basophils

Basophils are granulocytes packed with granules that stain basic (purple with H&E). The nucleus is bilobed, and the metachromatic granules contain sulfated glycosaminoglycans as well as vasoactive compounds – histamine and proteoglycans. Lysosomal arylsulfatase is found in granules of developing basophils in bone marrow. Basophil granules are surrounded by a unit membrane and contain particles which are uniform in size across the same granule yet vary in size in different granules within the same cell. Some granules reveal a homogeneous texture and/or "myelin" figures. [s] Though basophils and mast cells share morphological features, the appearance of most basophil granules differs from the ultrastructure of human mast cell granules.

Activated basophils release proinflammatory histamine and proteoglycans from granules, and synthesize then secrete leukotrienes and cytokines (particularly IL-4). Histamine and IL-4, which is associated with production of IgE, are involved in allergic reactions.

Basophils are the least common granulocyte, representing about 0.5% to 1% of circulating leukocytes. A low basophil count combined with a low neutrophil count almost always portends leukemia.

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tags [Immunology][leukocytes]

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granulocytes

Granulocytes are granulated leukocytes generated by granulopoiesis in the bone marrow. Because their nuclei vary in shape (usually 3-lobed) granulocytes are also termed polymorphonuclear leukocytes (PMN, PML). Usually, the term polymorphonuclear leukocyte is used to designate neutrophil granulocytes, which are the most abundant of the granulocytes.

The granules are named according to their staining characteristics with hematoxylin and eosin (H&E) :
● Basophil granules stain purple
● Eosinophil granules stain red
● Neutrophil granules stain a neutral pink

Mast cells contain metachromatic granules that store inflammatory mediators, including rich in histamine and heparin. Mast cells react to allergens and other degranulating agents with sequential exocytosis (description). Mast cells function in wound healing, in autoimmune disorders, and in the inflammatory and immune responses, including allergic reactions and anaphylaxis. (image at left - click to enlarge).

Mast cells are morphologically similar to basophils and both express CD34, though they may have different bone marrow precursors. Unlike basophils, masts cell circulate in an immature form, maturing only in tissue sites where they are resident. Mast cells are found in connective tissue of most organs, and in mucosal tissue.

Mast cells can be activated when an allergen binds to the IgE that is already coating the cell. Allergens are typically proteins or polysaccharides that bind to the Fab segment of the cognate IgE coating the mast cell surface. Crosslinking of two or more IgE molecules is required to activate the mast cell when steric changes disturb the cell membrane structure, triggering an intracellular cascade that activates the mast cell. Although this reaction is most often encountered in allergic reactions, it apparently evolved as a defense system against intestinal parasitism, such as tapeworm infestations.

Mast cells express pattern-recognition receptors plus high-affinity receptor (FcεRI) for IgE, which ligates specific IgE molecules irreversibly.
On activation, mast cells degranulate, releasing preformed mediators into the interstitium:
● histamine
● proteoglycans, mainly heparin (anticoagulant)
● serine proteases
● freshly synthesized lipid mediators (eicosanoids)  Eicosanoid Actions :
_ ● prostaglandin D2 (PGD2)
_ ● leukotriene C4 (LTC4)
● cytokines

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tags [Immunology] [WBC] [granulocyte]

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leukocytes

Leukocytes are white blood cells (WBCs) – left b=basophil, e=eosinophil, m=monocyte, n=neutrophil.

Leukocytes are subdivided according to presense or lack of granulation:

● granulocytes generated by granulopoiesis :
--- ● basophils → mast cells
--- ● eosinophils
--- ● neutrophils

○ agranulocytes
- ○ lymphocytes generated by lymphopoiesis
--- ○ B lymphocytes → plasma cells
--- ○ T lymphocytes → helper T cells, killer T cells
--- ○ natural killer cells → lymphoid dendritic cells

- ○ agranulocytes generated by monocytopoiesis
--- ○ monocytes → macrophages
----------------. → myeloid dendritic cells

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