Which cells are primarily responsible for the differentiation of B cells?

The correct choice highlights the crucial role that T helper cells play in the differentiation of B cells, which is a vital part of the adaptive immune response. When B cells encounter an antigen, they require assistance to fully activate and differentiate into plasma cells that produce antibodies. T helper cells, particularly those that express CD4, provide necessary signals through direct cell-cell interactions and by secreting cytokines. These signals not only promote B cell proliferation but also guide B cells in undergoing class switching, allowing them to produce different types of antibodies according to the needs of the immune response.

While other cell types, such as macrophages, can present antigens and contribute to the immune response, they do not have the specific role of directly facilitating B cell differentiation. Thymus cells are primarily involved in the maturation of T cells and do not differentiate B cells. Cytotoxic T cells are directly involved in killing infected or cancerous cells but do not participate in the differentiation of B cells, focusing instead on targeting and eliminating cells that display foreign antigens.