Anatomy & Organs

Granulocytes – structure, function & diseases

Granulocytes

Granulocytes are blood cells belonging to the leukocyte series . With a proportion of about 50 to 70% of the total number of leukocytes, they are even the most strongly represented fraction of this cell type.

What are granulocytes?

Basically, the granulocytes take over important tasks of the cellular immune system. They are further divided into several subgroups. These result from the microscopic appearance of the individual cells and their respective staining behavior and correlate with their specific functions. Specifically, there are polymorphonuclear neutrophilic granulocytes, which are differentiated into rod-nuclear and segmented neutrophilic granulocytes, as well as eosinophilic and basophilic granulocytes.

All granulocytes are members of the innate immune response system. This means the non-specific control of fungi , bacteria and parasites . In some cases, the granulocytes can even absorb pests phagocytically and render them harmless by destroying them. In adults, they are formed in the bone marrow . This process is technically referred to as granulocytopoiesis and begins with a multipotent hematopoietic stem cell that undergoes various transformation steps and ultimately becomes the corresponding cell type.

Physiologically, only then is the affected granulocyte released into the peripheral blood . If earlier stages of maturation can be detected in the blood, this can be an indication of a serious illness.

Anatomy & Structure

The majority of the granulocytes are made up of about 55 to 65% by the polymorphonuclear neutrophilic granulocytes. They are about 15 µm in size and have a cytoplasm that appears colorless to pale violet in microscopy . They are almost impossible to color with dyes. For this reason, they are also called “neutrophils” – they are neutral towards staining.

Based on the cell nucleus, there is an extended differentiation in the neutrophils: If it is band-shaped and has only minor incisions, then it is a matter of a rod-nucleated neutrophilic granulocyte. However, if there are incisions that cover more than two-thirds of the width of the cell nucleus, then a segmented neutrophilic granulocyte is present. These usually have cell nuclei consisting of two to five segments .

Eosinophils are rather rare, accounting for 2 to 4% of the total number of granulocytes. They closely resemble neutrophils in morphology, but their cytoplasm contains red-orange granules and their nucleus consists of only two segments. The basophils usually only have two core segments. Their cytoplasm contains numerous purple-colored granules. They make up 0 to 1% of the granulocytes.

Function & Tasks

All types of granulocytes are in the service of the immune system . Depending on the subgroup, they fulfill specific functions.

The polymorphonuclear neutrophilic granulocytes are responsible for phagocytosis and for the destruction of microbial pathogens. Half of the neutrophils circulate in the blood, while the other half sit on the walls of small blood vessels so that they are available quickly when needed. If they are activated, they migrate into tissue and exudates and do their job there. Their granules are very important for rendering pathogens harmless: they contain peroxidases and esterases, which have a cytotoxic effect on bacteria and fungi. Eosinophilic granulocytes act as immune regulators.

They are used, for example, when foreign proteins that act as allergens enter the body. They also defend against parasites and break down fibrin if fibrin formation has taken place as part of an inflammatory process. Like the neutrophils, the eosinophilic granulocytes fulfill their tasks primarily in the tissue and in inflammatory exudates .

The basophilic granulocytes become active in the immediate allergic reaction. This is also known as type I allergy and includes, for example, allergic rhinoconjunctivitis in hay fever . When the basophils are stimulated to activity, their granules empty. These are normally filled with mediators such as histamine , heparin , serotonin , prostaglandins and leukotrienes , which mediate the immune response.

Diseases

Changes in the number of granulocytes and their appearance can have congenital and acquired causes. Since the granulocytes are so important for the immune system, such variations sometimes cause drastic problems.

A pathological increase in neutrophilic granulocytes is referred to as neutrophilia . Their absolute number is over 8000 per microliter. Neutrophilia is mainly triggered by three pathomechanisms. On the one hand, there may be increased mobilization of the neutrophilic granulocytes that actually adhere to the vessel walls. This sometimes happens as a result of great stress . On the other hand, it may be that more blood cells are released from the bone marrow, for example as a reaction of the body to acute infections. It is also possible that granulocyte formation in the bone marrow itself is increased.

If the neutrophil count is less than 1500/µL, neutropenia is present. If it is even below 500/µl, there is a particularly critical condition known as agranulocytosis. If the neutrophil count is below 200/µl, there is an acute danger to life, since the immune system cannot then defend itself efficiently. This can be caused by bone marrow failure, allergies or autoimmune diseases .

Elevated eosinophil and basophil counts, i.e. eosinophilia or basophilia, are mostly caused by immune reactions. However, they can also be caused by a malignancy . Since eosinophils and basophils already physiologically make up a very small proportion of the granulocyte count, reductions are difficult to diagnose. For this reason, the neutrophil count is the decisive factor for assessing the granulocytes. Overall, the granulocytes fulfill central tasks of the immune system, which is why deviations urgently require further clarification.

Typical & common blood diseases

Lisa Newlon
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Hello! I am Lisa Newlon, and I am a medical writer and researcher with over 10 years of experience in the healthcare industry. I have a Master’s degree in Medicine, and my deep understanding of medical terminology, practices, and procedures has made me a trusted source of information in the medical world.