Granulocytic cells

Introdction to Granulocytes:

Granulocytes are a type of leukocyte, or white blood cell. These cells are part of the immune system, and are involved in several different types of immune reaction. There are three types of granulocytes: neutrophils, basophils, and eosinophils.[1]

All three types of granulocyte develop and differentiate in the marrow of long bones such as the femur. All granulocytes start as myeloblasts, immature cells type which has the potential to differentiate into any type of granulocytic cell. Each cell progresses through several stages before emerging from the bone marrow as a mature granulocyte. In the blood, mature neutrophils account for around 55% of white blood cells, while 3% to 6% are eosinophils, and less than 0.5% are basophils. The proportions of these cell types in the blood are fairly constant; measuring blood granulocyte count is therefore a useful tool in diagnosing many types of disease.[1]

As mature white blood cells, granulocytes circulate in the blood until they receive chemical signals that provide further instructions. These signals come from sites of infection orinflammation, and may be chemicals secreted by pathogens or by other cells of the immune system. Each cell type responds to slightly different chemical signals.[1]

Neutrophils, Eosinophils and Basophils on the basis of cellularmorphology and cytoplasmic staining characteristics

Neutrophils

The neutrophil has a multilobed nucleus and a granulated cytoplasm that stains with both acid and basic dyes; it is often called a polymorphonuclear leukocyte (PMN) for its multilobed nucleus.[2]

Neutrophils are produced by hematopoiesis in the bone marrow. They are released into the peripheral blood and circulate for 7–10 h before migrating into the tissues, where they have a life span of only a few days. In response to many types of infections, the bone marrow releases more than the usual number of neutrophils and these cells generally are the first to arrive at a site of inflammation. The resulting transient increase in the number of circulating neutrophils, called leukocytosis, is used medically as an indication of infection. Movement of circulating neutrophils into tissues, called extravasation, takes several steps: the cell first adheres to the vascular endothelium, then penetrates the gap between adjacent endothelial cells lining the vessel wall, and finally penetrates the vascular basement membrane, moving out into the tissue spaces. [2]

Phagocytosis by neutrophils is similar to that described for macrophages, except that the lytic enzymes and bactericidal substances in neutrophils are contained within primary and secondary granules. Neutrophils also employ both oxygen-dependent and oxygen-independent pathways to generate antimicrobial substances.[2]

Source: Chapter 2, Cells and Organs of Immune System, Immunology, Kuby, 5^th Edition

EOSINOPHILS

Eosinophils, like neutrophils, are motile phagocytic cells that can migrate from the blood into the tissue spaces. Their phagocytic role is significantly less important than that of neutrophils, and it is thought that they play a role in the defense against parasitic organisms . The secreted contents of eosinophilic granules may damage the parasite membrane.[2]

Source: Chapter 2, Cells and Organs of Immune System, Immunology, Kuby, 5^th Edition

BASOPHILS

Basophils are nonphagocytic granulocytes that function by releasing pharmacologically active substances from their cytoplasmic granules. These  substances play a major role in certain allergic responses.[2]

Source: Chapter 2, Cells and Organs of Immune System, Immunology, Kuby, 5^th Edition

MAST CELLS

Mast-cell precursors, which are formed in the bone marrow by hematopoiesis, are released into the blood as undifferentiated cells; they do not differentiate until they leave the blood and enter the tissues. Mast cells can be found in a wide variety

of tissues, including the skin, connective tissues of various organs, and mucosal epithelial tissue of the respiratory, genitourinary, and digestive tracts. Like circulating basophils, these cells have large numbers of cytoplasmic granules that

contain histamine and other pharmacologically active substances. Mast cells, together with blood basophils, play an important role in the development of allergies.[2]

DENDRITIC CELLS

The dendritic cell (DC) acquired its name because it is covered with long membrane extensions that resemble the dendrites of nerve cells. Dendritic cells can be difficult to isolate because the conventional procedures for cell isolation tend to

damage their long extensions. The development of isolation techniques that employ enzymes and gentler dispersion has facilitated isolation of these cells for study in vitro. There are many types of dendritic cells, although most mature dendritic cells have the same major function, the presentation of antigen to TH cells. Four types of dendritic cells are known:

Type of dendritic cells

Source: Chapter 2, Cells and Organs of Immune System, Immunology, Kuby, 5^th Edition

Despite their differences, they all constitutively express high levels of both class II

MHC molecules and members of the co-stimulatory B7 family. For this reason, they are more potent antigen-presenting cells than macrophages and B cells, both of which need to be activated before they can function as antigen-presenting cells

(APCs). Immature or precursor forms of each of these types of dendritic cells acquire antigen by phagocytosis or endocytosis; the antigen is processed, and mature dendritic cells present it to TH cells. Following microbial invasion or during inflammation, mature and immature forms of Langerhans cells and interstitial dendritic cells migrate into draining lymph nodes, where they make the critical presentation of antigen to TH cells that is required for the initiation of responses by those key cells.[2]

Another type of dendritic cell, the follicular dendritic cell does not arise in bone marrow and has a different function from the antigen-presenting dendritic cells described above. Follicular dendritic cells do not express class II MHC molecules and therefore do not function as antigenpresenting cells for TH-cell activation. Although they do not express class II molecules, follicular dendritic cells express high levels of membrane receptors for antibody,which allows the binding of antigen-antibody complexes. The interaction of B cells with this bound antigen can have important effects on B cell responses.[2]

Refrences:

[1]What are granulocytes, www.wisegeek.com/what-are- granulycte.htm

[2] Chapter 2, Cells and Organs of Immune System, Immunology, Kuby, 5^th Edition.