Mast Cells and Basophils

Author: V. Dimov, M.D., Fellow, Creighton University Division of Allergy & Immunology
Reviewer: S. Randhawa, M.D., Fellow, LSU (Shreveport) Department of Allergy & Immunology

Mast cells and basophils are the major cells in the early phases of immediate hypersensitivity reactions. Mast cells are fixed in the tissues while basophils circulate in the blood. Both are derived from CD34+ bone marrow progenitor cells.

CD differentiation (captions in German). Image source: Wikipedia.


Blood cell lineage. Image source: Wikipedia.


Mast cells (mind map).

Cell Morphology and Subpopulations

Mast cells are fixed in tissues and are bigger than basophils. They are 10 μm in size and have round nuclei. Mast cells have many more granules (1000 per cell) than basophils (80 granules) but their granules are 6 times smaller (0.2 µm vs. 1.2 µm).

Mast cells are subdivided into 2 types based on proteinase content:

TC mast cells - Tryptase and Chymase in granules, Kit+/CD88+
T mast cells - Tryptase only granules, Kit+/CD88-

TC
Tryptase/Chymase, Kit+/CD88+

T
Tryptase only, Kit+/CD88-

D
Dermis
Dual granules - Tryptase/Chymase
Dual surface markers - Kit+/CD88+

M
Mast cells
Major cell (10 µm) -- 2 times bigger than basophils (5 µm)
Minute granules (0.2 µm)
More granules (1000) than basophils (80)
Monophorm (round) nucleus

Mast cells. Image source: Wikipedia.

B
Basophils
Blood circulation
Bigger granules (1.2 µm)
Bilobar nucleus

Basophil granulocyte. Image source: Wikipedia.

Mast-Cell Signaling and Degranulation

Mast cells have FcεRI receptors which bind specific IgE. When a multivalent allergen binds to IgE, this leads to crosslinking of FcεRI receptors. Crosslinking, via signaling, leads to mast cell degranulation and production of cytokines, chemokines, and other mediators.

Role of IgE and mast cells in allergy. Image source: Wikipedia.

Mast Cell Mediators

The mediators fall into 3 groups remembered by the mnemonic ONC.

ONC:
Old
New
Cytokines

Old (pre-formed) Mediators in Granules of Mast Cells

Histamine is the classic example of a granule-associated mediator. Serine proteases (Tryptase/Chymase) are also pre-formed mediators in the granules of mast cells. The participate in airway remodelling in asthma.

Histamine structure. Image source: Wikipedia.

Structure of trypsin, a typical serine protease. Image source: Wikipedia.

Hereditary angioedema (HAE) is an autosomal dominant condition associated with episodic attacks of nonpitting edema. Patients with HAE have low levels of C1 inhibitor (a serine protease inhibitor).

Newly-generated Cell Mediators

Mast cells quickly generate different mediators from the metabolism of arachidonic acid: leukotrienes and prostglandins (LTC4, LTB4, PGD2). These substances are produced within minutes of IgE-receptor crosslinking on the surface of mast cells.

Eicosanoid synthesis. Image source: Wikipedia.

Eicosanoids are signaling molecules made by oxygenation of 20-carbon essential fatty acids. There are 4 families of eicosanoids (PP-LT): prostaglandins (PG), prostacyclins (PGI), leukotrienes (LT) and thromboxanes (TX).

Arachidonic acid. Image source: Wikipedia.


LTC4 is a cysteinyl leukotriene, as are D4 and E4. Image source: Wikipedia.

LTB4. Note the four double bonds, three of them conjugated. This is a common property of A4, B4, C4, D4, and E4. Image source: Wikipedia.

Prostaglandin D2. Image source: Wikipedia.

Leukotriene receptors mnemonic

Leukotriene
B4
BLT 1, 2 receptors

Leukotriene
C4, D4, E4
CysLT 1, 2 receptors

Cytokines

Different cytokines are produced by mast cells including interleukins, TNF, FGF, GM-CSF.

Tumor necrosis factor-alpha (TNF-α) is a proinflammatory cytokine that has been implicated in the airway pathology in asthma. TNF-α may play an important role in severe refractory asthma unresponsive to inhaled CS.

Etanercept (Enbrel) is a recombinant human TNF-α receptor fusion protein (anti-TNF-α). Its therapeutic potential is based on the fact that TNF-alpha is the "master regulator" of the inflammatory response in many organ systems.

References

Allergy and Immunology MKSAP, 3rd edition.
Targeting TNF-α: A novel therapeutic approach for asthma. JACI, Volume 121, Issue 1, Pages 5-10 (January 2008).

Related Reading

FIT Corner Q & A from 5th edition of Cellular and Molecular Immunology, edited by Abul K. Abbas and Andrew H. Lichtman. ACAAI, 2004.

Published: 12/13/2007
Updated: 05/12/2009