gradual transformation of a part of the red bone marrow into more fat, yellow bone marrow occurs.

Most red blood cells

There are most red blood cells. In one liter of blood, there are four to five trillion red blood cells. Almost a quarter of all body cells are blood cells. Each red blood cell is very small, only seven thousand millimeters. Mature red blood cells lack nucleus and cell organs, so-called organelles, which are normally found in cells. Therefore, red blood cells have no metabolism unlike other cells.

The red blood cells are round, flat and very malleable. Therefore, they can easily progress into the smallest blood vessels, the so-called capillaries, which are smaller than the red blood cells.

More than two million new red blood cells are formed every second. The formation is controlled by the hormone erythropoietin which is secreted from the kidneys. When there is too little oxygen in the blood, the kidneys feel it and produce more of the hormone. Then more red blood cells are formed that can carry oxygen.

Hemoglobin carries oxygen and carbon dioxide

The red blood cells contain hemoglobin, which binds and transports oxygen and carbon dioxide. The carbon dioxide can also chemically react with water inside the red blood cells and is transported as bicarbonate. Hemoglobin is a protein that contains iron, and it gives the blood the red color. Acid-rich blood is lighter red while oxygen-poor blood is darker red.

As the blood passes through the lungs, it releases the carbon dioxide that is formed in the body’s cells during metabolism. Instead, the blood takes up oxygen, which is then transported to the cells in the body. If there is a lot of carbon dioxide in the blood, it becomes acidic. The red blood cells transport the carbon dioxide to the lungs, where it is removed by breathing. It also helps to regulate how acidic it is in the body, the so-called acid-base balance.

Lives for four months

A red blood cell lives about 120 days. As the blood cell ages, it becomes less malleable and breaks easily as it passes through the splenic capillaries of the spleen. The dying red blood cell is then broken down into the spleen and liver. The iron is taken care of and transported back to the bone marrow to be used to form new red blood cells. The body cannot reuse all residual products, but part of the hemoglobin is converted to the dye bilirubin. It is included in the bile and gives the stools the brown color. A little bilirubin is also excreted in the urine and makes it yellow.

White blood cells

The most important task of the white blood cells is to defend the body against infections. There are several different types of white blood cells. A distinction is made between granulocytes, lymphocytes and monocytes. The white blood cells have cell nucleus and cell organs like normal cells, unlike the red blood cells.

Granulocytes “eat up” bacteria

The granulocytes actually consist of three different types of cells, which have different tasks. The different cells can be separated by containing small grains that are stained differently if treated with a dye in a laboratory.

The white blood cells called neutrophil granulocytes defend the body against bacteria by “eating” them. The white blood cells not only circulate in the blood but can also get out into the tissues during a bacterial attack, for example in the skin. In the tissues, the neutrophil granulocytes move with the help of small “feet” formed by the interior of the cell.

Another type of white blood cell is eosinophilic granulocytes. If you get an allergic reaction or a parasitic disease, the number of eosinophilic granulocytes in the blood increases.

Finally, there are basophilic granulocytes. They contain plenty of histamine that starts the allergic reaction if you are hypersensitive.

In total, there are three to eight billion granulocytes per liter of blood. The neutrophilic granulocytes are most common, while the eosinophilic and basophilic granulocytes are significantly fewer. Granulocytes have a lifespan of a few days.

Lymphocytes destroy invaders

Lymphocytes are another type of white blood cell that forms in the bone marrow. The cells then mature into finished lymphocytes in the so-called lymphatic tissue. Lymphatic tissue is found mainly in the lymphatic organs, that is, in the lymph nodes, the spleen and the bile, or the thymus. Lymphatic tissue is also found elsewhere in the body, including in the tonsils, pharynx and tongue tonsils, and in the walls of the intestines. Mature lymphocytes circulate in the blood, but they are also found in the lymph, which is the fluid that circulates in the lymphatic system.

The lymphocytes are of two types, T and B lymphocytes. The T lymphocytes are most common. They are formed in the bone marrow, but are then brought with the blood to the bumps where they mature. When they come into contact with foreign microorganisms, the T lymphocytes are transformed into so-called killer cells that can harm the invaders. Some T lymphocytes remain as T memory cells, which recognize the substance that triggered the body’s defense response.

B lymphocytes are also formed in the bone marrow. The B lymphocytes are transformed into plasma cells when they come into contact with foreign substances that create a defense response in the body, so-called antigens. The plasma cells form antibodies that fight the antigens. Read more about it in the chapter on the immune system. Also, in B lymphocytes, a small proportion develops into B memory cells.

The lymphocytes live longer than the other blood cells. Some lymphocytes remain throughout their lives.

Monocytes function as a purifier

The monocytes are the largest of the white blood cells. There are about half a billion monocytes per liter of blood. The monocytes are present in the blood for about a day. Then they migrate into the tissues and transform into cells called macrophages. Macrophages mean big eaters. The cells can “eat up” invaders, such as bacteria. They also take care of dead tissue and therefore function as a sort of “cleaning patrol” in the body. The cells also form different substances that activate the T and B lymphocytes. The macrophages can live for several years.

The white blood cells are thus found not only in the bloodstream, but also in the lymphatic system and in the tissues.


Platelets, so-called platelets, are not actual cells but only part of a large cell called megakaryocyte. The megakaryocytes are found in the bone marrow and leave the small cell sections, which then circulate in the blood. A megakaryocyte can form up to 6,000 platelets. The platelets live only a few days.

Although the platelets are not complete cells, they have very important information. They contain various substances needed for the blood to survive in an injury, coagulate. In case of damage to a blood vessel, the platelets clump together and adhere to the walls of the vessel. This creates a plug that stops the bleeding. There are approximately 150-400 billion platelets per liter of blood.

Blood plasma

What remains if you remove all the blood cells from the blood is called blood plasma. Blood plasma is a pale yellow liquid that consists of 90 percent water. The rest are various proteins, salts and some other substances in small amounts.

Substances that dissolve in water, such as hormones and other neurotransmitters, can be transported around the body by means of blood plasma. Nutrients, such as glucose in the form of glucose, are also transported by the blood. Some substances first bind to the proteins in the blood plasma to make transport easier. In this way, even non-water-soluble fats can be exported to the body’s cells.

Blood plasma also contains bicarbonate, which is very important for the blood to maintain a normal level of acidity.

Plasma proteins have different tasks

The proteins in the blood plasma are of different kinds and have different tasks. In total, there are about 70 grams of protein per liter of plasma. Some of the most important plasma proteins are:

  • Albumin, the most common protein, which is designed to prevent the blood in the blood from leaving the bloodstream. If the albumin did not retain the water, it would instead release into the tissues, which would then become swollen. Albumin also transports some hormones and fat.
  • Transferrin, which is an important protein for blood formation because it transports iron to the bone marrow where it is needed to form red blood cells.
  • Immunoglobulins, which are antibodies and are important for the body’s immune system. Read more about it in the next chapter.
  • Coagulation factors, which are necessary for the blood to be able to live so that a bleeding can be stopped. The most important coagulation factor is called fibrinogen. The amount of fibrinogen increases in various inflammatory reactions and is measured by the blood test called lowering. When blood coagulates, fibrinogen is converted to fibrin. If blood cells and fibrin are removed, the remainder is called blood serum.
  • Hormones, which are proteins that regulate many important mechanisms in the body.

The blood contains salts

The salts in the blood are called electrolytes and have many important tasks. The salt that is most abundant is saline, which consists of so-called sodium ions and chloride ions.

The blood plasma also contains salts of potassium, calcium, phosphate and magnesium. Each substance has its own specific task in the body.

The body stops bleeding

In case of injury, such as a cut wound, not only the tissues but also the blood vessels are damaged, and then the bleeding begins. If bleeding was not stopped, large amounts of blood would be lost. Therefore, there is an ingenious system that causes blood to clot or coagulate. 1. The blood vessel is damaged and it begins to bleed.

2. The blood vessel walls contract. It reduces bleeding. Platelets accumulate upon injury until a plug is formed that seals the hole in the blood vessel.

3. Various substances found in the blood cause it to coagulate.

First, the blood vessels contract

In case of injury, the muscle in the blood vessel walls automatically contract. It reduces bleeding. Thereafter, platelets adhere to the damaged vessel wall. In the injury area, the body forms substances that cause the platelets to become sticky. Various substances are released from the platelets, which means that more and more platelets are collected until a plug is formed that seals the hole. The substances released from the platelets also affect the smooth muscle in the walls of the blood vessels so that the vessel is contracted even more.

The blood lives on the injury

In order for the bleeding to be stopped effectively, the blood must also solidify, coagulate. Therefore, there are several different proteins called coagulation factors in the blood. They operate in a specific order. One substance affects the next, which in turn affects the next and so on. The chain reaction is triggered by substances that are released during the damage. The last step in the chain is that the plasma protein fibrinogen is converted to fibrin. Fibrin forms strong strands and strengthens the plug of platelets. It is said that a clot has formed, or that the blood has dissipated. The clot prevents bleeding from continuing until the vessel wall has healed. In order for the blood to thrive, it must contain a certain amount of calcium.

Another system in the blood prevents coagulation from continuing unimpeded. There is always a well-balanced balance between the two systems. Once the vessel wall has healed, the regulatory system dissolves the blood clot that is formed.

Blood groups

Although all people’s blood has about the same content, there is a difference in the type of blood one has.

There are two common systems for dividing the blood groups: the AB0 system and the Rh system.

In addition to these systems, there are hundreds of variants that must be taken into account in some cases.


According to the AB0 system, you can have any of the blood groups

  • A
  • B
  • AB
  • 0 (zero).

What distinguishes the different groups is small proteins, called antigens, that sit on the surface of the blood cells. The antigens can be compared to markers, showing what this type of red blood cell is. If you have blood group A, you have A antigen on the surface of blood cells, while people with blood group B have B antigen. In blood group AB, there is both A and B antigen on the surface of the blood cells. If both antigens are missing, there is a blood group of 0. The different antigens on the surface of the blood cells prevent the blood from being mixed anyway because antibodies are formed that attack the foreign blood.

In addition to antigens on the surface of blood cells, there are also proteins called antibodies in the blood plasma. The antibodies are directed against blood cells with a certain type of antigen on their surface. If you have blood group A, there are antibodies against B blood cells in the plasma. These antibodies are called anti-B. People with blood group B have the antibodies anti-A in their plasma. Blood group AB lacks antibodies directed against A or B blood cells, while people with blood group 0 have both anti-A and anti-B.

Blood group A is most common in USA, followed by blood group 0. Blood groups AB and B are relatively uncommon.

The Rh antigen must also match

There is not only A and B antigen on the surface of the blood cells, but also several other antigens. One of these is called Rh antigen. If you have this antigen you are said to be Rh-positive (Rh +), if you lack the antigen you are Rh-negative (Rh-). Most Europeans are Rh positive. Sometimes the terms D + and D- are used instead of Rh + or Rh- because the D antigen is the most important in the Rh system.

Important with the right blood group for blood transfusion

Because we have different blood groups, you cannot receive blood from anyone. Before a blood transfusion, both the donor’s and the recipient’s blood must be carefully tested. The blood group is determined and furthermore it is checked that the blood of the donor matches with the recipient, so that no reaction between the antigens and the antibodies occurs when the blood from two persons is mixed. If the wrong kind of blood is given to a person, the red blood cells clump and burst, which can be life threatening.

These combinations are possible within the AB0 system:
A can give blood to: A and AB
0 can give blood to: All blood groups
B can give blood to: B and AB
AB can give blood to: AB

These combinations are possible within the Rh system:
Rh + can give blood to: Rh +
Rh can give blood to: Rh + and Rh

In addition, it is also checked that the donor’s blood does not carry any serious infection. In USA tests are made for HIV, hepatitis B and C and syphilis. Before the tests are complete, the blood must not be released from the transfusion blood center.

At the blood center, the blood is separated so that the different types of blood cells can be supplied separately and the blood plasma separately. If you suffer from a major bleeding, you first need red blood cells. In case of even greater bleeding, blood plasma and sometimes platelets are also supplied. Platelets may also need to be given to cancer patients treated with cell toxins. Blood plasma can also be given to patients with severe burns.

Red blood cells of the type 0 Rh- in an emergency can be received by anyone. Therefore, in the operating departments and emergency rooms, there is usually such blood that can be given in really emergency situations before having tested the blood of the donor and the recipient. However, as far as possible, attempts are made to perform the tests to avoid unwanted complications.

The immune system

Without an effective immune system, we can’t do it. The body would quickly be attacked by bacteria, viruses and other foreign organisms with serious infections as a result. In addition, the body would be much more difficult to fight cancer at an early stage.

The description of the various immune reactions has been very simplified here. In fact, these are extremely complicated events with many mechanisms involved.

Both external and internal defense

The body is defended both by an external and an internal infection defense that cooperates. The external defense includes the skin and mucous membranes as well as various types of secretions, such as saliva, sweat, tears and gastric juice. If skin and mucous membranes are undamaged, bacteria and other microorganisms have difficulty penetrating the body. If they still pass and cause an infection, the internal defense enters. This includes the white blood cells and some other cells that also destroy and remove foreign matter as well as dead and erroneous cells, such as cancer cells.

In order for the immune defense cells to find which cells to fight and which to leave, there are markers on the surface of all cells. The markers are called antigens and show whether the cell belongs to the body or if it is foreign. In normal cases, the immune system attacks only foreign cells.

In some disease states, so-called autoimmune diseases, the defense does not function normally but also attacks the body’s own cells. Examples of such diseases are rheumatoid arthritis, certain thyroid disorders, type 1 diabetes and multiple sclerosis, MS.

Different defense reactions

The cells that are part of the internal immune system attack the foreign organisms in different ways. It is usually said that there are various immune reactions. Here belongs

  • inflammation
  • antigen-antibody reactions
  • killer cells
  • large-eating cells, called macrophages.


When tissue, such as a muscle or skin, is damaged, inflammation occurs. The damage can be due to many things. It can be, for example, an inflammation caused by a wound or an overload, or an infection caused by bacteria, viruses or fungi. Inflammation and infection are thus not the same thing. An inflammation can be caused by an infection, but inflammation can also be caused by many other causes.

When you get an inflammation, the diseased area is affected by redness, pain, swelling, heat increase and impaired function.

When tissue is damaged, the small blood vessels, the so-called capillaries, dilate. Therefore, the area becomes red and warm. At the same time, the permeability of the capillaries increases, so that the white blood cells, especially the neutrophil granulocytes, are easier to detect. Also, serum leaks into the tissue which becomes swollen. The swelling causes pain and thus the function is impaired. An area that is swollen and tender is preferably kept in silence. In case of severe inflammation you can also get a fever, as substances from the damaged tissue affect the brain’s temperature center.

Substances from the damaged tissue stimulate the white blood cells to accumulate during the injury to begin fighting the invading bacteria. Neutrophilic granulocytes and large-eating cells formed by monocytes also take care of dead tissue.

Sometimes you can see that it was formed in an infected wound. The formation of war is a result of the effects of the immune system. The yellow-green fluid that is formed mainly consists of dead bacteria, dead broken tissue and white blood cells.

Antibodies defend

Another way of attacking the foreign organisms is the antibodies. B lymphocytes recognize the antigens found on the surface of other cells or on microorganisms and may also bind certain antigens. In that case, the antigen must fit exactly to the lymphocyte, much like the key in a lock. When a foreign antigen mates and binds to the lymphocyte, a complicated course of events begins. On the one hand, other white blood cells are alerted to the invader with the foreign antigen on its surface, and on the other hand, the B lymphocyte is converted into a plasma cell. Plasma cells are specialized cells that can form antibodies. The antibodies are made up of proteins with the ability to fight precisely the antigen that caused the B lymphocyte to go to defense. The antibody binds to the antigen which is thereby destroyed.

If this is the first time the body hits an antigen, it will take some time before it has formed enough antibodies to fight the invader. If the same intruder tries a new attack, there are B lymphocytes in the body that remember the previous event and can be activated quickly. These cells are called memory cells. In some diseases, such as measles, you become immune for the rest of your life and thus cannot get the disease again.

It is also this memory function of B and T lymphocytes that is used in vaccinations. You get vaccinated with a weakened virus that does not cause infection. The vaccination causes the immune system to “remember” and responds quickly so that an infection can be prevented if infected by a real virus. 1. The B lymphocytes have specific recipients, called receptors, that recognize the antigen present on the surface of the bacteria and bind to them, like the key in a lock.

2. Then the B lymphocyte is converted to a plasma cell while alerting other white blood cells to the invaders. Plasma cells form antibodies that bind to the antigen on the bacterium, which is thereby destroyed.

Killer cells

T lymphocytes also have the ability to recognize foreign antigen on the surface of other cells. When the antigen fits and binds to the T lymphocyte, it activates and forms killer cells. The killer cells can destroy cells with the antigen on the surface that activated the T lymphocyte. The killer cells primarily attack cells that have viral antigen on their surface, foreign cells that have been added to the body during a transplant, or cells that have “traced out” and become cancer cells. Here too, memory cells are formed which can be activated quickly if needed.

Large-eating cells

Cells called large eaters, so-called macrophages, are in a way similar to killer cells in that they can destroy other cells and even microorganisms. In contrast, macrophages cannot differentiate between different antigens in the way that lymphocytes can. One type of large-eating cell is formed by the white blood cell monocyte as it exits into the tissues. Neutrophilic granulocytes also function as large eaters. There are also other types of large-eating cells. 1. A particular recipient, known as a receptor, on the T lymphocyte recognizes the antigen found on the surface of virally infected cells and binds them to it. The T lymphocytes are activated and form so-called killer cells that destroy the invaders.2. T-lymphocytes also activate large-eating cells, called macrophages, that harm virus-infected cells by “eating them up”.

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