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The Reticuloendothelial System (RES)—now more accurately referred to as the Mononuclear Phagocyte System (MPS)—is a functional and anatomical system of phagocytic cells, primarily involved in:

Phagocytosis of cellular debris and pathogens
Immune surveillance and antigen presentation
Recycling of old red blood cells and iron </aside>
It includes monocytes in blood, macrophages in tissues, and specialized phagocytes in organs like the liver, spleen, lymph nodes, and bone marrow.

Components

Organ / Site	Resident Cell Type	Function
Liver	Kupffer cells	Remove bacteria and senescent RBCs
Spleen	Splenic macrophages	Filter blood, remove abnormal RBCs, immune response
Bone marrow	Marrow macrophages	RBC recycling, hematopoiesis support
Lymph nodes	Sinus histiocytes	Antigen presentation, immune filtering
Lungs	Alveolar macrophages	Phagocytose inhaled particles
CNS	Microglia	CNS immune surveillance
Blood	Monocytes	Precursor to tissue macrophages

Physiologic Functions

Phagocytosis of pathogens, dead cells, and particles
Iron metabolism: Macrophages recycle heme iron from RBCs
Immune activation: Antigen presentation and cytokine release
Tumor surveillance
Lipid metabolism (foam cells in atherosclerosis)

Imaging

Nuclear Medicine

Liver–Spleen Scintigraphy with Tc-99m sulfur colloid:

Colloid is phagocytosed by RES cells in liver (~85%), spleen, and bone marrow
Used to assess:
- Liver function and reticuloendothelial integrity
- Focal liver lesions (e.g., hepatocellular carcinoma or metastases → cold spots)
- Accessory spleen vs splenosis
- Diffuse liver disease (e.g., cirrhosis → decreased liver uptake)

MRI & CT

Splenic and hepatic imaging assess RES organs:
- Cirrhosis → heterogeneous or nodular liver
- Hypersplenism (seen in portal hypertension)
- Bone marrow reconversion or infiltration assessed on MRI