The histocompatibility complex was a feature of cell membranes.
In 2365, when offering advice about how to approach an Allasomorph passenger, Data believed it would be as easy as simply asking her, as they were likely biologically compatible. He speculated that there might be a difference between Wesley Crusher's histocompatibility complex and Salia's. It was more information than Crusher was looking for; "Data, I want to meet her, not dissect her. "(TNG: "The Dauphin")