Allergen immunotherapy (AIT) shows great promise not only in treating allergies but also in preventing the development of new allergic conditions, which has recently gained scientific attention. The aim of this review is to summarize current knowledge on how AIT modulates immune responses and its potential as a preventive tool for allergic diseases. By targeting the underlying immunological mechanisms, AIT offers the possibility of not only alleviating symptoms but also preventing the progression of new allergies. This review is based on literature published in English, sourced mainly from PubMed and Google Scholar. Only full-text original studies, reviews, and meta-analyses were included. Animal studies, conference abstracts, and non-peer-reviewed content were excluded. AIT reshapes immune responses by shifting from Th2 dominance to a more balanced Th1/regulatory pattern. This includes reduced IgE production, increased IgG4, and limited activation of effector cells. Some studies have shown AIT reduces the risk of new sensitization in children and adults, although results vary depending on allergen type, patient age, and treatment method. AIT appears to be a promising strategy for allergy prevention, offering more durable and disease-modifying effects than other methods such as elimination diets, probiotics, or allergen avoidance. However, more research is needed to define its optimal use, ensure long-term safety, and personalize treatments based on individual risk profiles.