Other components that can enter the lumen without passing through the epithelial cells are leukocytes (discussed in Mastitis Module). The leukocytes comprise the vast majority of the somatic cells in the milk. These cells pass between the epithelial cells and in the process they "break open" the tight junctions between the epithelial cells and enter via the paracellular pathway. Of course, this also allows other extracellular components like salts to diffuse into the lumen and milk components to diffuse out of the lumen into the extracellular fluid. [This is one reason why there is a change in electrical conductivity in the milk during mastitis].