Final answer:
The predominant isotype switch in Peyer's patches is to IgA, which is critical for mucosal immunity in the gut.
Step-by-step explanation:
In Peyer's patches of gut epithelium and lymphoid tissues that line mucosa, the predominant isotype switch is to IgA. These patches play a crucial role in the immune system by sampling potential pathogens from the intestinal lumen through specialized cells called microfold or M cells. The pathogens are then presented to lymphocytes within the Peyer's patches, leading to the activation of B cells, which undergo isotype switching and produce IgA antibodies. These antibodies are essential as they are secreted into the intestinal lumen, providing a primary defense against ingested pathogens by neutralizing them and preventing their attachment and entry into the body's tissues.