What type of cell is required to activate a B cell when dealing with a T dependent antigen?

To activate a B cell in response to a T-dependent antigen, the primary requirement is the presence of T helper cells. T-dependent antigens typically require additional signals provided by T helper cells for optimal B cell activation and differentiation. When a B cell binds to a T-dependent antigen through its B cell receptor, it internalizes and processes this antigen. Subsequently, it presents peptide fragments of the antigen on its surface using Major Histocompatibility Complex (MHC) class II molecules.

The T helper cell recognizes the presented peptide via its T cell receptor, leading to a crucial interaction that involves co-stimulatory signals. This interaction triggers the T helper cell to release specific cytokines, which provide the essential signals that promote B cell activation. As a result, the B cell undergoes proliferation and differentiates into plasma cells or memory B cells, ultimately contributing to a robust adaptive immune response.

In contrast, other cell types listed do not play this specific role in activating B cells. While B cells themselves are responsible for producing antibodies once activated, they do not have the capability to independently activate without T helper cells. Cytotoxic T cells are primarily focused on killing infected or cancerous cells rather than activating B cells. Natural killer cells are involved in innate immunity and