Morphotropic

A fantastic exploration of an alternative biology. In our world, the cells in an organism belong to it: attempts to directly transfer cells from one organism to another usually result in rejection by the immune system. But in this story, groups of cell (cytes) communicate with each other and with other cells to determine their course of action. Given the right signals, the cells can decide as a group to, for example, leave a person for another person which provides a better environment.

The story starts with two main characters: one who wakes up with parts of her body missing, her cells deciding to leave her for unknown reasons, leading her to become a researcher stuying how cytes communicate and decide on their roles in people's bodies. The other is a Swapper, a person who actively seeks out others to exchange their cytes, hoping to find a better combination of cytes that may eventually let them live as Flourishers, a group of people whose cytes cooperate so well that they can live for hundreds of years.

Both would eventually meet, in unusual circumstances, when the research leads to the discovery of a set of signals that can persuade the cytes to change their roles. But danger lurks, for there is another group that wants to use the research for their own purposes and are not above using murder to stop others.

Through the characters, we get to see how people live in this world, where evolution has lead to a world that looks much like our own: but on the level of cells, groups of them can decide to migrate to another body, or even change their role in the body. This has repercussions in the act of reproduction, but you'll have to read the story to discover how this happens in this alternate world.