Your original question said "casual," but I am assuming you meant "causal." A causal argument essay is one in which you will take a position about something with which some people would not agree, or at least something which has more than one side.
In the case of The Immortal Life of Henrietta Lacks by Rebecca Skloot, there are many possibilities. While the book is designed to be informative, the story is full of ethical and moral issues which are up for debate. The primary one, of course, is whether the doctor had the right to take Henrietta's cells tissue without her permission. The only document she signed was this one:
[Henrietta] signed a form with the words operation permit at the top of the page. It said:
I hereby give consent to the staff of The Johns Hopkins Hospital to perform any operative procedures and under any anaesthetic either local or general that they may deem necessary in the proper surgical care and treatment of: ______________________________.
Henrietta printed her name in the blank space. A witness with illegible handwriting signed a line at the bottom of the form, and Henrietta signed another.
It is clear that she did not give permission for anything that was not connected to her treatment, and it is equally clear that when Doctor Lawrence Wharton Jr. took tissue samples (both healthy and cancerous), he was doing so for research rather than treatment--and possibly for profit
Your argument against these actions is that he did not tell her what he was doing and therefore had no right to take them without permission; your argument that this was a perfectly acceptable act is that this tumor was unusual enough that the doctor thought it might be useful for research. Unfortunately, race may also have been a factor: did he not ask because she was black and therefore did not matter or because he did not think she would agree to it? In any case, he did not get permission for the procedure, and that is a problem.
The second major ethical, and undoubtedly legal, issue is whether anyone should have profited from this illegal tissue harvesting. Certainly the family is proud of Henrietta's astonishing contribution to science and research; however, they knew nothing about it for years and received no compensation for their mother's literal body tissue. Selling something that is illegally obtained is clearly against the law; the other side of this is that the illegal material was (presumably) only used for research and therefore a worthy cause. However, research leads to pharmaceuticals, and they are hugely profitable. Should others be allowed to profit from this when the family got nothing? Lots of ethical issues here.
The truth is that Henrietta might have signed whatever anyone put in front of her; however, no one asked her for permission to take her tissue and, even worse, those who took it made a profit from this unauthorized act without any compensation to her family. The ethical and even legal issues are many, and they would all be excellent for a causal argument essay.
To make a decision about what you will argue, think about the things you were outraged by or the things which seemed perfectly justified to you as you read. Use the links below for excellent advice on how to go from there.