What are Basophils?
Definition of Basophils
Basophils are nonphagocytic granulocytes that are the least common leukocytes occurring in the blood circulation and are critical to the immune response against parasites.
- Basophils are similar to eosinophils in that these are produced in response to parasitic infections; however, in the absence of parasitic infections, basophils are involved in allergy symptoms.
- Even though basophils account for a small percentage (0.5%) of the total circulating blood cells, the number can increase rapidly in response to inflammatory signals.
- Basophils are formed from the granulocyte-monocyte progenitors present in the bone marrow, which then populate the peripheral blood as mature cells.
- Studies related to the functions, roles, and their responses have been difficult due to their short lifespan and scarcity.
- Basophils are developmentally related to mast cells as both the cells express high-affinity receptors for IgE and can release a similar spectrum of mediators in response to IgE binding.
- Some of the common markers that can be used to distinguish basophils include ckit–FcεRI+, CD11b+, IL-3Rhi, etc.
- As a result of their involvement in allergic reactions, a basophil activation test has been prepared that detects allergic reactions to drugs, food, and venom in patients.
- Basophils are considered circulating innate cells that not only elicit microvascular permeability to enable the transmigration of leukocytes into the inflamed tissues but also contribute to the Th2 immune response.
- The paucity of basophils in circulating blood is a regulatory mechanism in innate and acquired immunity rather than hypersensitivity and atopic response.
Structure of Basophils
- Basophils are the smallest granulocytes with a diameter ranging between 10-14 µm. These polymorphonuclear cells with polylobed nucleus and prominent, brightly metachromatic cytoplasmic granules.
- Even though most basophils are rounded cells, some elongated, narrow uropods or tails have been described in humans.
- The nucleus contains condensed nuclear chromatin, but the nucleolus is absent.
- The cytoplasm contains organelles like mitochondria, vesicles, glycogen, and granules. The mature granules usually contain dense particles.
- Usually, human basophil cytoplasmic granules are round to angular, membrane-bound structures with sizes ranging up to 1.2 µm.
- The substructure of these granules contains dense particles in a less dense matrix occasional complex membrane.
- The cytoplasm has a complex vesicular system that is involved in basophil degranulation in the presence of an appropriate stimulus.
- The structure of basophils is similar to mast cells, and the differentiation can be made on the basis of glycogen content and the plasmalemma ridges.
- Basophils have a higher glycogen content in the cytoplasm, whereas these lack the plasmalemmal ridges and folds that are present in mast cells.
How do Basophils work against pathogens? (Immunity)
- Basophils are formed from the granulocyte-monocyte progenitor cells of the bone marrow, which are then released into the peripheral blood as fully differentiated cells.
- The differentiation of precursor cells into the basophil lineage is dependent on the transcription factor, C/EBPα.
- The level of basophils in the blood remains maintained under normal conditions, but the number increases as a result of parasitic infections or allergies.
- The exact nature of stimuli responsible for basophil development during parasitic infections is not clear yet, but the hematopoietic cytokine IL-3 has been suggested to play an essential role.
- Other parasite-associated molecules like proteases, glycoproteins, or structural components like chitin can also act as stimuli for the differentiation of basophils.
- Basophils, like mast cells, are activated by the cross-linkage between the surface IgE receptor, FcεRI, and the IgE antibodies present in the blood.
- The binding of IgE to basophils results in an activation signal which causes a rapid release of intracellular mediators like histamine and leukotrienes. The increased level of these compounds causes increased secretion of cytokines like IL-4 by basophils.
- Besides, IL-4-independent pathways involving IL-3 have also been described, which result in basophil activation.
- Activated basophils produce a large amount of IL-4, which might even be greater than that produced by T cells.
- The IL-4 produced by basophils induces the activation of parasite-specific naïve T cells to Th2 type effector T cells.
Functions of Basophils
The following are some of the functions of basophils:
- The most important function of basophils is the induction of immune response against parasitic infections, especially infections involving helminths.
- Studies have also indicated that basophils can function as professional antigen-presenting cells required for T cell differentiation. Basophils, once bound to IgE, lead to the differentiation of antigen-specific CD4 T cells into Th2 cells.
- Basophils work with other immune cells via cross-talk to maintain an orchestrated mechanism of allergy.
- Basophils also decrease hypersensitive response by the modulation of releasibility of either histamine or lipid mediators.
- Basophils activation test is a test developed to test allergic reactions to different foods, venoms, and drugs.
- One of the important physiological roles of basophils is the B cell maturation, resulting in IgE production.
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