“ MHC overall are the antigen presentation centres of cells and play a role in immune cell activation. MHC class I is found on all cells with a nucleus, MHC class II is only found on APCs. MHC class I not only activates cytotoxic T cells, it also acts as a way for those T cells to distinguish self from non self. You see during a viral infection your infected cells start to make viral proteins since they hijack the nucleus which is what viruses take over and use to make more of themselves. So as they are making those proteins, they take tiny pieces and present some of them on their surfaces to immune cells for inspection. This plays a vital role in viral clearance AND cancer clearance. It lets cytotoxic T cells know something is wrong and that the cell has gotta go. And that’s exactly what happens as soon as the TCR recognizes the viral/cancerous antigen on an MHC class I molecule. Meanwhile MHC class II plays more of a role in the activation of T cells, specifically helper T cells. Macrophages, dendritic cells and B cells all express MHC class II. They are therefore APCs that take in antigen from the outside and essentially break it down into pieces and present it to incoming naive helper T cells for activation.”

The input correctly states that MHC molecules are central to antigen presentation and immune cell activation. MHC class I molecules are indeed expressed on nearly all nucleated cells and present endogenous peptides (including viral or cancer-derived peptides) to cytotoxic CD8+ T cells, enabling these T cells to distinguish self from non-self and target infected or malignant cells for destruction. This is a fundamental mechanism in viral clearance and cancer immunosurveillance. The description of infected cells presenting viral peptides on MHC class I aligns with established immunology.

MHC class II molecules are primarily expressed on professional antigen-presenting cells (APCs) such as macrophages, dendritic cells, and B cells. These APCs process exogenous antigens and present them via MHC class II to CD4+ helper T cells, which is crucial for initiating adaptive immune responses.

However, the input's statement that viruses hijack the nucleus to make viral proteins is an oversimplification. While some viruses (like DNA viruses) replicate in the nucleus, many RNA viruses replicate in the cytoplasm. Viral protein synthesis generally occurs in the cytoplasm using host ribosomes. The core concept that viral peptides are presented on MHC class I remains correct.

Overall, the explanation captures key immunological principles accurately but with minor simplifications typical of a general overview.