Which Statement Accurately Describes a Function of the Immune System

Which statement accurately describes a function of the immune system?

A) When an antigen enters the body, the body responds by producing antibodies.
B) The antigen-antibody reaction releases chemical mediators known as allergies.
C) An allergic reaction may occur at the first exposure to an antigen.
D) Normally, the body is not able to distinguish "self" from "not self."