I MMUNITY AND THE I MMUNE S YSTEM

The functioning of the immune system is considered in a separate essay, along with the means by which that system responds to foreign invasion. Also included in that essay is a discussion of allergies, which arise when the body responds to ordinary substances as though they were pathogens, or disease-carrying parasites. The body cannot know in advance what a pathogen will look like and how to fight it, so it creates millions and millions of different lymphocytes, a type of white blood cell. The principal types of lymphocyte are B cells and T cells. These cells recognize random antigens, or substances capable of requiring an immune response.

Certain researchers believe that while some B cells and T cells are directed toward fighting an infection, others remain in the bloodstream for months or even years, primed to respond to another invasion of the body. Such "memory" cells may be the basis for immunities that allow humans to survive such plagues as the Black Death of 1347-1351 (see Infectious Diseases). Other immunologists, however, maintain that trace amounts of a pathogen persist in the body and that their continued presence keeps the immune response strong over time.

I MMUNOLOGY

Immunology is the study of how the body responds to foreign substances and fights off infection and other disease-causing agents. Immunologists are concerned with the parts of the body that participate in this response, and this investigation takes them beyond looking merely at tissues and organs to studying specific types of cells or even molecules.

From ancient times, humans have recognized that some people survive epidemics, when the majority are dying. About 1,500 years ago in India, physicians even practiced a form of inoculation, as we discuss later. The modern science of immunology, however, had its beginnings only in 1798, when the English physician Edward Jenner (1749-1823) published a paper in which he maintained that people could be protected from the deadly disease smallpox by the prick of a needle dipped in the pus from a cowpox boil. (Cowpox is a related, less-lethal disease that, as its name suggests, primarily affects cattle.)

Later, the great French biologist and chemist Louis Pasteur (1822-1895) theorized that inoculation protects people against disease by exposing them to a version of the pathogen that is harmless enough not to kill them but sufficiently like the disease-causing organism that the immune system learns to fight it. Modern vaccines against such diseases as measles, polio, and chicken pox are based on this principle.

HUMORAL AND CELLULAR IMMUNITY.

In the late nineteenth century, a scientific debate raged between the German physician Paul Ehrlich (1854-1915) and the Russian zoologist Élie Metchnikoff (1845-1916) concerning the means by which the body protects against diseases. Ehrlich and his followers maintained that proteins in the blood, called antibodies, eliminate pathogens by sticking to them. This phenomenon and the theory surrounding it became known as humoral immunity. Metchnikoff and his students, on the other hand, had noted that certain white blood cells could swallow and digest foreign materials. This cellular immunity, they claimed, was the real way that the body fights infection. In fact, as modern immunologists have shown, both the humoral and cellular responses identified by Ehrlich and Metchnikoff, respectively, play a role in fighting disease.