Although basophils and mast cells share biochemical
and functional characteristics, they are not identical. In humans,
basophils are the least frequent of the three granulocytes, typically
accounting for less than 0.5 percent of blood leukocytes. Basophils
circulate as mature cells and can be recruited into tissues, particularly at
sites of immunologic or inflammatory responses, but they ordinarily
do not reside in tissues. By contrast, mast cells typically are derived
from blood precursors that lack many of the characteristic features
of the mature cells and complete their maturation in the tissues.
The mature mast cells can reside in tissues for long periods of
time. Mast cells are particularly abundant near blood vessels and
nerves and in connective tissues beneath surfaces that are exposed
to the external environment, such as the skin, gastrointestinal
and urogenital tracts, and respiratory system. Tissue mast cell
numbers can increase at sites of parasite infection or in association
with certain chronic allergic diseases or other forms of pathology,
by recruitment and local maturation of blood precursors and by proliferation
of resident mast cells.
Mast cells and basophils express the high-affinity
receptor for immunoglobulin (Ig) E (FcεRI) on their surface.
Both cell types can be triggered to release potent mediators in
response to activation via FcεRI, for example,
when their cell-bound IgE recognizes bivalent or multivalent allergens.
Accordingly, mast cells and basophils have long been regarded as
important effector cells in asthma, hay fever, and other allergic disorders.
It is thought that the cells’ cytoplasmic granule-associated
preformed mediators, including histamine and certain proteases,
their lipid mediators (such as prostaglandin D2 and leukotriene
C4), which are generated upon activation of the cells, and
their cytokines, growth factors, and chemokines contribute to many
of the characteristic signs and symptoms of these diseases. However,
several lines of evidence indicate mast cells and basophils also contribute
to protective host responses associated with IgE production, especially
those directed against parasites. In mice, mast cells also can contribute
to host defense in innate immune responses to certain bacterial
infections and to pathology in certain T-cell–associated
immunologic disorders not thought to involve IgE, including some
autoimmune diseases. Mast cells and basophils also may express immunoregulatory
functions through cytokine production and other mechanisms.
Although a variety of systemic disorders have been
associated with changes in the numbers of blood basophils and many
pathologic processes can be associated with changes in the numbers
of tissue mast cells, patients with primary deficiencies in basophils appear
to be exceedingly rare (if they exist at all). Patients with a primary
deficiency of tissue mast cells have not been reported. By contrast,
neoplastic processes can affect both of the lineages. Increased
numbers of basophils may be present in association with myeloproliferative
neoplasms and several forms of myeloid leukemia. Increased numbers
of basophils, sometimes to levels of 20 to 90 percent of blood leukocytes,
occur in virtually all patients with chronic myelogenous leukemia.
The basophils associated both chronic myelogenous ...