Eosin is an acidic dye; thus, the structure being stained is basic.
Eosinophilic describes the appearance of cells and structures seen in histological sections that take up the staining dye eosin. This is a bright-pink dye that stains the cytoplasm of cells, as well as extracellular proteins such as collagen.
Such eosinophilic structures are, in general, composed of protein.
The stain eosin is usually combined with a stain called hematoxylin to produce a hematoxylin and eosin-stained section (also called an H&E stain, HE or H+E section). This is the most widely used histological stain in medical diagnosis; for example, when a pathologist examines a biopsy of a suspected cancer, the biopsy will have been stained with H&E.