How do macrophages in the spleen help defend against blood-borne microorganisms?

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Diagram of the human lymphatic system highlighting lymphatic vessels and major lymphoid organs, including lymph nodes, adenoids, tonsils, thymus gland, spleen, appendix, bone marrow, and ducts. — lymphatic system The human lymphatic system, showing the lymphatic vessels and lymphoid organs.

The spleen functions as the primary filter for the blood within the lymphatic system, acting similarly to how lymph nodes filter lymphatic circulation.1

Macrophages play a critical role in this defense by residing in both the red and white pulp of the spleen. When microorganisms and other antigens enter the bloodstream, these phagocytic cells perform two essential functions:

Removal of Foreign Material: Macrophages actively clear foreign substances and pathogens directly from the blood as it passes through the splenic tissue.1
Immune Activation: They initiate an immune response that leads to the production of antibodies, helping the body recognize and combat specific threats.1

By serving as a reservoir for these cells, the spleen ensures a rapid response to blood-borne infections, which is why individuals who have undergone a splenectomy often face an increased risk of infection.1