Final answer:
Immune cells use MHC class I and II molecules to present antigens from pathogens, signaling the immune system to respond appropriately to 'non-self' invaders.
Step-by-step explanation:
Immune related cells, such as antigen-presenting cells (APCs), display two classes of major histocompatibility complex (MHC) molecules on their surface: MHC class I and MHC class II. MHC class I molecules are present on all nucleated cells and are vital for signaling the immune system about the cell's health, presenting both normal self-antigens and those of invading pathogens. When cells become infected, they can present foreign antigens on MHC I, alerting cytotoxic T cells to destroy the infected cell. In contrast, MHC class II molecules are expressed specifically on macrophages, dendritic cells, and B cells. These cells present pathogen fragments for the activation of helper T cells. When a pathogen invades, fragments resulting from the pathogen's breakdown and processing are loaded onto MHC molecules and presented on the cell surface, indicating the presence of 'non-self' entities that must be targeted by the immune response.
Immune cells have two major histocompatibility class (MHC) molecules, MHC I and MHC II, which bond to fragments of invading pathogens. MHC I molecules are found on all nucleated cells and present abnormal or nonself pathogen antigens to T cells involved in cellular immunity. On the other hand, MHC II molecules are only found on macrophages, dendritic cells, and B cells, and they present abnormal or nonself pathogen antigens for the initial activation of T cells.