Stevens Immunology-Serology Practice Test

MHC class II molecules are expressed mainly on professional antigen-presenting cells, such as B cells, macrophages, and dendritic cells, and they present exogenous peptide antigens to CD4+ T helper cells. This expression pattern is what makes the statement about being found on B cells and macrophages true—these cells specialize in taking up, processing, and presenting extracellular antigens to activate helper T cells and coordinate broader immune responses, including B cell antibody production.

In contrast, MHC class I molecules are found on almost all nucleated cells and present endogenous peptides to CD8+ T cells, which helps you see why the other points aren’t correct. Class II genes aren’t located at a single locus; they’re encoded by multiple HLA genes (DP, DQ, DR) within the major histocompatibility complex on chromosome 6, not chromosome 9.