IBD approved Biologics (remicade, humira, simponi, entyvio, stelara, cimzia) are all biologic monoclonal antibodies. They're large proteins molecule that act on the outside surface of cells. As biologics are antibodies, your immune system can create antibodies against them. Biologics are generated by well established cellular lineages (mouse, hamster, or several strains of bacteria) that are reprogrammed to be protein factories producing medicine. There's no mouse parts in Biologics.
Xeljanz is a tiny molecule that acts inside of cells. Xeljanz technically isn't a biologic, but it is a very strong medicine similar in effectiveness as biologics so usually talked about together. Your immune system cannot create antibodies against xeljanz.
Interesting, thank you. I'll have to look up how Xeljanz stops working then if antibodies aren't created. I suppose like other drugs like mesalamine stop working as well.