Re: Can two different types of cancer cell exchange genetic material?

Great question!
	To my knowledge, while there are bateria (prokaryotes) which routinely 
exchange genetic material, there are no human cancers which do so.  This 
type of exchange is performed to enhance genetic diversity of some bacteria 
and does not normally occur in higher eukaryotes.  If you would like to 
learn more about exchange of genetic material in prokaryotes textbooks of 
microbiology are a good place to start.
	At the heart of your query is the nature of a cancer cell.  What is a 
cancer cell?  Cancer cells are cells found normally in one's body.  A cancer 
arises when a cell is mutated in such a way that it can grow and divide 
abnormally.  While it is true that many cell types in our bodies can grow 
and divide, cancer cells grow better than neighboring cells and their growth 
is unregulated.
	Because the cancer cell was part of our bodies before it became a 
cancer cell, it is very difficult for the immune system to fight it.  Almost 
every cell in the body displays a molecule on its surface that notifies the 
immune system that it belongs in our body {it is "self"}.  Cancer cells 
still possess this molecule.  When a cell does not possess this molecule or 
it has been mutated, then the immune system is alerted that something that 
is "not self" or dangerous is present.
	The solution you have proposed to B's cancer has many problems.  First, 
unless A and B are identical twins, B's normal cells (and, therefore, B's 
cancer too) will be recognized as "not self".  This will produce a vigorous 
rejection of B's cells by A's immune system.  Second, the transplant from B 
might contain immune cells from B, which would recognize any of A's cells as 
"not self".  This could cause the reverse reaction called "graft-versus-host 
disease".  {This is a particular problem in bone marrow transplants.}  This 
a very painful and potentially fatal reaction.
	Cancer or tumor immunobiology is a burgeoning and very exciting field 
of immunology.  The question you have posed calls upon many different 
aspects of immunology such as development of recognition of "self" versus 
"nonself", transplantation/graft rejection, and cancer immunotherapy.  Many 
lines of research are now underway to learn how we can manipulate the immune 
system to selectively kill cancer cells.  However, as I mentioned above, 
since cancer cells look like "self", this is quite difficult.
	If you would like to learn more about how this amazing system works in 
your body there are several introductory immunology textbooks available in 
university libraries or in some online bookstores.