Immunologic Aspects of Organ Transplantation

Susan Smith MN, PhD

Disclosures

June 17, 2002

Functions of the Mature Immune System

The word immune is derived from the Latin term immunis, which translates as "free from taxes or free from burden."[1] And indeed, the human immune system (Figure 1) functions to protect us from the burden of injury related to potentially harmful environmental substances and organisms. The mature immune system consists of millions of cells capable of performing 3 general types of functions: defense, homeostasis, and surveillance.

Figure 1.

The immune system.

In providing defense, resistance to infection is facilitated by both nonspecific phagocytic mechanisms and more specific immune responses that not only destroy, but also remember, foreign antigens. Maintaining immunologic homeostasis encompasses keeping a balance between immune protective and immune destructive responses and the removal of senescent or dead immune cells from the body. Although the function of the immune system is inherently protective, there are conditions in which immune responses become destructive to the host, such as autoimmune diseases and anaphylactic reactions. Surveillance involves the recognition of microorganisms bearing foreign antigens. Some of the immune cells, lymphocytes in particular, are highly mobile and travel throughout the vascular and lymphatic systems in surveillance of potentially harmful antigens. Some types of cancer cells in particular are sought out and destroyed by immune cells.

Immunocompetence, then, is the possession of a mature immune system that functions in at least 3 ways to: recognize, destroy, and remember foreign antigens. A lack of this essential property of immunocompetence is termed anergy, which is manifested by an inability to mount an effective immune response to a foreign antigen. Immune responses can be classified into 2 major types: (1) nonspecific responses, which are also called natural or innate responses, and (2) specific responses, which are the acquired responses. Both types of responses play critical roles in host defense.

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