The HeLa cell line is the first studied immortal cell line. It was taken from a tumor in Henrietta Lacks without her knowledge, consent or property transfer agreement. It has developed into an important tool in medical research.
It is available to buy from multiple sources, for example ATCC. While it is not quite clear under what terms you are purchasing them, I think it is under this material transfer agreement (MTA). It states:
ATCC and/or its Contributors shall retain ownership of all right, title and interest in the ATCC Materials, including such ATCC Materials contained or incorporated in Modifications.
This indicates that there are some rights, title and interest beyond physical possession that applies to these cells, that they are currently owned by ATCC, and that the rights extend to child cells of those originally provided (despite those derivative cells including a small proportion of the actual atoms that were originally provided, in much the same way as the cells will include very little of the actual atoms that were part of Henrietta Lacks).
Are there any such rights? If so, how did companies such as ATCC come to possess them? The wiki mentions the case of Moore v. Regents of Univ. of Cal., 51 Cal.3d 120, 271 Cal.Rptr. 146, 793 P.2d 479, 483 (1990) which seems to establish that individuals do not have rights to a share in the profits earned from commercial products or research derived from their cells, but does not seem to explain how anyone else gains those rights, other than from possession or via a consent form (that John Moore signed but Henrietta Lacks did not). In particular the finding that 'At the very least, Moore had the "right to do with his own tissue what the defendants did with it"'. This would seem to indicate that the estate of Henrietta Lacks would have any rights that ATCC is claiming, but not what those rights are or from where ATCC gained them.