Page 178 - The Cell in 40 Topics

lthough the cells of a developing embryo, preparing to set-
tle in the walls of the womb are genetically different to those
of the mother, they are not rejected in the same way that an
organ or tissue transplanted into the body will be. This fact remained a
mystery for a long time. G. L. Flanagan sets out the answer in his book,
Beginning Life:

... The cell cluster suppresses its genetic markers and instead gives out
special signals that can be compared to a universal password. This pass-
word is the same for all people and is the same one that the mother’s cells
expressed when she herself was just such a cluster. Therefore, her cells do
not now mobilize defences against the new arrivals, because they biologi-
cally recognize the nesting cluster as universal friend, not foe. 29
There’s one very important point here. The way a collection of cells
can send a universal message, as Flanagan puts it, and that their message can
be understood by other collections of cells, which then know whether
they’re dealing with friend or invader, is a truly great miracle. It must not
be forgotten that the “societies” in question consist not of human beings,

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