Symmetry of IP complexity class
There is no relativizing technique to show that $\mathrm{IP}$ is closed under complement.

Clearly, $\mathrm{NP}\subseteq\mathrm{IP}$.

Fortnow and Sipser gave an oracle relative to which $\mathrm{co}$-$\mathrm{NP}$ does not have interactive proof system.

So, you have to study the arithmetization technique that settles down this conjecture (long ago).

Are there interactive protocols for CO-NP