Passage I

Table 1 lists various components of the human immune system and some of their actions. An antigen is a foreign substance that elicits an immune response. An antibody is a molecule that binds specific antigens.

Figures 1 and 2 show the time periods during which cell and/or antibody concentrations are typically above normal during the 28 days after first exposure to an antigen.

Figure 3 shows the stages of infection in humans exposed to the human immunodeficiency virus (HIV), the virus that causes AIDS.

Table 1 and Figure 3 adapted from Neil A. Campbell, Biology, 3rd ed.

©1993 by the Benjamin/Cummings Publishing Company, Inc.

Figure 1 and 2 adapted from Peter H. Raven and George B. Johnson,

Understanding Biology. ©1988 by Times Mirror/Mosby College Publishing.

1. According to Figure 3, approximately how many years after a patient's first exposure to HIV will the patient's helper T cell concentration fall below 40 cells/cm³？

Answer and Explanation

Your Answer is

Correct Answer is D

Explanation

Look at the helper T cell concentration curve (dotted line) in figure 3, when the abscissa is greater than 7 years, the ordinate is less than 50 cells/cm3;

So when When the concentration is 40 cell/cm3, the abscissa must be greater than 7 years, and option D meets