Self And The Phenomenon Of Life: A Biologist Examines Life From Molecules To Humanity

The Animal Self: Molecular Recognition 121

“9x6” b2726 Self and the Phenomenon of Life: A Biologist Examines Life from Molecules to Humanity

component); (2) for reasons beyond the scope of this book, each indi-
vidual’s MHC is unique (in amino acid sequence) and is not shared by
any other person, even among siblings, much like every person has his
own set of finger prints; (3) the T cells of a person will mount an attack
on any cell that carries a non-self MHC. In this sense, MHC serves as a
stringent molecular marker for the physical self.^10
Recall the story in the beginning of this chapter, in which a trans-
plant recipient rejected her brother’s skin graft but kept her own, lead-
ing Medawar to suspect that organ rejection is a form of immune attack.
For this historical reason, the term “major histocompatibility complex”
(MHC) was coined with reference to tissue transplantation (“histo” is
the Greek equivalent of “tissue”), before it was later realized to be also
involved in anti-infection.
The truth is, pathogens and foreign tissues have one thing in com-
mon: they both are molecular non-self intruders and should be fero-
ciously rejected by the host. Individual variation in MHC is exceedingly
intricate, and it is extremely rare for two genetically unrelated persons
to have the same MHC protein. In practice, only identical twins make a
perfect match in transplantation. All other matches, including siblings,
show various degrees of incompatibility, requiring immunosuppressive
agents to mitigate rejection. Table 6.3 shows rejection of a skin graft when
the donor comes from a person (or animal) of different genetic makeup.

Table 6.3. Outcomes of Skin Transplantation

Donor Recipient Outcome

Syngeneic First time Acceptance

Allogeneic First time Rejection

Allogeneic Second time Accelerated rejection

This table shows tolerance of self (syngeneic donor) and

rejection of non-self (allogeneic donor) in skin grafting.

“Syngeneic” refers to two individuals with identical genetic

makeup; “allogeneic” refers to those belonging to the same

species but different in genetic makeup. The third row

shows that repeated transplantation of foreign skin from the

same donor leads to accelerated rejection of the graft.