History of Immunology

The science of immunology arose from the knowledge that those who survived one of the common infectious diseases of the past rarely contracted the disease again. As early as 430 bc, during the plague in Athens, Thucydides recorded that individuals who had previously contracted the disease recovered and he recognized their “immune” status.

Beginning about 1000 ad, the Chinese practiced a form of immunization by inhaling dried powders derived from the crusts of smallpox lesions. In the 15th century, powdered smallpox “crusts” were inserted with a pin into the skin. When this practice became popular in England, it was discouraged at first, partly because the practice of inoculation occasionally killed or disfigured a patient.

Louis Pasteur is generally considered to be the Father of Immunology. Table 1-1 lists some historic benchmarks in immunology.

What is immunology?

Immunology is defined as resistance to disease, specifically infectious disease. Immunology consists of the following: the study of the molecules, cells, organs, and systems responsible for the recognition and disposal of foreign (nonself) material; how body components respond and interact; the desirable and undesirable consequences of immune interactions; and the ways in which the immune system can be advantageously manipulated to protect against or treat disease (Box 1-1). Immunologists in the Western Hemisphere generally exclude from the study of immunology the relationship among cells during embryonic development.

The immune system is composed of a large complex set of widely distributed elements, with distinctive characteristics. Specificity and memory are characteristics of lymphocytes (see Chapter 4). Various specific and nonspecific elements of the immune system demonstrate mobility, including T and B lymphocytes, immunoglobulins (antibodies), complement, and hematopoietic cells.

Cells of the Immune System

Cooperation is required for optimal functioning of the immune system. This cooperative interaction involves specific cellular elements, cell products, and nonlymphoid elements.

Cells of the immune system consist of lymphocytes, specialized cells that capture and display microbial antigen, and effector cells that eliminate microbes (see Color Plate 1). The principal functions of the major cell types involved in the immune response are as follows:

Function of Immunology

The function of the immune system is to recognize self from nonself and to defend the body against nonself. Such a system is necessary for survival. The distinction of self from nonself is made by an elaborate, specific recognition system. Specific cellular elements of the immune system include the lymphocytes. The immune system also has nonspecific effector mechanisms that usually amplify the specific functions. Nonspecific components of the immune system include mononuclear phagocytes, polymorphonuclear leukocytes, and soluble factors (e.g., complement).

Nonself substances range from life-threatening infectious microorganisms to a lifesaving organ transplantation. The desirable consequences of immunity include natural resistance, recovery, and acquired resistance to infectious diseases. A deficiency or dysfunction of the immune system can cause many disorders. Undesirable consequences of immunity include allergy, rejection of a transplanted organ, or an autoimmune disorder, in which the body’s own tissues are attacked as if they were foreign. Over the last decade, a new concept, the danger theory, has challenged the classic self-nonself viewpoint; although popular, it has not been widely accepted by immunologists (see Chapter 4).

Before a pathogen can invade the human body, it must overcome the resistance provided by the body’s first line of defense (Fig. 1-1). The first barrier to infection is unbroken skin and mucosal membrane surfaces. These surfaces are essential in forming a physical barrier to many microorganisms because this is where foreign materials usually first contact the host. Keratinization of the upper layer of the skin and the constant renewal of the skin’s epithelial cells, which repairs breaks in the skin, assist in the protective function of skin and mucosal membranes. In addition, the normal flora (microorganisms normally inhabiting the skin and membranes) deter penetration or facilitate elimination of foreign microorganisms from the body.