Science

It's not chemicals but unique cellular patters (like flagella, peptidoglycan, and LPS to name a few) which are recognized as not being apart of self. Training occurs before B and T Cells ever see a foreign pathogen. It occurs in the bone marrow for B Cells and in the Thymus for T Cells. The biggest aspect of training is not recognizing pathogens but actually not binding to self. This is what helps to prevent autoimmunity. Only a small percent of B and T Cells pass this training as their B and T Cell receptors are made entirely at random. So, even if a B or T Cell passes training, there's a chance it'll never be used as it doesn't bind to a single pathogen epitope. Basically the production of our adaptive immune response is super energy intensive due to soo many cells failing training, plus another portion that pass not being useful for the pathogens we encounter. However, this process is also the reason a small percentage of folks were immune to COVID before ever encountering the virus thanks to the random nature of B and T Cell receptor formation. The immune system is super intriguing and it's 100% my favorite subject to study!