Platelets, erythrocytes, and leukocytes together make up the blood cells. They perform functions in blood clotting, oxygen transport, and immunological processes. In diseases such as leukemia, white blood cells change into tumor cells and spread throughout the body.
What are blood cells?
Blood cells or hemocytes are all cells found in the blood of a living being. In vertebrate blood, the subgroups of platelets, erythrocytes, and leukocytes make up the totality of all cells in the blood. Leukocytes are all blood cells with a nucleus. They can be further divided into granulocytes, lymphocytes, macrophages and megakaryocytes. They play an important role in the immune system. The erythrocytes transport oxygen and thus serve as a transport medium in lung respiration. The platelets provide wound closure. All blood cells arise in hematopoiesis from stem cells such as those found in the bone marrow. They are involved in the formation of new blood. Billions of new blood cells are formed in humans every day because the life span of erythrocytes and platelets is limited.
Anatomy and structure
Erythrocytes are disc-shaped. They carry glycoproteins on the cell surface. They are composed of a network of spectrin filaments. Their limbs are connected by actin filaments, and they are highly deformable. About 90 percent of them consist of hemoglobin, which gives blood its red color. There are about 24 to 30 trillion of them in the blood. Platelets number between 150,000 and 380,000 in each µl of blood. They have mitochondria and a special form of rough endoplasmic reticulum, also known as the canalicular system. Their plasma membrane contains the protein tissue factor. Leukocytes number about 4,000 to 10,000 in each µl of blood. The subgroups of leukocytes differ anatomically. Granulocytes, for example, have a collapsed nucleus and carry tiny particles in their cytoplasm.
Function and tasks
Erythrocytes provide oxygen transport in the blood system. In the capillaries of the lungs, they take up oxygen and transport it as a transport medium to the individual organs, where they release it again. Inside them is hemoglobin, which is capable of binding oxygen. In part, they transport carbon dioxide out of the cells and thus support cellular respiration. Erythrocytes are produced again and again in the so-called red bone marrow, since their life span is limited to four months. Production is controlled by the hormone EPO, which is formed in the kidneys. In the case of platelets, the hormone thrombopoietin is involved in their formation. Platelets play a major role in blood clotting. During wound closure, they change their disc shape due to the action of substances such as ADP, collagen and thrombin, thus increasing their surface area. Due to fibrin-mediated binding, platelets bind to each other during blood clotting, thus closing the wound. Red blood cells live for eight to twelve days. Leukocytes or white blood cells are involved in immunological processes. The individual subgroups of leukocytes have different tasks in these processes. They move through the blood system along the tissue and scan this tissue to detect harmful changes. For example, they detect cancer cells or the invasion of parasites. Granulocytes are involved in allergic reactions and ward off viruses, bacteria and fungi. They trigger itching and inflammatory processes. Some leukocytes mark antigens and initiate an immune response. The B-cell group of white blood cells, on the other hand, specializes in producing antibodies. The T-cell group slows down the immune response when needed, but also activates killer cells that attack tumor cells and infected cells.
Diseases
Several diseases can affect the function of blood cells. These include anemia, such as can occur in the form of iron deficiency. The number of erythrocytes is greatly reduced in anemia. In polyglobulia, on the other hand, there are too many erythrocytes in the bloodstream. The blood becomes thick and the risk of thrombosis increases. On the other hand, if erythrocytes are broken down more, jaundice occurs and pigment stones form in the bile. With regard to erythrocytes, however, mutations can also occur.In sickle cell anemia, for example, erythrocytes mutate into a sickle shape so that they can no longer travel through small capillaries. Leukocytes can also be affected by disease. In leukemia, for example, subsets of leukocytes become tumor cells. At the level of the bone marrow, tumor cells thereby come into contact with blood and spread throughout the body via the blood system. The mutated leukocytes inhibit the formation of the other blood cells, so that there is a tendency to bleed. Because they are flushed throughout the organ system, they cause symptoms in every organ system. In HIV, on the other hand, the number of T-helper cells decreases, i.e. the total number of leukocytes decreases. In the final stage, this leads to a complete breakdown of the immune system. An abnormal number of platelets, on the other hand, can occur in the context of syndromes such as Gaucher’s disease, TAR syndrome or Jacobsen’s syndrome. Abnormal concentrations may also occur when the spleen is removed, as this organ is responsible for the breakdown of platelets. Platelet deficiency is also known as thrombocytopenia. In thrombocytopathies, on the other hand, platelet function is impaired.
