Immune System

Historical Context

In ancient times it was believed that a disease was a result of supernatural forces imposing a punishment for evil. In those times the Latin word immunis (immunity) meant dismissal from tax, military service, and other public services. The Greek historian Thucydides (ca. 460–395 B.C.E.) is considered to be the first to leave a written description of the concept of immunity. In 430 B.C.E., during an epidemic in Athens, he noticed that people who had fought off the disease could look after the ill and the dying without threat of renewal of the disease. The Islamic physician Al-Razi (865–925 C.E.) is believed to be the first to describe immunity in a clinical way in Kitab fi al-Jadari wa-al-Hasbah (A Treatise on Smallpox and Measles). He concluded that contact with a pathogen contributed to the arousal of immunity in the body.

The great development in immunology came toward the end of the 19th century as a result of research on humoral and cellular immunity. The German physician and scientist Paul Ehrlich (1854–1915), with his side-chain theory, made a special contribution to this branch of medical science. He explained the specificity of the antigen-antibody reaction. In 1908, for his research on humoral immunity, together with Russian biologist Elie Metchnikoff (1845–1916), the founder of cellular immunology, he was awarded the Nobel Prize in Physiology or Medicine.

Types of Immunity

Figure 1 presents types of immunity. There is nonspecific immunity, which is innate, and acquired specific immunity. Nonspecific immunity is genetically based and consists of various mechanical barriers. The first of them is the skin, a kind of natural barrier for most unwanted organisms, and its secretions. Mucus, saliva, tears, sweat, and urine are some examples of them. Pyrogen increases body temperature, lysozyme destroys bacterial cell walls, and interferon represses the process of creation of viruses. There is also a crucial role for reflexes like coughing, sneezing, vomiting, or diarrhea that remove pathogens from the body.

Figure 1Types of immunity

Another kind of immunity is a specific immunity, which is acquired. There are two kinds of specific immunity: active, which develops slowly, and passive, which develops immediately. Active immunity is based on immune memory and is connected with formation of specific antibodies. This kind of immunity is activated after contact with a foreign antigen. There are two types of such contact: natural and artificial. If an organism is infected and fights disease, naturally acquired active immunity is induced; but if an organism is in contact [Page 587]with a substance that contains antigen, like a vaccine, artificially acquired active immunity is induced. Passive immunity, in turn, is not based on immune memory, nor is it connected with the formation of antibodies. These antibodies and lymphocytes come from and are preformed in another host. Passive immunity also may be classified into two kinds: naturally acquired and artificially acquired. Naturally acquired passive immunity is formed during the fetal period and refers to the antibodies passing through the placenta from the mother to the baby. Artificially acquired passive immunity is a kind of short-term immunization induced by the transfer of antibodies, like antisera, blood plasma, pooled human immunoglobulin for intravenous (IVIG) or intramuscular (IG) use, and in the form of monoclonal antibodies (MAb).

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