Question-all ssri's/snri's have different chemical structure but is their at least 1 chemical compound in all of them that could cause allergic react?

Complicated: To best answer you, it would be necessary to know specifically what symptoms you're calling "allergic react." for instance, some people might call nausea an allergic reaction, but it really isn't -- it's a physiologic side effect that all ssri's can cause. Also, generic ssri's may contain different colorings and caking agents that can cause allergic responses. These differ between preparations.