Final answer:
In the context of the immune system, professional antigen-presenting cells such as macrophages, dendritic cells, and B cells present antigens on class II MHC molecules, which activate T cells rather than marking the cell for destruction.
Step-by-step explanation:
Normally, when cells express endogenous foreign antigens on class I MHC (Major Histocompatibility Complex) proteins on their cell membrane, they are marked for destruction. This is not true for professional antigen-presenting cells such as macrophages, dendritic cells, and B cells. On these cells, the presentation of antigens acts as an activation signal for T cells. These professional antigen-presenting cells have the unique ability to present extracellular antigens on class II MHC molecules, unlike all other nucleated cells which present antigens only on class I MHC molecules. The T cells, specifically helper T cells, are then activated to coordinate an immune response.