Of all the organs origins, the origin of the lymph nodes immunitary system of vertebrate pluricellular animals is the most complex one, together with the origin of the central nervous system (see divulgation blogs 13, 16, 17 and 48).

Both immunitary and nervous systems share a primitive model, a lymph node chain present in marine annelids (600 million years ago) rope ladder type, as organic origin mantaining in the case of the nervous system a sensory function and in the case of the renal-lymphatic system a function of chemical and biological filtration.

Annelids nephridia are distributed organs, in pairs, for each body ring whose function is to filter and remove metabolic wastes. We can find two types of nephridia: metanephridia, with outside exit, and protonephridia, without outside exit.

It is known that some metanephridia evolved through the different types of animals (annelids, flatworms, arthropods and molluscs), to give rise to vertebrates kidney and that these kidneys had, originally, immune functions.

However, apart from the nephridia pair that resulted in kidneys, most metanephridia and protonephridia chain pairs organized phylogenetically, in a bilateral and symmetric network branched with ducts, capillars, lymphoid tissues associated to digestive and respiratory mucoses, lymph nodes and some specific organs (thymus and spleen).

Ducts and capillars are the result of the evolution of channels connecting nephridia with the outside while, lymphoid tissues, without capsule and mucose-associated, are the first sign of tissue organization in the respiratory and gastrointestinal mucoses.

Lymph nodes constitute specialized tissues compound by an area connected to the outside (post-metanephridia) and another blind area without external connection (post-protonephridia), and they are located along the right and left lymph ducts. They are composed of cortex, marrow (post-metanephridia and post -protonephridia) and vessels.

These post-metanephridia and post –protonephridia associated constitute the primitive lymph nodes acting as a filtration system specialized in filtrating biological agents, unlike the kidney which is specialized in a chemical filtration system.

Therefore kidneys and lymph nodes share functionality (filtration), external and internal morphology and their complex organic structure appeared before the blood circulatory system. Thus the kidney, in the pronephros portion or head kidney, behaves as a primary lymphoid organ in fish and some amphibians, reptiles have primitive lymph nodes around great vessels as aorta and cava; birds, with a different evolutionary line than mammals exhibit lymphoid formations equivalent to nodes but without capsule (Peyer's patches, ileocecal valves), and mammals, already manifest capsule and germinal centers or secondary lymphoid follicles in lymph nodes.

These chains of lymph nodes are particularly important because they are the source of the sentinel lymph lumboaortic chain of which depend defense side chains of the intestine, kidney, spleen, liver, stomach, bronchi, joints, subcutaneous spaces, ears, eyes and trachea. Their infection by bacteria, such as Actinobacillus, constitutes the mechanism by which the infection will cause immunosuppression.