you hit it right on the button...it IS an immune reaction. My onc explained that to me. i have been off of herceptin now for two months, but my rings still don't slide easily over my knuckles, and i am always stiff after sitting or lying down for periods of time. yoga helps immensely, and for the rest of the day after practicing, i feel great. sweating, yoga, any kind of stretching is great for this joint achiness/stiffness...