Very interesting that your irritable bladder has more-or-less disappeared following rituximab treatment. As you say, it does suggest that irritable bladder may have a similar B-cell / autoantibody etiology.
Interstitial cystitis (IC), which is related to irritable bladder (IB), is a known co-morbid condition of ME/CFS, being statistically associated with ME/CFS. I have often speculated that these bladder problems may have a similar pathophysiology to ME/CFS. This is why I think if you already have IC or IB, it may predispose you to getting ME/CFS. Irritable bowel syndrome (IBS) is another condition that is co-morbid with ME/CFS, and so having IBS probably also predisposes you to ME/CFS.
I myself had both irritable bladder and Irritable bowel syndrome before I developed ME/CFS from a viral infection, and my guess is that if I did not have IB and IBS beforehand, I would not have come down with ME/CFS.
All these conditions of IC, IB, IBS and ME/CFS may well have a similar pathophysiology, probably involving the production of damaging autoantibodies, and so an individual suffering from several of these conditions together may get more ill more because these similar conditions "gang up" together to worsen each other.
(In
this thread, I speculated that autoantibodies to muscarinic acetylcholine receptors might underpin ME/CFS, IC and IBS. If there is any mileage in this speculation, certainly rituximab would reduce these damaging autoantibodies, and then alleviate these conditions).