Leukocytes are also known as “white blood cells”. They make up the majority of the immune cells in the human body and, measured as a blood value, are an important benchmark in medical diagnostics and therapy.

According to their place of formation and their function, a distinction is made between different subclasses of leukocytes, which can also be measured individually as a so-called “differential blood count” in the laboratory. The granulocytes make up a large part, which in turn are largely divided according to their staining behavior into neutrophils, eosinophils and basophils granulocytes and are particularly important 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 marrow”), and T lymphocytes, which are produced in the thymus (hence “T”). The thymus is an important human organ with a poor lobby – hardly known to anyone outside of the medical profession – which is located in the upper chest behind the breastbone.

In childhood, the thymus is the site of formation of those T-cells, which in turn are specialized as T-killer cells or T-helper cells and play an important role in the defense against viruses and the formation of the immunological memory ( childhood diseases , vaccinations, etc.). In adulthood, the thymus degenerates more and more and turns into a functionless fat body – maybe that’s why it’s 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 called monocytes, when they then escape into the tissue, they take up their main work as macrophages or giant scavenger cells and lurk everywhere in the tissue of the skin , intestines , lungs and the rest of the body for pathogens and foreign material.

The natural killer cells (NK cells), which play a role in the defense against viruses and tumor cells, have a nice name . Dendritic cells and mast cells also belong to the leukocytes, but strictly speaking they are not found in the blood but in surface tissues such as skin and intestines and are therefore part of the outermost line of defense of the innate immune system.

Measure blood values, blood test & leukocytes

The creation of the blood count was originally carried out under the microscope and initially separates red blood cells (erythrocytes) from leukocytes and blood platelets (thrombocytes). 

This is quite simplistic since erythrocytes are actually red due to their hemoglobin content and leukocytes are not, while platelets are much smaller and have a distinctive shape. Furthermore, this “complete blood count” can be further subdivided by creating a “differential blood count” in which the individual leukocyte subclasses are then broken down individually. For this purpose, various stains can be carried out, which then show the leukocytes in various shades of red, blue and violet.

Nowadays, at least the “complete blood picture” is made by machine. There are also automated procedures for the “Diff-BB”, but due to the greater susceptibility to errors and measurement inaccuracies, the laboratory doctor often looks through the microscope himself.

The normal values ​​for leukocytes in general are 4000-10000/microliter, of which 50-75 percent are neutrophils, 20-45 percent are lymphocytes, 2-8 percent are monocytes, 2-5 percent are eosinophils and 0-1 percent are basophils (memory: ” N ever l et monkeys e at bananas “ ).

Function, effect & tasks

The function of the leukocytes can basically be summarized as the “body’s defense system”. The cells patrol the blood and, if necessary, migrate into the tissue, either to replace outdated “sentinel cells” (eg the dendritic cells) there or, in the event of an acute need, attracted by messenger substances.

In concrete terms, a defense reaction could look like this: A pathogen penetrates through a wound in the skin and is eaten by the macrophages that are constantly settling there. The phagocyte releases messenger substances and uses them to ensure that other defense cells are lured to the scene – there could be other pathogens there. If the pathogen is a bacterium, it is mainly neutrophils that then migrate and eat everything foreign that gets in their way.

If it is a virus, T-lymphocytes are attracted. Some of these can become active themselves as scavenger cells 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 similar to the original pathogen and thus inactivate and destroy the virus Prepare phagocytes for feeding.

Diseases

A measurement of the leukocyte value is actually part of every decent basic diagnosis when admitted to hospital or in the outpatient medical field. An increase in the leukocyte count can be an indication of an infection that the body is dealing with at the time. 

If you then add a differential blood count, you can possibly use the increase in neutrophils or lymphocytes to make a rough assessment of whether it could be a bacterial or a viral infection.

However, this is very imprecise and merely the starting point or reason for further diagnostics. In the case of severe blood poisoning or special individual infections, the leukocyte count can sometimes be reduced.

If the leukocyte count is increased in an obtrusive manner, this can be an expression of leukemia . In many cases, this is the first indication of blood cancer when the person concerned still feels relatively healthy. Here, too, the Diff-BB provides information about the origin and type of leukemia. And here, too, the whole thing is often ambiguous and there are many leukemias in which the leukocyte value is normal or slightly reduced.

The scene of another disease are the leukocytes in the case of infection with the human immunodeficiency virus ( HIV ): Here the T helper cells in particular are infected by the virus and become inoperable as a result. Since the disease lies dormant in the body for years without any external signs before the full picture of AIDS breaks out, measuring the T cells plays an important role here in order to be able to assess the progress of the disease and the success of the therapy.