Final answer:
The antigen-presenting molecule found exclusively on macrophages, dendritic cells, and B cells is MHC II, which plays a vital role in the immune system by presenting antigens to helper T cells.
Step-by-step explanation:
The type of antigen-presenting molecule that is found only on macrophages, dendritic cells, and B cells is known as MHC II (Major Histocompatibility Complex class II). Unlike MHC I, which is found on all nucleated cells, MHC II is exclusively expressed on professional antigen-presenting cells (APCs). These APCs process and present antigens to T cells, particularly helper T cells, which are crucial in the immune response. The binding of antigens presented with MHC II molecules to the T cell receptor (TCR) of helper T cells initiates a cascade of immune responses that lead to the activation and proliferation of cells in the adaptive immune system.