I've been studying narrow-band UV-B phototherapy used for treatment of psoriasis and eczema. Generally, the administering physician will begin at a low enough UV dose to avoid burning, and gradually increase dose treatment to treatment. If this is done carefully, the dose can be brought to several times what an unadapted person can handle without severe burning, without any noticeable tanning.
This would seem to indicate that the body has ways of adapting to UV exposure other than melanin production, and moreover that these other ways are the primary method of UV defense, considering that tanning only occurs if UV dose is increased too quickly. What are these other mechanisms?
Edit: I asked this question a few years ago, back when my knowledge of the topic was far weaker. As three years later, I'm still very much interested in an answer, I updated the question slightly to reflect better knowledge of the subejct matter.