The immune system is designed to recognize foreign invaders. To do so it generates on the order of 10¹¹ [100,000,000,000] different kinds of immunological receptors so that no matter what the shape or form of the foreign invader there will be some complementary receptor to recognize it and effect its elimination.

One type of cell that our body uses to fight invaders is the macrophage; its name means “big eater,” which is fitting because it devours foreign substances in our blood. After eating an invading virus, the macrophage breaks it into small fragments. It then displays some protein from the virus. This bit of marker protein serves as a red flag to our immune system, sounding the alarm that foreign organisms are on the loose inside us. If another cell in the immune system, the helper T cell, recognizes the virus protein, it exchanges chemical signals with the macrophage. These chemicals are themselves extraordinary proteins that have a bewildering array of functions, regulating and boosting our immune system’s response to invasion. This process results in a vigorous fight against the specific type of virus. As a result, we usually manage to overcome infections.