Where are MHC Class II molecules primarily found?

MHC Class II molecules are primarily found on professional antigen-presenting cells, which play a crucial role in the immune response. Dendritic cells are key components of the immune system and are particularly proficient in capturing, processing, and presenting antigens to T helper cells. This presentation is essential for the activation of CD4+ T cells, which subsequently orchestrate the adaptive immune response.

Dendritic cells express MHC Class II molecules on their surface and thus are capable of presenting exogenous antigens in association with these molecules. This function is fundamental for initiating a robust immune response against pathogens and is critical for both cellular and humoral immunity.

In contrast, other options like aortic endothelial cells, platelets, and red blood cells do not play a primary role in antigen presentation or express MHC Class II molecules as a typical function. Aortic endothelial cells may express MHC Class I molecules, while platelets lack MHC molecules altogether. Red blood cells also do not express MHC Class II, as they are primarily involved in oxygen transport and do not participate in immune responses directly. Thus, the presence of MHC Class II molecules is a defining characteristic of dendritic cells, which is why this is the correct answer.