The human body’s special regions
The topic of a seminar which I attended at my university sometime ago dealt with the problem of how to preserve small pieces of donor tissue for an eventual future use in replacing diseased or non-functioning body components.
The major problem, the audience was told, was to keep the donor tissue alive until it might be needed years later. And then there was also the problem of rejection of the foreign tissue. When during question time I was suggesting that one could perhaps deposit the foreign cells in the anterior eye chamber, I saw a lot of surprised faces amongst the largely non-medical audience. The anterior eye chamber of mammals, I knew, is indeed immune-privileged.
Privileged sounds a bit like “superior or favoured” and immune means “untouchable, unsusceptible“. Eye surgeons have long known about the special position the anterior eye chamber with its content of aqueous humour (a clear liquid released into the chamber by the ciliary body) holds and usually need not be worried about rejection problems following corneal transplants. Pieces of pancreatic, thyroid, skin and even nerve cells placed experimentally into the anterior eye chamber were not just tolerated, but continued to grow, albeit ever so slowly. What is going on and why is this region of the human body so different?
Almost everywhere else in the human body infection-like rejection reactions between foreign and local tissue ensue when tissue grafts are detected by the body’s defences as foreign. However, no inflammatory immune reactions are elicited in the anterior eye chamber, the brain, cheek pouches of the golden hamster, the testicles and ovaries. As evident from this list, all of these immune-privileged areas with the exception of the anterior eye chamber, are difficult and inconvenient to reach in the human.
It is believed that immune privilege represents a kind of mechanism to limit lateral damage, a term all too well known from the battlefield. If, for example, during the struggle for dominance in the fight against bacteria (or cells of tissues stemming from another individual) inflammatory reactions occur, the latter could leave behind irreparable and possibly disastrous consequences. It follows that in order to safeguard such vitally important functions like vision, reproduction, and mental capacity the alternative to an all out fight is to set aside some areas where foreign cells and tissues can be tolerated. It was once believed that the foreign cells will perhaps not even get into the anterior eye chamber, but this has meanwhile been shown to be erroneous.
In fact defensive cells of the host body that manage to get into the immune-privileged regions are forced to undergo self-destruction and in the anterior eye chamber are filtered out by the trabecular meshwork that the aqueous liquid passes through on its way to the canal of Schlemm and out of the eye. The bottom line is that the unwillingness to induce an immune fight is protective and avoids incalculable and unpredictable consequences like ruinous inflammations, temporary deficiencies or total losses. A strategy that I suppose politicians and military advisors could take something away from.
© Dr V.B. Meyer-Rochow and http://www.bioforthebiobuff.wordpress.com, 2016.
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