Primary cells are cells taken directly from living tissue and established for growth in vitro. Primary cells have a finite lifespan and are widely used because they retain many of the markers seen in vivo. Primary cells from different species allow you to highlight potential differences between humans and preclinical test species. Before in vivo studies, using primary cells can refine doses and reduce the number of animals required for preclinical toxicology. Human primary cells can be used to determine accuracy of extrapolating human data from an animal model.
A cell line is a population of cells that has undergone a genetic transformation, resulting in indefinite growth potential. In practice, cell lines can be cultured through a very high number of subcultures, although some further genotypic, and therefore phenotypic, changes may occur at very high passage numbers.
Generally, cell lines lack many of the markers seen in vivo and also show very different marker profiles than primary cells.