"So far the evidence for objective placebo effects is weak. Another hypothesis is that the placebo effect is only subjective, but that these subjective effects can indirectly lead to objectively measurable effects. For example, if you are in pain and the placebo effect decreases your perception of that pain, you might expect your pulse and blood pressure to drop. If you are asthmatic and are wheezing, any psychological factor that reduces your anxiety level or helps you relax might indirectly ease your breathing symptoms and even allow your constricted bronchi to dilate. In this view, the placebo effect doesn’t really cause objective effects, it allows you to have a different experience of your symptoms and it is that different experience that indirectly affects your physiology. Semantic quibbling, perhaps."
An interesting article by a medical doctor about various lines of research on placebo effects. The comments to the submitted article on the original site are interesting too.