Granulocytes belong to the white blood cells (leukocytes) and are the most important part of the innate immune system and the so-called "first line of defiance". They are characterized by the presence of many granules in their cytoplasm and a polymorphic nucleus (different forms of the nucleus, which is typically lobulated into three segments). They are therefore also referred to as polymorphonuclear leukocytes (PMN, PML or PMNL). This distinguishes them from mononuclear cells, which are also called PBMC (peripheral blood mononuclear cells - T cells, B cells, NK cells and monocytes).
There are three main types of granulocytes, which are distinguished by their appearance under the Wright stain:
- Neutrophil granulocytes
- Eosinophil granulocytes
- Basophilic granulocytes
Neutrophil granulocytes are normally found in the bloodstream and are the main type of phagocyte, accounting for about 50% to 75% of the total circulating leukocytes. One litre of human blood contains about five billion (4-5x109) neutrophils,] which have a diameter of about 6-10 µm. After neutrophil activation, neutrophils take about 30 minutes to leave the bloodstream and travel to the site of infection. After their ‘work’, the neutrophils do not return to the blood; they turn into pus cells and die. Mature neutrophils are smaller than monocytes and have a segmented nucleus with several segments (two to five segments); each segment is connected by chromatin filaments. In contrast to other leukocytes, the average lifespan of inactivated human neutrophils in circulation has been reported to be 5 to 90 hours by different approaches. After migration into the issue, they survive only 24-48 hours. Translated with DeepL.com (free version)
Eosinophils also have kidney-shaped lobed nuclei (two to four lobes). The number of granules in an eosinophil can vary as they tend to degranulate while in the bloodstream]. Eosinophils play a crucial role in killing parasites (e.g. enteric nematodes) as their granules contain a unique, toxic basic protein and cationic protein. Surface receptors on the eosinophils that bind to IgE are used to assist in this task. Eosinophils also have a limited ability to engage in phagocytosis. They are professional APC, they regulate other functions of immune cells (e.g. CD4+ T cell, dendritic cell, B cell, mast cell, neutrophil and basophil functions). Eosinophils can be involved in the destruction of tumour cells and they promote the repair of damaged tissue. Translated with DeepL.com (free version)
The eosinophil granulocytes can be isolated together with the neutrophil granulocytes using the leuko spin density gradient media. (PBMC Spin - Leuko Spin Density Gradient Cascade)
Basophils are one of the smallest cell populations in the bone marrow and blood (they occur in <2% of all leucocytes). Like neutrophils and eosinophils, they have lobulated nuclei. They have only two lobes, and the chromatin filaments that connect them are not very visible. Basophils have receptors that can bind to IgE, IgG, complement and histamine. The cytoplasm of basophils contains a variable number of granules, these granules are usually numerous enough to partially obscure the nucleus. The granule content of basophils is abundant with histamine, heparin, chondroitin sulphate, peroxidase, platelet-activating factor and other substances. Translated with DeepL.com (free version)