Final answer:
Helper T cells require assistance from antigen-presenting cells like macrophages to recognize antigens. Macrophages provide this 'help' by presenting antigens on their surface with MHC proteins, facilitating T cell activation.
Step-by-step explanation:
A macrophage, which is a phagocytic cell, provides this 'help' by digesting pathogens and displaying part of the pathogen on its surface with the help of Major Histocompatibility Complex (MHC) proteins; thus functioning as an antigen-presenting cell. Helper T cells, upon interacting with these antigens, become activated and can then facilitate the activation of other immune cells, including B cells and cytotoxic T cells. In contrast, Natural killer cells do not require such help as they are capable of recognizing and killing cells that do not express 'self' proteins or that contain abnormal markers.