Proto Shield is one of the four health types of Corpus, possessed by special units. This type of shield is significantly stronger than the standard Shield type and can be considered the more advanced version of standard shields.
Toxin damage has a +25% bonus against Proto Shield. However, in the vast majority of cases the Toxin damage bypasses shields entirely, rendering the damage bonus pointless. Currently this benefit only comes into play against the Kyta Raknoid.