Introduction & Taxonomy of Immunology Practice Test

Natural killer cells are identified by their surface marker profile: they lack CD3, which is a classic T cell marker, and they express CD56. Among the markers listed, CD56 is the one that signals NK cells in this context, while CD3 marks T cells, CD4 marks helper T cells, and CD19 marks B cells. So, NK cells are typically characterized as CD3− CD56+, distinguishing them from T cells (CD3+) and B cells (CD19+).