Indicate whether each of the following cell-surface proteins is expressed mainly by B cells (B), dendritic cells (D), or T cells (T). Your answer would be a four-letter string composed of letters B, D, and T only, e.g. DBBB.
( ) Co-stimulatory protein B7, which is recognized by CD28
( ) Invariant CD3 complex
( ) Inhibitory CTLA4 protein
( ) CD40 receptor, which recognizes the co-stimulatory protein CD40 ligand on a helper T cell