What is an important characteristic of MHC class 2 molecules?

MHC class II molecules are crucial for the adaptive immune response, particularly in their role in presenting antigens. One of their key characteristics is their ability to present extracellular antigens to CD4+ T helper cells. This process involves the uptake of extracellular proteins by antigen-presenting cells (APCs) like dendritic cells, macrophages, and B cells, which then process these proteins into peptides. Once processed, these peptides are loaded onto MHC class II molecules, which then travel to the cell surface.

When a CD4+ T cell encounters an MHC class II molecule displaying a peptide that matches its T cell receptor (TCR), it binds to the MHC-peptide complex. This interaction is essential for the activation of CD4+ T cells, leading to their differentiation and subsequent help in orchestrating an immune response, such as helping B cells produce antibodies or activating other immune cells.

The other characteristics mentioned, such as expression by all nucleated cells or binding directly to CD8+ T cells, are traits associated with different classes of MHC molecules or different cell types. MHC class I molecules, not class II, are expressed by all nucleated cells and are responsible for presenting intracellular antigens to CD8+ T cells