Immune System

I

Introduction

Immune System, group of cells, molecules, and organs that act together to defend the body against foreign invaders that may cause disease, such as bacteria, viruses, and fungi. The health of the body is dependent on the immune system’s ability to recognize and then repel or destroy these invaders.

II

Immunity: Innate and Adaptive

Most animals have systems that resist disease. The disease resistance provided by these systems is called immunity. There are two types of immunity: innate and adaptive. Innate, or nonspecific, immunity is the body’s first, generalized line of defense against all invaders. Innate immunity is furnished by barriers such as skin, tears, mucus, and saliva, as well as by the rapid inflammation of tissues that takes place shortly after injury or infection. These innate immune mechanisms hinder the entrance and spread of disease but can rarely prevent disease completely.

If an invader gets past this first line of defense, the cells, molecules, and organs of the immune system develop specifically tailored defenses against the invader. The immune system can call upon these defenses whenever this particular invader attacks again in the future. These specifically adapted defenses are known as adaptive, or specific, immunity.

Adaptive immunity has four distinguishing properties: First, it responds only after the invader is present. Second, it is specific, tailoring each response to act only on a specific type of invader. Third, it displays memory, responding better after the first exposure to an invader, even if the second exposure is years later. Fourth, it does not usually attack normal body components, only those substances it recognizes as nonself.

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Adaptive immune responses are actually reactions of the immune system to structures on the surface of the invading organism called antigens. There are two types of adaptive immune responses: humoral and cell mediated. During humoral immune responses, proteins called antibodies, which can stick to and destroy antigens, appear in the blood and other body fluids. Humoral immune responses resist invaders that act outside of cells, such as bacteria and toxins (poisonous substances produced by living organisms). Humoral immune responses can also prevent viruses from entering cells.

During cell-mediated immune responses, cells that can destroy other cells become active. Their destructive activity is limited to cells that are either infected with, or producing, a specific antigen. Cell-mediated immune responses resist invaders that reproduce within the body cells, such as viruses. Cell-mediated responses may also destroy cells making mutated (changed) forms of normal molecules, as in some cancers.

III

Components of the Immune System

The ability of the immune system to mount a response to disease is dependent on many complex interactions between the components of the immune system and the antigens on the invading pathogens, or disease-causing agents.

A

Macrophages

White blood cells are the mainstay of the immune system. Some white blood cells, known as macrophages, play a function in innate immunity by surrounding, ingesting, and destroying invading bacteria and other foreign organisms in a process called phagocytosis (literally, “cell eating”), which is part of the inflammatory reaction. Macrophages also play an important role in adaptive immunity in that they attach to invading antigens and deliver them to be destroyed by other components of the adaptive immune system.

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