Antigen is a substance which

An antigen is any molecule or substance that can be recognised by the immune system and provoke an immune response; specifically, antigens stimulate lymphocytes (B cells) to produce antibodies tailored to that antigen [3]. Educational and clinical sources describe antigens as “antibody generators” because exposure to an antigen leads to production of specific antibodies which bind the antigen and mark it for destruction or neutralization by the immune system [2]. Vaccines use harmless antigenic parts of germs to train the immune system to form antibodies without causing disease, illustrating the antigen’s role in stimulating antibody formation [2].

Sources

1. [1] https://egyankosh.ac.in/bitstream/123456789/101521/1/Unit-3.pdf
2. [3] https://www.britannica.com/science/antigen