Abstract Description Oysters, rich in essential nutrients, are renowned in traditional Korean medicine, as noted in the Donguibogam, for their medicinal properties such as tonic effects, calming benefits, osteoporosis prevention, and enhancement of skin and immune health. Compounds derived from oysters are increasingly applied in food, pharmaceuticals, and medical fields for their antioxidant, anti-inflammatory, antimicrobial, and osteogenic activities. Oyster shells, consisting of over 90% calcium carbonate along with other minerals and amino acids, have been traditionally used as antacids in East Asia. Because their influenced immune responses are not fully understood, we investigated the role of ostreae testa water extract (OTW), derived from oyster shells, in modulating T cell activity. Our study reveals that OTW significantly activates T cells in mice splenocytes, with this activation being mediated by dendritic cells and macrophages through cytokine secretion with direct cell-cell interactions. These findings suggest that OTW possesses novel immune-enhancing properties by activating antigen-presenting cells, thereby enhancing T cell-mediated immune responses. OTW-stimulated T cells effect reduced tumor growth in a mouse model, suggesting its potential as an adjuvant in cancer therapy by promoting T cell activation through APCs activation. This study underscores OTW’s potential as a therapeutic agent for enhancing immune function, especially in immunological disorders. Topic Categories Immune Response Regulation: Cellular Mechanisms (IRC)