Well-characterized cells and tissues are employed not only as substrates to produce biological products such as vaccines, but increasingly as therapeutic products themselves. A number of biological medicinal products are derived from cells that either naturally produce the substance desired, are genetically modified to do so, or, as is the case with viral vaccines, are infected in order to produce large quantities of virus. As growth substrates, the uniformity of these materials and their freedom from adventitious agents is essential. Traditionally animal cells have been used for the production of viral vaccines, although one of the commercially available recombinant human papillomavirus virus-like particle vaccines is produced in insect cells.
There are a number of cell types used for the production of vaccines and other biological products: primary cells or tissues (used without passage in tissue culture); diploid cells (cells with a finite lifespan and passage in tissue culture) or continous cell lines (immortal, neoplastic cells with unrestricted passage in tissue culture which are non-tumorigenic). Cell substrates also differ by their host range (ability to be infected or transfected by microorganisms) and degree of differentiation.