B

Dendritic cells are professional antigen-presenting cells that express abundant MHC and co-stimulatory molecules and are uniquely efficient at providing the two signals required to prime naïve T lymphocytes; macrophages or B cells generally stimulate previously primed or memory T cells, and NK cells are not APCs [Ni & O'Neill, 1997, PMID 9243286; Knight & Stagg, 1993, PMID 8347299].