As it turns out, humans have an evolutionary connection with these creatures. Certain characteristics of sea squirts might, therefore, represent early forms of similar characteristics in humans. When two colonies of Botryllus touch, their blood cells mix and a kind of natural transplantation occurs. This is followed by one of two consequences. 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.