Immunology (Lippincott Illustrated Reviews Series) 2nd Edition
Sense of Being: The Concept of Self and Self/Nonself Recognition
ΓΝΩΘΙ ΣΑΥΤΟΝΙ (“Know thyself”)
—Words originally inscribed in gold on the pronaos of the
Temple of Apollo at Delphi
This dictum—short in length but deep in meaning—encapsulates a basic need for all forms of life.
In a way, most organisms in our world live alone. 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. Distinguishing “I” or “us” from “them” became increasingly complex: Is that adjoining cell, which seems so different from “I,” really a part of “us,” or is it an intruder from “them”?
The development of commensal arrangements between organisms (e.g., moss and fungi combining to form lichens, humans and normal bacterial flora in the gut and on the skin) required yet more questions: If there is an intruder, does it represent a threat or can it safely be ignored? If it represents a threat, what should be done to eliminate it?
These questions are the starting points from which the immune system operates. The human immune system uses various methods to ask and answer these questions. Some of these methods have been widely used for eons; others have been developed more recently by more restricted groups of organisms. This unit introduces how the human immune system deals with these questions.