There are HeLa cells, which were taken from sick cancer patient and now is growing worldwide in different laboratories for experiments. HeLa cells can be kept because they are immortal and so they are an instance of immortal cell line.
What about stem cells? They are also immortal. Such cells can be taken from normal healthy person and also kept forever. Do scientists do this? Why don't we hear about it then? For example, they are distinguished in Wikipedia article, referred above.
Are there some special properties of stem cell lines, making them different? For example, being low differentiated they may be useless for experiments.