Final answer:
B7 expression on APCs is stimulated by their interaction with foreign antigens and is essential for T cell activation, providing a second signal alongside TCR engagement with the presented antigen.
Step-by-step explanation:
The expression of B7 on the surface of an Antigen-Presenting Cell (APC) is upregulated in response to the encounter with foreign antigens. APCs, which include dendritic cells, macrophages, and B cells, play a critical role in the immune response by displaying antigens via Major Histocompatibility Complex (MHC) class II molecules to helper T cells. Once a foreign antigen is processed and presented on the surface in the context of an MHC II molecule, the APC can be stimulated by external signals such as cytokines or interaction with T cells, leading to the increased expression of co-stimulatory molecules like B7. The presence of B7 is crucial for the proper activation of T cells, which occurs through the binding of B7 with the CD28 receptor on naive T cells. This interaction, along with the T Cell Receptor (TCR) engagement with the antigen-MHC II complex, provides the necessary second signal for T cell activation.