Immunology by Doan T, et al
They are composed of single cells or particles, and as such, their need to distinguish themselves is seemingly simple. Their single cell or particle is “I,” and all else is “them.” They need to sense which of “them” is appropriate to mate with or perhaps to congregate with, but otherwise their version of self is limited by their own membrane. Multicellular organisms faced a new problem as they evolved. They gave up some of their independence to reap the advantages of being part of a greater whole—an organism composed of multiple semi-independent units. Initially, any such unit was pretty much like every other one within the greater structure, so extending the concept of self to include others that were essentially identical was perhaps a relatively small leap. “I” became “us” but only as multiples of “I.” As organisms became more complex and the different cells within a single organism began to engage in a division of labor, they generated an array of cells with different forms and functions.
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