What is Myeloid Lineage?

Myeloid progenitor cells produce phagocytic cells and the remaining accessory cells. Different phagocytic cells of myeloid lineage can be classified into polymorphonuclear phagocytes and mononuclear phagocytes with reference to the nucleus structure.

Monocytes and macrophages are included in mononuclear phagocyte cell group. Neutrophils, basophils, eosinophils are included into polymorphonuclear phagocytes group because of varying shapes of their nucleus.

The accessory cells are the cells that help immune system in various ways such as processing and presenting the antigens to immunocompetent T cells, enhancing the immunologic response through secretion etc.

Different cells of myeloid lineage are as follows:

Granulocytes:

About 60-70% of total leucocytes are granulocytes. They contain multilobed nucleus and many granules. Hence they are referred as polymorpho nuclear granulocytes (PMN or PML). They are short lived with a life span of about 2-3 days.

They are not specific for antigens but play an important role in immunity. On the basis of morphology and staining characters the granulocytes are classified into three types namely neutrophils, eosinophils and basophils.

Neutrophils:

These are the most abundant type of granulocytes found in the peripheral blood and about 90% of circulating granulocytes are neutrophils. They are motile and phagocytic in nature. Mature neutrophils have multi lobed nuclei and granular cytoplasm. They are about 10 micrometers in diameter and live approximately five days.

One liter of human blood contains about five billion neutrophils. Mature neutrophils are smaller than monocytes. The granules of neutrophils are different. The large denser azurophilic granules contain peroxidase, lysozyme and various hydrolytic enzymes and the smaller granules contain collagenase, lactoferrin and lysozyme.

The collective function of the granules helps in digestion and elimination of pathogen phagocytised by the cell. The chemotactic action of cytokines at the site of inflammation initiates marginalization and extravasculation or diapedisis in neutrophils to attract near to the site of infection

Once neutrophils have received the appropriate signals, they take about thirty minutes to leave the blood and reach the site of infection. Once they reach the site of inflammation they do not return to the blood; they turn into pus cells and die.

Production of neutrophils increases in response to many infections and during severe infection neutrophil precursors called myelocytes and promyelocytes are released in to the circulation. In medical terminology the raise of neutrophils is known as leucocytosis.

Marginalization:

Cells flowing in the circulation move nearer to the endothelial lining (margin) of blood vessels to squeeze out.

Extravasculation or Diapedisis:

Emigration of cells through the gaps of endothelial cells of blood vessel.

Eosinophils:

Eosinophols with bilobed nucleus and granular cytoplasm are motile phagocytes. They occupy about 2-5% of total blood leucocytes in normal healthy individuals. They are acidophilic in nature and stain with acid dye eosin.

Like neutrophils they also exhibit extravasation and phagocytosis during inflammation reactions. Basically they help in the eradication of big parasites such as round worms, pin worms etc., because their granules contain a unique toxic basic protein and cationic protein (e.g. cathepsin) and the receptors that bind to IgE are used to help in this task.

In addition to that they help in the removal of immune complexes through phagocytosis and regulate functions of other immune cell (e.g. CD₄ ⁺ T cells, dendritic cells, B cells, and mast cells, neutrophils, and basophils functions).

In healthy individuals raise of eosinophils is a general immunological response to intestinal parasites. Unlike neutrophils the granules of eosinophils are membrane bound. An appropriate stimulus induces fusion of cytoplasmic granules with plasma membrane to release their toxic protein contents and histamine in the surroundings of parasite, when the parasite is too big for phagocytosis.

The released substances of eosinophils bind to the membrane of the parasite to kill and eliminate. In general atopic persons show high levels of eosinophils (Refer Chapter 14 for further details of hyper sensitivity).

Eosinophils respond to chemotactic factors released by basophils and mast cells. In healthy individuals secretions of eosinophils suppress the activity of mast cell secretions to regulate the immunological response.

Basophils:

Basophils are one of the least abundant cells in blood (comprise less than 0.2% of total leucocytes count). Unlike neutrophils and eosinophils they are non phagocytic in nature. They contain granules that are receptive to basic stains hence they were named as basophils. They work by releasing their pharmacologically active substances such as histamine, sertonine, heparin, peroxidase, platelet activating factor etc.

When basophils are induced they release histamine; this contributes to the inflammatory response that helps in fighting with invading organisms. In general the released substances of basophils result in Type I hypersensitivity/ allergic reactions.

Basophils have receptors that allow binding of IgE, IgG, complement, and histamine. Since they have receptors for Fc region of Ig E antibodies the IgE antibodies produced against an allergen bind to basophils.

Interaction of antigen with the basophil bound IgE antibody induces release of basophil contents, histamine in the released substances causes dilation and increased permeability of capillaries close to the basophil.

Another substance called prostaglandins contributes to an increased blood flow to the site of infection. Both of these mechanisms allow blood clotting elements to be delivered at the infected area, which prevents further spread of pathogens. Increased permeability of the blood also allows more phagocyte migration to the site of infection so that they can devour the microbes.