I have sle. is it likely that a tattoo will cause a flare up? if so, how? (i.e activate immune system?)

No: It will not cause a flare of the lupus but be careful about infection if you are on Prednisone or immunosuppressants. Also remember that Prednisone delays wound healing. All in all, i would tell you to not have it done.

Not likely: Technically, any stress could cause a lupus flare, but a tattoo done by a professional using sterile equipment is not likely to cause a flare if your lupus is well controlled. Do not attempt a tattoo if you are in poor health or have frequent skin infections.

It may: Your skin is actively involved in sle and putting the ink in your skin could trigger a flare. Also the ink can intensify the sun effect and cause a further reaction. All in all, probably not a good idea.