Final answer:
A B cell is stimulated by antigen presented on an MHC class II molecule of an APC. This action is necessary for T cell-dependent activation of B cells, which involves helper T cells and leads to a strong immune response with the development of memory.
Step-by-step explanation:
A B cell is first stimulated by antigen presented on an MHC class II molecule of an Antigen-Presenting Cell (APC). The presentation of antigen by MHC class II is crucial for the activation of B cells by helper T cells. The T Cell Receptor (TCR) of a helper T cell binds to antigens presented with MHC II molecules. This implies that the APCs, such as macrophages and dendritic cells, process the foreign antigen and present it on their surface with MHC II. Once a B cell has internalized and processed a protein antigen, it presents this antigen with MHC II, which is then recognized by the TCR of a helper T cell, facilitating B cell activation.
It is important to note that while all nucleated cells present antigens with MHC I, only APCs present antigens with MHC II. This distinction is essential for the proper immune response. When a B cell presents an antigen with MHC II, it allows for the interaction and activation of T helper cells, which is a part of the T cell-dependent activation of B cells. The T cell-dependent activation leads to a robust immune response and the development of immune memory, making it an essential aspect of adaptive immunity.