**3.3 MLN**

Lymph nodes are tissues located at the junction of the lymphatic system and higher organs [57]. The vast lymphatic vasculature collects lymph from tissues and returns it to the blood. MLNs are the lymph nodes in the intestinal mucosal immune system [58]. When an antigen enters the body through the intestinal mucosal system, it encounters the lymphatic system and is recognised; the antigen-presenting cells are then activated. These cells carry the antigen to the MLN, perform the antigen presentation reaction, and finally activate appropriate T and B cells [59–61].

## **3.4 Relationship between intestinal immune response and Igs**

GALT macrophages have different characteristics from macrophages in other parts of the body; that is, they have good phagocytic and bactericidal abilities [62]. CD4+ FOXP3+ regulatory T (TReg) cells are located in the regulatory layer of the intestinal mucosa. T cells differentiate into TReg cells in the presence of TGF-β. The balance between functional T and TReg cells highly affects the homoeostasis of intestinal mucosal immune response [63].
