Abstract
Hyaluronic acid fillers are one of the most widely used and versatile fillers worldwide. Although traditionally regarded as immunologically inert, many currently available products have been substantially modified to improve longevity and to optimize properties for specific indications. Such modifications, either alone or in combination with other factors (such as the immune status of the patient, immune-triggering events, and bacterial contamination), may lead to the development of late-onset inflammatory nodules in some patients. This article discusses the clinical presentation of late-onset adverse inflammatory reactions to hyaluronic acid injections, describes their likely triggers, and presents the author's treatment algorithm for successful resolution.