Immunology (Lippincott Illustrated Reviews Series) 2nd Edition
The Innate Immune System
“. . . in order to kill the enemy, our men must be roused to anger . . .”
—Sun Tzu, The Art of War, circa 500 BCE
Our initial immune defenses rely on types of cells and molecules that have performed admirably for hundreds of millions of years. Early in the history of life, organisms developed mechanisms to ask whether a particular cell was “self or nonself” and “friend or foe.” As life diversified, different groups of organisms developed specialized molecules with restricted distribution. For example, bacteria expressed molecules that were not expressed by protozoa or by algae—or by trees or by humans. Over time, these group-specific markers enabled one group (e.g., multicellular animals) to encode and synthesize receptors able to recognize and bind molecules that are characteristic of other groups (e.g., bacteria). As a result, organisms encoded within their genomes a series of “hard-wired” receptors capable of a type of self or nonself distinction.
On recognizing and binding to a nonself intruder, initiate a series of enzymatic reactions that might directly destroy the intruder or at least render it more susceptible to some other means of destruction. Other receptors are placed on the surface of certain host cells that move around in the body. These cells, generically termed phagocytes, often have janitorial duties: clearing the body of debris. But when, in the course of their duties, their receptors detect the presence of nonself, phagocytes undergo a change of personality. They become angry and aggressive. Like a mild-mannered Clark Kent, they “step into a telephone booth” and emerge as Superman, with powers to attack and destroy the intruders they have intercepted. It is on these soluble and membrane-bound hard-wired receptors that the human innate immune system is built.