Leukocytes are one of the three important cell series of human blood, along with erythrocytes and platelets. As a component of our immune system, they are responsible for the defense against pathogens and carry out this activity far beyond the boundaries of the blood vessels. Leukocyte is therefore not leukocyte – there are a whole lot of colorful subtypes.
What are leukocytes?
A blood test is used by doctors to further diagnose various diseases. Leukocytes are also called “white blood cells.” They make up the majority of immune cells in the human body and, when measured as a blood value, are an important benchmark in medical diagnosis and therapy. A distinction is made between different subclasses of leukocytes according to their place of formation and function, which can also be measured individually in the laboratory as a so-called “differential blood count“. Granulocytes make up a large part of the leukocytes, which in turn are divided into neutrophils, eosinophils and basophilic granulocytes largely according to their staining behavior and are important primarily for the defense against bacteria and parasites, but also play a role in the development of allergies and autoimmune diseases. They can also be counted as part of the non-specific innate immune system and are formed in the bone marrow. Another important group are the lymphocytes, which belong to the specific immune system. Here again, a distinction is made between B lymphocytes, which also come from the bone marrow (“B” for “bone mark”), and T lymphocytes, which are produced in the thymus (hence “T”). The thymus is an important human organ with a bad lobby – hardly anyone outside the medical profession knows about it – located in the upper chest behind the breastbone. In childhood, the thymus is the site of formation of those T-cells, which in turn become specialized as T-killer cells or T-helper cells and play an important role in the defense against viruses and the formation of immunological memory (childhood diseases, vaccinations, etc.). In adulthood, the thymus degenerates more and more and transforms into a functionless fat body – perhaps that is why it is so little known. Meanwhile, the B lymphocytes from the bone marrow are the cells that produce the antibodies and thus carry the specific immune defense of the human organism. Another very important group of leukocytes: the macrophages. In the blood, they are initially still called monocytes, but when they emerge into the tissues, they take up their main work as macrophages or giant phagocytes, lurking everywhere in the tissues of the skin, intestines, lungs and the rest of the body for pathogens and foreign material. Meanwhile, Natural Killer Cells (NK cells), which play a role in defending against viruses and tumor cells, have a nice name. Dendritic cells and mast cells are also among the leukocytes, but strictly speaking they are not found in the blood but in surface tissues such as the skin and intestines, and thus belong to the outermost line of defense of the innate immune system.
Measuring blood values, blood testing and leukocytes
Blood work was originally done under a microscope and first separates red blood cells (erythrocytes) from leukocytes and platelets (thrombocytes). This is quite simple because erythrocytes are actually red due to their hemoglobin content and leukocytes are not, while platelets are much smaller and have a characteristic shape. Furthermore, one can further subdivide this “Big Blood Count” by doing a “Differential Blood Count” in which the individual leukocyte subclasses are then broken down individually. For this purpose, various stains can be performed, which then show the leukocytes in different shades of red-blue-violet. Nowadays, at least the “complete blood count” is done by machine. There are also automated procedures for the “Diff-BB”, but because of the greater susceptibility to error and measurement inaccuracies, the laboratory physician often still looks through the microscope himself. The standard values for leukocytes in general are 4000-10000/microliter, of which 50-75 percent are neutrophils, 20-45 percent lymphocytes, 2-8 percent monocytes, 2-5 percent eosinophils, and 0-1 percent basophils (mnemonic: “Never let monkeys eat bananas”).
Function, action, and tasks
The function of leukocytes can be basically summarized as “the body’s defense system.”The cells patrol the blood and migrate into the tissues as needed, either to replace obsolete “sentinel cells” (e.g., the dendritic cells) or in acute need, attracted by messenger substances. In concrete terms, a defense reaction could look like this: A pathogen enters through a wound in the skin and is eaten by macrophages that are permanently located there. The macrophage releases messenger substances and uses them to attract other immune cells to the site of the incident – there could be other pathogens there. If the pathogen is a bacterium, it is mainly neutrophils that migrate and eat everything that gets in their way. If it is a virus, T lymphocytes are attracted. Some of these can become active as phagocytes themselves or help (as “T helper cells”) the B cells to produce antibodies, which are then spread in the blood and on mucous membranes and mark any foreign particle resembling the original pathogen, thereby inactivating it and preparing it for the phagocytes to eat.
Diseases
A measurement of the leukocyte count is actually part of every decent basic diagnostic procedure during hospital admission or in the outpatient medical setting. An increase in the leukocyte count can be an indication of an infection, which the body is dealing with at the time. If a differential blood count is also taken, the increase in neutrophils or lymphocytes may also provide a rough estimate of whether the infection is bacterial or viral. However, this is very imprecise and only a starting point or reason for further diagnostics. In the case of severe blood poisoning or specific individual infections, the leukocyte count may also be reduced at times. If the leukocyte count is obstrusely increased, this may be an expression of leukemia. In many cases this is the first indication of a blood cancer, if the person concerned still feels relatively healthy. Here, too, the Diff-BB gives clues about the origin and type of leukemia. And again, the whole thing is often ambiguous and there are many leukemias in which the leukocyte value is normal or slightly decreased. The leukocytes are also affected by the human immunodeficiency virus (HIV). In this case, the T-helper cells in particular are affected by the virus and become non-functional. Since the disease lies dormant in the body for years without external signs before the full-blown AIDS breaks out, measurement of T cells plays an important role here in assessing the progress of the disease and the success of therapy.
