This is surfacing the somewhat fuzzy terminology here.

You could argue bitcoin is centralized by that same logic because it depends of the internet.

The immune system and the ways in which the body distinguishes “good” and “bad” stuff, based on my super limited understanding, seems like it’s based on peer information and a sort of voting rather than relying on a central definitive source of information about what’s bad so as to avoid catastrophic failure that would result from corruption of that definitive source.

Given how insanely complex the body is and how many problems its solving I think you could find different systems and pieces that were decentralized, centralized, distributed, etc.

The brain isn't centralized; there is no ultimate authority to arbitrate which loci get control over which stream of consciousness at which time. Every locus operates independently on every connected loci's outputs. Even worse, the interconnects between loci are not centrally managed either, and can strengthen or fade away over time.