Either the colonies fuse, producing a single organism, or the cells recognize each other as being too different and the colonies grow apart after a rejection reaction. Humans also have the ability to accept or reject transplanted tissue. This is a function of the immune system, an elaborate network of blood proteins and specialized cells that protect the body against infectious disease and tumors. In humans, cells from every individual carry a set of unique surface markers, or transplantation antigens, that define those cells as belonging to that individual alone. By reading these markers, the immune system distinguishes self from non-self, and in this way decides to accept or reject a transplant.
Sea squirts distinguish self from non-self in much the same way.
Because I was curious whether transplantation in sea squirts might represent an early form of human immunity, I traveled to the MBL in 1980 to conduct research on Botryllus. It wasn't long before my colleagues and I confirmed that transplantation in these animals is controlled, as it is in human grafting, by defined genes that can be studied in a laboratory. Given our evolutionary link to sea squirts, these genes might actually be simple versions of those that control human grafting. Studying how they work might tell us something about their human counterparts, and might there, help solve puzzling problems of human biomedicine - problems like AIDS AIDS is a disease of the human immune system, but the story of how sea squirts won a role in AIDS research begins not with immunity but with fertilization. Like us, Botryllus, and other sea squirts begin life as a single fertilized egg.