Lymph nodes are part of the lymphoid system. They are secondary or peripheral lymphoid organs. They are distributed throughout the body (most commonly in the neck) and number about 800. The lymph nodes are located along the lymphatic network. A distinction is made between the deep networks (abdomen, thorax, neck) and the superficial networks.

The lymph nodes are the site of proliferation and differentiation of immune cells and thus of activation of the immune system. Lymph nodes are composed of the peripheral cortex where B cells multiply and the deep cortex or paracortex where T cells and dendritic cells are found. The lymph nodes are divided into three functional compartments: the sinus compartment, the blood compartment and the parenchymal compartment. The lymphoid tissue is made up of reticular tissue and free cells, mainly lymphocytes, but also plasma cells and macrophages.