In the context of antigen presentation, which cells express MHC Class II molecules?

MHC Class II molecules are primarily expressed by professional antigen-presenting cells (APCs), which includes B cells, dendritic cells, and macrophages. These cells are crucial for the immune system's functioning, as they present processed antigens to CD4+ T helper cells, facilitating the activation of adaptive immune responses.

B cells are important because they not only process and present antigens but are also capable of producing antibodies in response to T-cell help. Dendritic cells play a key role in the capture and presentation of antigens to T cells, and they are critical for initiating T cell responses in the lymph nodes.

While helper T cells are essential players in the immune response, they do not express MHC Class II molecules; instead, their function relies on recognizing antigens presented by other cells that do express these molecules. Additionally, all nucleated cells express MHC Class I molecules but not MHC Class II, which is specific to APCs. Red blood cells lack a nucleus and therefore do not express either class of MHC molecules.

Thus, the unique role of B cells and dendritic cells in antigen presentation and the specific expression of MHC Class II molecules make this answer correct.