(1) (2) (3)A. Dendritic cell engulfs antigen, processes it, and displays its fragments on class II MHC protein. B. Immunocompetent CD4 cell recognizes antigen-MHC complex. Both TCR and CD4 protein bind to antigen- MHC complex. C. CD4 cells are activated, proliferate (clone), and become memory and effector cells..