What are antigens, and how do lymphocytes “recognize” them?

What are antigens ? - Antigens is a toxin or other foreign substance that induces an immune response in the body, especially the production of antibodies.

How do lymphocyte "recognize" them ? - However, the T-cell receptor differs from the B-cell receptor in an important way: It does not recognize and bind antigen directly, but instead recognizes short peptide fragments of pathogen protein antigens, which are bound to MHC molecules on the surfaces of other cells.