It is also known as MALT (mucosa associated lymphoid tissue) and is lymphoid tissue that is not encapsulated or is incompletely encapsulated by the connective tissue. The cells in this tissue are immediately beneath the lining epithelium in the lamina propia of the GI, GU, and Respiratory tracts because their lumens are exposed to the external environment. The cells are placed here strategically to intercept antigens and to initiate an immune response. They contact an antigen, they move to a lymph node in the region, proliferate and then differentiate. These new cells then return to the original location as the effector B and T cells. The tissues do not have a medullary or cortical region and they do not filter the blood or lymph.