25-OH Vitamin D3-BSA is a conjugate of 25-OH Vitamin D3 with bovine serum albumin (BSA), serving as an antigen-adjuvant complex. Conjugating the antigen with a protein adjuvant boosts the production of antigen-specific antibodies in vaccine models. This conjugate does not disrupt protein folding or alter primary epitopes, while it enhances cross-presentation and the generation of antigen-specific T cells.