The lymphoid system is part of the reticuloendothelial system. The framewoks of the organs consits of a capsule of dense collagenous tissue with trabeculae penetrating inside where it becomes continuous with the reticular fibers that make up the stroma of the gland. Attached to the reticular fibers are primitive reticular cells (reticuloendothelial cells) and layer fixed macrophages (littoral cells). The lymphoid tissues are immunological in natrue. Cells formed and or stored here are lymphocytes, macrophages, antigen presenting cells and plasma cells.
B-lymphocytes are produced in the bone marrow from which they circulate to tissues of all types and to lyphoid organs where they concentrate in nodular tissue. When they encounter an antigen they divide, often forming nodules. Some may change to antibody producing plasma cells. This is called humorol immunity.
Cell mediated immunity comes from T-cells which are lymphocytes that kill invaders by touching them or poisoning them with lymphokines. T-lymphocytes produced in the thymus. These lymphocytes occur mostly in diffuse lymphatic tissue. Many types of T-cells and lymphokines exist. Both B and T lymphocytes are capable of producing a memory response against reinfection by the same antigen.
Diffuse lymphatic tissue has many fibers and few cells and dense has very many cells. Nodular tissue is the richest in cells.

Peyer's patches are aggregate lymphatic nodules found in the small intestine.
So many nodules are aggregated together that they are visible beneath the epithelium of the intestine. Germinal centers are usually obvious.