Practice question

CD3 is the signaling component of T cell receptor, exclusive to T lymphocytes. CD4 is classically a T helper marker but also expressed at lower levels on monocytes, macrophages and dendritic cells where CD3 is absent. A population lacking CD3 but expressing CD4 cannot be conventional T cells, nor B cells which are CD19 positive, nor NK cells which are CD56 positive. Monocytes show CD3 negative CD4 dim phenotype with CD14 coexpression and characteristic scatter, fitting this specific immunophenotypic profile.