The reticuloendothelial system (RES), part of the immune system, consists of the phagocytic cells located in reticular connective tissue, primarily monocytes and macrophages. Since phagocytosis is their primary role, mononuclear phagocytic system has been suggested as an alternative name. 
The Reticuloendothelial System is divided into primary and secondary lymphoid organs.
RES cells function in the secondary organs: lymph nodes, spleen, and MALT (mucosa-associated lymphoid tissue).
MALT is further divided into the GALT (gut-associated lymphoid tissue) and the BALT (bronchus-associated lymphoid tissue).
The secondary lymphoid structures function to survey all entering or circulating antigen and to mobilize an immune response against foreign antigen upon its discovery. The GALT and BALT are privy to the myriads of antigen entering the gastrointestinal and respiratory tracts, respectively. All extracellular fluid must filter through lymph nodes as it traverses the lymphatics on its way back to the systemic circulation. Antigen residing in the interstitium is thus swept to the lymph nodes for processing. Finally, the spleen filters the blood in search of antigen. Upon the discovery of foreign antigen, all of these tissues react in a similar manner to amass an appropriate and multifaceted immune response.
In patients with hemochromatosis, the reticuloendothelial system is a site of iron accumulation.
- The Reticuloendothelial System: CLS 311/312 Lecture, University of Mississippi Department of Diagnostic and Clinical Health Sciences, 30 March 2010