Granulocytes are blood cells that belong to the leukocyte series. In fact, they are the most represented fraction of this cell type, accounting for about 50% to 70% of total leukocytes.
What are granulocytes?
Basically, granulocytes perform important tasks in cellular immune defense. They are further subdivided into several subgroups. These result from the microscopic appearance of the individual cells as well as their respective staining behavior and correlate with their specific functions. In detail, there are polymorphonuclear neutrophil granulocytes, which are differentiated into rod-nuclear as well as segment-nuclear neutrophil granulocytes, and eosinophil and basophil granulocytes. All granulocytes are members of the innate immune response system. This is understood as the non-specific fight against fungi, bacteria and parasites. In some cases, granulocytes can even phagocytose pests and render them harmless by destroying them. In adults, their formation takes place in the bone marrow. This process is technically called granulocytopoiesis and begins with a multipotent hematopoietic stem cell, which undergoes various transformation steps and ultimately becomes the corresponding cell type. Physiologically, only then is the relevant granulocyte released into the peripheral blood. If earlier maturation stages are detectable in the blood, this may indicate serious disease.
Anatomy and structure
Polymorphonuclear neutrophil granulocytes make up the majority of granulocytes, approximately 55 to 65%. They are about 15µm in size and have a cytoplasm that appears colorless to pale purple under microscopy. They are almost impossible to stain with dyes. For this reason they also bear the name “neutrophils” – they are neutral to staining. On the basis of the cell nucleus, neutrophils can be further differentiated: If the nucleus is ribbon-shaped and has only minor incisions, it is a rod-nucleated neutrophil granulocyte. If, however, incisions are found that account for more than two-thirds of the width of the nucleus, then a segment-nucleated neutrophil granulocyte is present. These usually have nuclei consisting of two to five segments. Eosinophils are rather rare, accounting for 2 to 4% of the total number of granulocytes. In their morphology they strongly resemble neutrophils, but their cytoplasm contains red-orange granules and their nucleus consists of only two segments. Basophils also usually possess only two nuclear segments. Their cytoplasm contains numerous purple granules. They account for 0 to 1% of granulocytes.
Function and tasks
All types of granulocytes are in the service of immune defense. They perform specific functions in this regard, depending on the subgroup. The polymorphonuclear neutrophil granulocytes are responsible for phagocytosis as well as destruction of microbial pathogens. To ensure that they are quickly available when needed, half of the neutrophils circulate in the blood, while the other half are located on the walls of small blood vessels. When activated, they migrate into tissues and exudates to perform their task. Their granules are very important for rendering pathogens harmless: these contain peroxidases and esterases that have a cytotoxic effect on bacteria and fungi. Eosinophil granulocytes act as immune regulators. They are used, for example, when foreign proteins that act as allergens enter the body. Furthermore, they take over the defense against parasites as well as the fibrin degradation, if a fibrin formation has taken place in the context of inflammatory events. Like the neutrophils, the eosinophilic granulocytes fulfill their tasks primarily in tissue and in inflammatory exudates. The basophilic granulocytes become active during the immediate allergic reaction. This is also referred to as type I allergy and includes, for example, allergic rhinoconjunctivitis in hay fever. When the basophils are stimulated into activity, their granules empty. These are normally filled with mediators such as histamine, heparin, serotonin, prostaglandins, and leukotrienes, which mediate immunologic defense processes.
Diseases
Changes in granulocyte numbers as well as their appearance can have congenital and acquired causes. Because granulocytes are so important for immune defense, such variations sometimes cause drastic problems.A pathological increase of neutrophil granulocytes is called neutrophilia. Here, their absolute number is above 8000 per microliter. Neutrophilia is mainly caused by three pathomechanisms. First, there may be an increased mobilization of the neutrophil granulocytes that are actually adherent to the vessel walls. This occasionally occurs 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. In addition, it is possible that granulocyte production in the bone marrow is increased per se. If the neutrophil count is below 1500/µl, neutropenia is present. If it is even below 500/µl, a particularly critical condition known as agranulocytosis is present. If the neutrophil count is below 200/µl, there is an acute danger to life, as no efficient immune defense can then take place. This can be caused by bone marrow failure, allergies or autoimmune diseases. Elevated eosinophil and basophil counts, i.e. eosino- and basophilia, respectively, are usually 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 determining factor for the assessment of granulocytes. Overall, granulocytes perform central immune defense tasks, and therefore abnormalities urgently require further clarification.
Typical and common blood disorders
- Acute lymphoblastic leukemia
- Acute myeloid leukemia
- Chronic lymphocytic leukemia
- Chronic myeloid leukemia
- Blood poisoning
