if you already had the chiken pox you can be arournd a person with the disease without no fear of becoming sick. what feature of the immune system explains this?

a. acquired immunity
b. cell- mediated immunity
c. humoral immunity
d. primary immune system

http://www.niaid.nih.gov/topics/immunesystem/Pages/immunity.aspx

The correct answer is a. acquired immunity.

Acquired immunity is a feature of the immune system that develops after an individual is exposed to a pathogen, such as the chickenpox virus, and their immune system mounts an effective response against it. In the case of chickenpox, once a person has had the disease, their immune system develops memory cells, called B cells and T cells, that specifically recognize and remember the chickenpox virus.

If that person is exposed to the virus again in the future, their immune system quickly recognizes it and mounts a rapid and specific response to eliminate the virus before it causes a full-blown infection. This acquired immunity is what allows a person who has had chickenpox to be around someone with the disease without fear of becoming sick again. The immune system's memory cells provide protection by recognizing and eliminating the pathogen more efficiently.