Three reasons:
* The poison/venom is contained (generated in its own self contained gland separated from the rest of the body)
* The poison/venom is generated on demand (e.g. created from different components that are combined when neeed)
* It has a natural immunity to it. Either it contains the proper chemical or enzyme that can break down the poison/venom or it lacks the biological component that the poison/venom interacts with to cause its debilitating effects.
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