What are Mast Cells?
What are Mast Cells? Mast cells are cells of the hematopoietic lineage immune system that remain widespread in vascularized tissues throughout the body.
- In contrast to other immune cells, mast cells do not remain in the blood, but rather remain located throughout the body in the mucous membrane and epithelial tissue.
- These are among the first cells to interact with environmental antigens and invading pathogens or microorganisms. Mast cells are therefore considered to be one of the first cells in the immune system to interact with external antigens.
- Mast cells are primarily involved in IgE-dependent responses, but these can also respond to a variety of other innate signals from pathogens, complement compounds, and other host cell types.
- Mast cells are also known to express many costimulatory molecules with immunoregulatory activities that help regulate immune responses in the body.
- These are made from the bone marrow and released into the blood as undifferentiated cells. These cells then mature after leaving the blood and reaching the specialized tissues.
- In the bone marrow, mast cells originate from pluripotent CD34 + / CD117 + progenitor cells, and the maturation of these cells depends on KIT activation.
Structure of Mast Cells
- Mast cells are mononuclear cells made up of small secretory granules that are between 0.2 and 0.8 micrometers in size. In some cells, the granules are dense enough to hide the appearance of the nucleus.
- The cells are oval or irregular in shape with a single central nucleus. Dense peripheral chromatin can be observed within the nucleus.
- Most of the cytoplasm is occupied by cytoplasmic granules, but only by a few secondary lysosomes. The cells also have small, finger-like protrusions from the cell membrane.
- The granules store inflammatory mediators such as histamine, herein, cytokines, and proteases.
- The mast cell plasma membrane has IgE receptors that bind to the IgE antibodies in the Fc region of the circulatory system to induce degranulation of these mast cells.
- Mast cells share many characteristics with basophils, but these can be differentiated by their mononuclear morphology and monoclonal antibodies.
- Mast cells exist in three different forms due to their structure; intact cells, spreading cells, and degranulated cells.
- Intact cells are mostly found in blood vessels and skin tissues. The granules in these cells are tightly packed together, preventing the observation of other cellular organelles. The cells are usually spindle-shaped.
- Spreading cells are cells found in the superficial connective tissue. The granules are less numerous than in the intact cell type.
- Degranulated cells are not metachromatic and have a clear cytoplasm with a prominent nucleus.
How do Mast Cells work against pathogens?
- The most important mechanism of action of mast cells is the IgE-mediated allergic reactions through the Fc receptor.
- The IgE antibodies are produced by mature B cells in response to activation by CD4 + Th2 cells. IgE is produced from IgM by class switching in response to activation of B cells in the presence of IL-4.
- The binding of IgE to the Fc receptors on the mast cells leads to the activation of mast cells, which activates the release of granules from the mast cells.
- The binding activates the LYN tyrosine kinase in the cells, which phosphorylates the tyrosine at the binding site to activate the motifs.
- The LYN also activates the Syk tyrosine kinase, which phosphorylates signal proteins such as LAT1 and LAT2.
- Phosphorylated PLC? hydrolyzes phosphatidylinositol-4,5-biphosphate to form inositol-1,4,5-triphosphate (IP3) and diacylglycerol (DAG).
- IP3 and DAG act as second messengers and mobilize calcium from the endoplasmic reticulum. The release of calcium from the ER causes the translocation of NF? B in the nucleus.
- The process is also followed by the release of cytokines such as IL-6, TNF & agr; and IL-13. This then regulates mast cell degranulation.
- Mast cell desensitization is another essential mechanism used in response to allergies to drugs or food particles.
Types of Mast Cells
- Mast cells in humans can be divided into two types; Mucosal and connective tissue mast cells.
- The connective tissue mast cells are mainly located in loose connective tissue and the skin, but also other connective tissues.
- The mucosal-type mast cell is located in the gastrointestinal mucosa and peripheral airways.
- The exact factor that causes mast cells to differentiate into the two types is not yet fully known.
- However, both types of mast cells can be activated by allergens and other non-allergic signals. Both types also have the IgE receptors, which are responsible for activating cell degranulation.
- The difference between the two mast cells can be observed in their response to non-allergic signals, the mediators and their release, their proteoglycan constituents, and the composition of the granular enzymes.
- The distinction between the two cell types can easily be made in rodents because these stains are of different sizes and are located in different tissues.
- The differentiation between mast cells in humans can be made by the presence of chymase, a chymotryptic protease.
- The protease is only present in connective tissue mast cells, but the enzyme tryptic protease or tryptase is present in all human mast cells.
Functions of Mast Cells
The following are some of the functions of mast cells in the body;
- Mast cells secrete various proangiogenic factors such as VEGF, bFGF, TGF-beta, and IL-8. The cells also release proteases and heparin, which induce the permeability of the microvasculature and induce angiogenesis.
- Mast cells are the first line of defense against antigens entering the body. These cells are therefore important for maintaining homeostasis in the body, especially the gastrointestinal tract.
- In innate immunity, receptors on mast cells bind to antigens and cause the release of inflammatory mediators such as IL-4, TNFα, and IL-6.
- These cells also help provide immunity to viral antigens through the release of IFN- & agr; and IFN-?.
- In adaptive immunity, mast cells process and present antigen via MHCI and MHCII, and these also activate dendritic cells, which are professional antigen-presenting cells.
- The most important and most important function of mast cells is the release of newly synthesized mediators, which are necessary for the immune response.
- Mast cells are also involved in an allergic reaction, in which the overactivation of mast cells can lead to hyperallergic reactions.