From human allergen immunotherapy to veterinary prophylactic vaccines: translational potential of Microcrystalline tyrosine as a vaccine adjuvant.

Microcrystalline tyrosine (MCT) is a biodegradable adjuvant composed of crystallized L-tyrosine that has been used for more than five decades in human allergen-specific immunotherapy. Despite its long-standing clinical use and well-documented safety profile, its application in veterinary prophylactic vaccination remains largely unexplored. This review critically integrates and compares current knowledge on the physicochemical properties, manufacturing principles, biodistribution, mechanisms of action, immunological effects, and safety profile of MCT, with aluminum oxyhydroxide considered as a reference adjuvant widely used in both human and veterinary vaccines. Available evidence indicates that MCT acts as a transient antigen depot while remaining biodegradable and rapidly resorbed, inducing mild and self-limiting local inflammatory responses. Experimental and clinical data suggest that MCT can promote balanced immune responses with favorable tolerability across multiple species. Although its use in veterinary vaccinology remains limited, preclinical studies in animal models and recent experimental data in sheep demonstrate its capacity to induce antibody responses against infectious agents, including bluetongue virus. Taken together, these findings highlight MCT as an underexplored adjuvant platform with translational potential for veterinary prophylactic vaccines. By consolidating available evidence and identifying key knowledge gaps, this review provides a framework for future research and supports the evaluation of MCT in veterinary vaccine development, with potential implications for both animal and human vaccinology.