Morphology

Human basophils have a polylobed nucleus with condensed nuclear chromatin and absent nucleoli. They have little endoplasmic reticulum and free ribo-somes. The cytoplasm contains a large number of granules that are heterogeneous in size. By electron microscopy some granules are filled uniformly with electron dense material, whereas others lack this dense material. When antigen is added in vitro, the basophils lose their oriented motility and extend pseudopodia. The degranulating basophils develop small cytoplasmic 'vesicles' that rapidly increase in size and coalesce. The granular membranes fuse with the plasma membrane, resulting in the development of narrow openings between the cell surface and individual granules. This results in the extrusion of membrane-free granular material through multiple openings in the circumference of the cell. However, there are some rare interconnected chains of granules opened to the exterior at a single point on the cell surface. The granule matrix is released as a whole to the outside, but the granule membrane is left behind. Frequently, membrane-free granular contents are seen attached to the cell exterior. The degranulation of mast cells is of the 'compound' type with granules fusing with each other, whereas in basophils most of the granules fuse directly with the plasma membrane (Figures 1 A & B).

Basophils infiltrate and occasionally constitute a significant proportion of the total cells at sites of delayed hypersensitivity reactions. Such reactions are termed cutaneous basophil hypersensitivity. The T cells in such reactions probably release factors that are chemotactic for basophils and might also induce the basophils to degranulate slowly: the granules never fuse with the cell membrane, but lose their matrix in a 'piecemeal' manner over a period of days. Small packages of granular content appear to bud from granule membranes, traverse the cytoplasm and fuse with the plasma membrane, leaving completely degranulated cells.

Basophil releasability

Releasability of basophils refers to the variation in the extent of histamine release from the cells of different donors. The extent of histamine release from the cells of any one individual can vary from none to values as high as 90-100% of the total cellular content. There is good positive correlation between antigen-induced histamine release in vitro with the

Figure 1 Human basophils; (A) non-activated; (B) after degranulation. Photomicrographs kindly provided by Dr. Connie Oliver.

Figure 1 Human basophils; (A) non-activated; (B) after degranulation. Photomicrographs kindly provided by Dr. Connie Oliver.

basophils of allergic patients and the extent of their symptoms. The releasability of the same cells can also vary with different secretagogues. There are also differences in the extent of release from basophils compared with mast cells. There is no correlation between the number of IgE molecules present on the surface of basophils and releasability. Recent evidence suggests that these differences in secretion are regulated by intracellular factors. For example, the cells of donors who are high responders release histamine with chemically cross-linked IgE dimers; whereas the cells of the donors who are low responders release with chemically cross-linked tri-mers, but very poorly with dimers. The sensitivity of cells to different stimuli can be modified by factors such as adherence of the cells to other cells or extracellular matrix proteins, by cytokines, by steroids or by dietary lipids that are incorporated into the membranes.

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