Which of the following is not true about the antigens?
a. they have immunogenicity which means they provoke immune responses
b. they have reactivity which means they react with the antibodies or cells they provoked
c. they are often large protein molecules
d. they always invade the human body from the external environment
e. they may be natural or man-made molecules

d. They always invade the human body from the external environment

Explanation:

Antigens are the molecules that have the ability to stimulate immune response and are mostly protein in nature. However, an antigen may be self or non-self molecule.

This means that antigen may enter the human body from the external environment (the non-self antigen) or any molecule present inside the body may serve as antigen (self-antigen). Self-antigens are responsible for autoimmune disorders.