This hematopoietic stem cell is a true stem cell in the sense that it can be transplanted using irradiated hosts many times. It is not clear if the regeneration potential of this stem cell has been exhausted. However, I will not expect a detailed knowledge of each factor and it's effect. It is sufficient to realize that events in far flung corners of the body can determine the need for increased or decreased numbers of a particular cell type, and the only way of communicating over these long distances is with specialized messenger molecules that report back to the bone marrow and regulate the various cellular differentiation steps along the way. In the beginning there were antigens, then came antibodies, and now there are immunogens, epitopes , paratopes , antigenic determinants, immunoglobulins, and the list grows.
Many names are sales gimmicks, though some are ultimately useful.
Only infectious agents and pathogens are real antigens because these were the real selection pressure in evolution that forced the development of the immune system. It was the rapid evolutionary potential of the parasites that forced any large slowly evolving vertebrate to find a defense mechanism that could on the one hand evolve as fast as the bugs, and on the other hand hijack all the preexisting defense mechanisms needed to actually rid the parasites. Experimentalists, with some justification, like to avoid pathogens because they are hard to handle, and any sloppy technique will likely be permanent. Then came the era of chemistry when proteins could be synthesized chemically and all manner of bizarre structures were made. It was clear from the earliest days that the immune system, then mostly thought to be antibodies because serum samples could be obtained easily and there was no way to take cells and test their activity in the test tube.