A GLIMPSE OF HISTORY
During the latter part of the 19th century, the studies of Louis Pasteur, Robert Koch, and others created much interest in microorganisms and the diseases that they caused. Once microorganisms were shown to cause disease, scientists worked to explain how the body defended itself against invasion by microorganisms. Elie Metchnikoff, a Russian-born scientist, theorized that there were specialized cells within the body that could destroy invading organisms. His ideas arose from observations that he made while studying the transparent immature larval form of starfish in Sicily in 1882. As he looked at the larvae in the microscope, he could see amoebalike cells within their bodies. He described his observations as follows:
"...I was observing the activity of the motile cells of a transparent larva, when a new thought suddenly dawned on me. It occurred to me that similar cells must function to protect the organism against harmful intruders....I thought that if my guess was correct a splinter introduced into the larva of a starfish should soon be surrounded by motile cells much as can be observed in a man with a splinter in his finger. No sooner said than done. In the small garden of our home...I took several rose thorns that I immediately introduced under the skin of some beautiful starfish larvae which were as transparent as water. Very nervous, I did not sleep during the night, as I was waiting for the results of my experiment. The next morning, very early, I found with joy that it had been successful."
Metchnikoff reasoned that certain cells present in animals were responsible for ingesting and destroying foreign material. He called these cells phagocytes, meaning cells that eat, and he proposed that these cells were primarily responsible for the body’s ability to destroy invading microorganisms.
When Metchnikoff returned to Russia, he looked for a way to study the ingestion of materials by phagocytes, called phagocytosis. The water flea Daphnia sp., which could be infected with a yeast, provided a way to do such studies. He observed phagocytes ingesting and destroying invading yeast cells within the experimentally infected transparent water fleas. In 1884, Metchnikoff published a paper that strongly supported his contention that phagocytic cells were primarily responsible for destroying disease-causing organisms. He spent the rest of his life studying phagocytosis and other biological phenomena and in 1908 was awarded the Nobel Prize in Physiology or Medicine for these early studies of immunity.
From before Elie Metchnikoff’s discoveries in the late 1800s to the present day, scientists have been studying ways in which animals, especially humans and other vertebrates, are able to protect themselves against invasion by foreign organisms or substances. The field of immunology is devoted to studying these many mechanisms of defense.
Although the science of immunology arose from studies of immunity, or protection, against infectious agents, it encompasses many other areas. Immune mechanisms operate not only against infectious agents, but also against cancers. They are also responsible for rejection of transplanted cells and organs. Under some conditions, immune responses may be directed against the cells of one’s own body, causing autoimmune responses. Sometimes they cause damage as well as provide protection, thereby resulting in allergic or hypersensitivity reactions. When a host lacks effective immunological defense mechanisms, even the most innocuous microorganisms may become deadly opportunists, taking advantage of an opportunity to cause disease, as seen in the current AIDS epidemic.
Immune reactions can be used in the laboratory to identify substances with a high degree of specificity. For example, immunological techniques are used to type blood and tissue cells, diagnose diseases, classify bacteria, and identify suspects in the investigation of criminal cases.