[Dylan16807]

"The role of immune dysfunction in BD is currently unclear, with low-grade chronic inflammation (increased plasma cytokines, soluble cytokine receptors, chemokines, acute phase reactants) and T-cell activation features that may be associated with BD, but the results are controversial"

If this is your best citation, then I don't know why you expect everyone to know about and be confident in this theory. And that's without even questioning whether bipolar and depression are the same thing.

Your search results are all over the place and don't show anything coherent.

[FollowingTheDao]

Sorry, wrong link.

https://www.cell.com/neuron/pdf/S0896-6273(20)30431-1.pdf

“It is now well established that dysregu- lation of both the innate and adaptive immune systems occur in depressed patients and hinder favorable prognosis, including antidepressant responses.”

[Dylan16807]

That sounds like evidence it's an important factor in treatment, but it doesn't mean we can be confident it's the cause.