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Immune Functions

根据以下材料，回答问题。

The immune system is equal in complexity to the combined intricacies of the brain and nervous system. The success of the immune system in defending the body relies on a dynamic regulatory communication network consisting of millions and millions of cells. Organized into sets and subsets, these cells pass information back and forth like clouds of bees flying around a hive (蜂巢). The result is a sensitive system of checks and balances that produces an immune response that is prompt, appropriate, effective, and self-limiting. At the heart of the immune system is the ability to distinguish between self and non-self. When immune defenders encounter cells or organisms carrying foreign or non-self molecules, the immune troops move quickly to eliminate the intruders (入侵者). Virtually every body cell carries distinctive molecules that identify it as self. The body's immune defenses do not normally attack tissues that carry a self-marker. Rather, immune cells and other body cells coexist peaceably in a state known as self-tolerance. When a normally functioning immune system attacks a non-self molecule, the system has the ability to "remember" the specifics of the foreign body. Upon subsequent encounters with the same species of molecules, the immune system reacts accordingly.

With the possible exception of antibodies (抗体) passed during lactation (授乳期) , this so called immune system memory is not inherited. Despite the occurrence of a virus in your family, your immune system must "learn" from experience with the many millions of distinctive non-self molecules in the sea of microbes (微生物) in which we live. Learning entails producing the appropriate molecules and cells to match up with and counteract each non-self invader.

Any substance capable of triggering an immune response is called an antigen (抗原) . Antigens are not to be confused with allergens (过敏源) , which are most often harmless substances that provoke the immune system to set off the inappropriate and harmful response known as allergy. An antigen can be a virus, a bacterium, or even a portion or product of one of these organisms. Tissues or cells from another individual also act as antigens; because the immune system recognizes transplanted tissues as foreign, it rejects them. The body will even reject nourishing proteins unless they are first broken down by the digestive system into their primary, non-antigenic building blocks.

An antigen announces its foreignness by means of intricate and characteristic shapes called epitopes (抗原表位) , which protrude (突出) from its surface. Most antigens, even the simplest microbes, carry several different kinds of epitopes on their surface; some may even carry several hundreds. Some epitopes will be more effective than others at stimulating an immune response.

Only in abnormal situations does the immune system wrongly identify self as non-self and execute a misdirected immune attack.
不定项选择题第1题Which of the following best expresses the main idea of this passage?
A.An antigen is any substance that triggers an immune response.
B.One of the immune system's primary functions is the allergic response.
C.The human body is an appropriate habitat for microbes.
D.The basic function of the immune system is to distinguish between self and non-self.