With the advances in understanding the relationships among biomaterials, the immune system and the skeletal system, the host responses elicited by implanted biomaterials can be balanced by properly designing material characteristics from the perspective of osteoimmunology. The immunoregulatory properties of bone tissue engineering scaffolds provide advantages for inducing macrophages from the proinflammatory M1 phenotype to the anti-inflammatory M2 phenotype and promoting osseointegration. Hydrogels are increasingly a focus in bone tissue engineering, and the immune response can be affected by different compositions of hydrogels, such as the sources, concentration, molecular weight, coupling with fibronectin, and the addition of cross-linking agents. Different physicochemical properties of modified hydrogels can trigger different host immune responses, modified by using soft photolithography to fabricate micropatterned hydrogels, adding enzyme-sensitive sequences, ester bonds and dynamic covalent chemistry to prevent rapid or slow degradation of the hydrogels, and adding porogens and 3D printing to modify the hydrogels with macroporous interconnective pore structures, soft and injectable hydrogels, etc. These optimized hydrogels can reduce proinflammatory factors, promote M2 macrophage polarization, and minimize foreign body reactions to evoke bone regeneration. However, the mechanism underlying the bone immune response is still poorly understood, and further study of the effects of hydrogels with different physicochemical properties on immune regulation is needed.

Periodontitis is a chronic infectious disease that occurs in periodontal support tissues. Plaque microorganisms are its initiating factor, while local inflammation and alveolar bone loss resulting from periodontitis are the most common causes of tooth loss. Interleukin-17 (IL-17), which plays an important role in the immune response to periodontitis, mostly originates from T helper cell 17 (Th17) and γδT cells. In periodontitis, the role of Th17 cells has been demonstrated broadly, but the role of γδT cells was not revealed until recent years. As a highly heterogeneous group of T lymphocytes, γδT cells are considered a link between innate immunity and adaptive immunity. Studies have found that γδT cells are mostly distributed in the oral epithelium near the biofilm, where they can interact with microorganisms to produce IL-17, recruit neutrophils, macrophages, etc., and participate in the host immune response to periodontitis. They also play a role in the association between periodontitis and relevant systemic diseases. In addition, γδT cells have been proven to produce tissue repair-related factors with a protective effect against periodontitis. The possible mechanism of γδT cells in periodontitis is discussed in this review.