The immune system utilizes Major Histocompatibility Complex (MHC) molecules to present antigens, which are crucial for the activation of T cells. There are two primary classes of MHC molecules: MHC class I and MHC class II, each serving distinct roles in immune response.

MHC class I molecules are present on all nucleated cells and are responsible for presenting endogenous antigens, which originate from within the cell. These antigens often include viral proteins produced by infected cells. The recognition of these antigens is facilitated by cytotoxic T cells (TC cells), specifically those expressing the CD8 marker. When a TC cell identifies an antigen presented by MHC class I, it can initiate an immune response to eliminate the infected cell.

In contrast, MHC class II molecules are found exclusively on antigen-presenting cells (APCs), such as dendritic cells, macrophages, and B cells. These molecules present exogenous antigens, which are derived from outside the host cell. Helper T cells (TH cells), marked by the CD4 marker, recognize these exogenous antigens. Upon recognition, TH cells activate and orchestrate the immune response, aiding in the activation of other immune cells.

It is important to note that while APCs possess both MHC class I and MHC class II molecules, the latter is specifically involved in presenting antigens to helper T cells. The interaction between T cells and MHC molecules is guided by their respective CD markers, ensuring that CD8+ TC cells interact with MHC class I and CD4+ TH cells interact with MHC class II.

In summary, MHC class I molecules are crucial for presenting endogenous antigens to cytotoxic T cells, while MHC class II molecules present exogenous antigens to helper T cells. This distinction is vital for the immune system's ability to recognize and respond to various pathogens effectively.

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