Final answer:
An antigen is a molecule that stimulates the production of antibodies and reacts specifically with them. They play a crucial role in both humoral and cellular immunity.
Step-by-step explanation:
An antigen is a molecule that stimulates the production of antibodies and reacts specifically with them. It is also known as an immunogen. Antigens play a crucial role in both humoral and cellular immunity. They stimulate B cells to produce antibodies, which bind to antigens and target them for destruction.