The cellular distribution of MHC class I versus MHC class II molecules is quite different, with MHC class II molecules generally expressed on a very limited set of cell types. This is because:

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MHC class II molecules are primarily involved in presenting antigens to CD4 T cells, which in turn secrete cytokines that stimulate macrophages and B cells to become more effective antigen-presenting cells and to participate in antibody production, respectively. Thus, MHC class II molecules are selectively expressed on cell types that are involved in immune responses to extracellular pathogens, such as macrophages, B cells, and dendritic cells that reside in peripheral tissues. By contrast, MHC class I molecules are ubiquitously expressed on nucleated cells and are involved in presenting antigens to CD8 T cells, which can kill infected host cells.

The Cellular Distribution of MHC Class I Versus MHC Class