Final answer:
MHC I molecules are found on all nucleated cells and are involved in presenting antigens to effector T cells. MHC II molecules are present only on antigen-presenting cells such as macrophages, dendritic cells, and B cells for activating helper T cells.
Step-by-step explanation:
The type of antigen-presenting molecule found on all nucleated cells is MHC I (major histocompatibility complex class I). MHC I molecules are essential for presenting normal self-antigens as well as abnormal or nonself pathogens to the effector T cells involved in cellular immunity. On the other hand, MHC II molecules are found only on specific antigen-presenting cells like macrophages, dendritic cells, and B cells, and they are instrumental in presenting pathogen antigens for the initial activation of helper T cells.