The lymphoid system a major part of the [wiki2="Immune-System"]immune system[/wiki2] that is a sort of diffuse and extensive system of several different types of [wiki2="White-Blood-Cells"]white blood cells[/wiki2]. The white blood cells (WBCs) constantly circulate in the blood and lymphatics, but they also congregate in certain structures whose function is to filter out and trap foreign invaders and provide a point of focus for the WBCs to come in contact with the foreign cells, become activated, proliferate, and attack them.
The major organs of the body that comprise the lymphoid system are primarily the spleen, liver (especially in fetuses), and the thymus. But in addition there are many smaller outposts of the lymphoid system scattered throughout the body: the tonsils, the lymph nodes, and in the gut Peyer's patches, and others. So like other lymphoid organs Peyer's patches serve as a physical and functional focal point for the immune response. So it makes sense that for a disease like Crohn's that is characterized by an over-activity of the immune response in the gut, Peyer's patches are likely to be involved in some way.
