What is the success rate of PRP, where they inject your scalp with your blood plasma to stimulate growth?

The short answer: a recent study showed excellent response in 60/61 patients. The long: Platelet-rich plasma has been utilized for years in wound healing and orthopedic conditions, but more recently it has gained traction in aesthetic medicine, especially with respect to hair loss. Several clinicians have had some success in treating alopecia of various etiologies (lichen planopilaris, alopecia areata, etc), but most of the limited data have been focused on PRP use for male- and female-pattern hair loss.
The mechanism of action involves platelets’ ability to aggregate together, activate, and release alpha-granules containing various growth factors, such as platelet derived growth factor, transforming growth factor beta, vascular endothelial growth factor, epidermal growth factor, fibroblast growth factor, and insulin like growth factor-1 (IGF-1). Their release increases angiogenesis and vascularization, providing resting telogen hairs the signal that they need to enter the anagen phase. This increased vascularization and mix of growth factors also fosters the proliferation of dermal papilla cells and inhibits apoptosis, helping to prolong the anagen phase.More specific to androgenetic alopecia, PRP may help to counteract certain effects of dihydrotestosterone (DHT). In a mouse model of androgenetic alopecia, DHT was shown to block IGF-1 contributing to hair loss and PRP is a known source of IGF-1.
Platelet-rich plasma is fully autologous and thus provides patients a more “natural” treatment, with side effects limited to procedural discomfort and bruising.
There is no standard system, quantity, or interval of treatment regarding PRP for hair loss, which is why we currently utilize a proprietary hyperconcentrated platelet rich plasma concentration and treat patients monthly 4 times, then reevaluate after 8 to 12 months. If a patient has a response, then retreatment at follow-up may result in continued efficacy if performed at a 6- to 12-month basis.

The majority of patients given that they are the right candidates will respond to PRP hair injections with up to 30% increase in hair density.  There are many variables including type of PRP used, technique of injection, and selecting the right candidates.  I don't think that adding ACell to the PRP treatments is worth the added expense.  Combining PRP injectins with other modalities such as LED light therapy, topical or oral Minoxodil and Propecia will add to the results and prolong its duration of effect.

In the scientific literature, PRP success rates vary, as do the definitions of success. PRP is a great option for many patients, but it is not going to put hair follicles into the scalp where no follicles remain. I believe that PRP can help to slow or reverse the process of hair miniaturization and progressive loss.
All the best,