Tissue engineering and regenerative medicine aim to restore or replace damaged tissues and organs using cells, biomaterials, and bioengineering techniques. These approaches promise to overcome donor shortages and revolutionize clinical treatments. Central to their success is the host immune response, with immunoassays playing a key role in monitoring graft integration and biological function. This chapter explores the interplay between innate and adaptive immunity in tissue engineering, highlighting the importance of immunological compatibility, biomaterial design, and strategies to prevent immune rejection. Multidisciplinary research continues to advance therapeutic and diagnostic applications in this transformative field.