Allergen immunotherapy (AIT) is a well-established treatment aimed at reducing allergen sensitivity by gradually exposing the immune system to increasing doses of allergens. This promotes desensitization and immune tolerance through multiple mechanisms. AIT offers long-term immune modulation and is considered a potentially curative certain forms of allergic diseases. Altered antibody responses is a key mechanism of AIT in the production of allergen-specific IgG4 antibodies, which act as blocking antibodies to prevent allergen binding to IgE on mast cells (MCs) and basophils. However, IgG4 responses are sometimes ineffective due to variations in antibody affinity and epitope targeting. Reverse class switching from IgE to IgG4 and selective depletion of IgE-producing B cells represent potential strategies to improve AIT efficacy. Tregs play a central role in AIT by suppressing Th2-driven allergic responses and promoting immune tolerance through anti-inflammatory cytokines interleukin (IL)-10 and transforming growth factor (TGF)-β. However, genetic and environmental factors may impair Treg function, leading to AIT failure. AIT reduces MC and basophil activation, leading to long-term suppression of allergic inflammation. It modulates IgE-FcεRI interactions and cytokine signaling pathways, but in some cases, anaphylactic reactions or resistance to MC desensitization may occur. Discussion and conclusions: While AIT is a highly effective allergy treatment, variability in immune responses can impact its success. Advances in biologic therapies offer potential synergies with AIT. Understanding these interactions will help refine AIT strategies and improve patient outcomes.