Which of the following best expresses the main idea of this passage?______A.An antigen is
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 immunes 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 nonself.