Page "Known Space" Paragraph 43

On Earth in the mid 21st century it became possible to transplant any organ from any person to another, with the exception of brain and central nervous system tissue.

Individuals were categorized according to their so-called " rejection spectrum " which allowed doctors to counter any immune system responses to the new organs, allowing transplants to " take " for life.

It also enabled the crime of " organlegging " which lasted well into the 24th century.