Answer:
The correct answer is - antigen.
Explanation:
Antigens are the substances or molecules which are capable to cause an immune response in an individual organism by activating white blood cells or lymphocytes which acts as infection-fighting cells of the body.
Antigen stimulates the immune response by producing antibodies. It occurs when the body does not recognize the substance and try to eliminate the substances. The antigen can be a virus, pollen, bacteria and many more.
Thus, the correct answer is - antigen.