The spleen is an organ located in the upper left abdomen, tucked behind the stomach. It works as part of the lymphatic system, filtering blood and managing the body’s immune defenses. The spleen clears out old and damaged cells while also producing the cells that protect you from infection.
The Spleen’s Two-Part Structure
The spleen is composed of two distinct types of tissue: the red pulp and the white pulp. The red pulp makes up the majority of the spleen’s tissue and its primary function is to act as a blood filter. As blood flows through its narrow passages, old, damaged, or misshapen red blood cells are identified and removed from circulation.
This filtration process is carried out by macrophages, which engulf and break down unfit blood cells. The spleen also recycles components like iron, storing it and returning it to the bone marrow where new red blood cells are made. This part of the spleen holds a reserve of blood that can be released if the body experiences significant blood loss.
In contrast, the white pulp is the immunological center of the spleen, resembling other lymphoid tissues like lymph nodes. This tissue is where the body’s adaptive immune responses are marshaled. When pathogens are detected in the bloodstream, the white pulp coordinates the defensive actions against these blood-borne invaders.
The Spleen’s Immune Cell Arsenal
The spleen houses a variety of specialized immune cells. These cells are positioned within both the red and white pulp to maximize their effectiveness. Each cell type has a specific role, contributing to the organ’s overall defensive capabilities.
Macrophages are a prominent cell type found throughout the spleen. In the red pulp, these large cells are responsible for the “quality control” of blood, consuming old red blood cells and pathogens. In the white pulp, their function shifts to acting as antigen-presenting cells, displaying fragments of a pathogen on their surface to initiate a more targeted response.
The core of the spleen’s adaptive immunity lies within its population of lymphocytes, which are concentrated in the white pulp. These are mainly B cells and T cells. B cells are organized in structures called follicles and, when activated, produce vast quantities of antibodies to mark pathogens for destruction.
T cells are found in areas surrounding the small arteries within the white pulp, in regions known as periarteriolar lymphoid sheaths (PALS). There are different types, including helper T cells that coordinate the immune attack and cytotoxic T cells that can directly kill infected body cells. They are activated when they recognize the specific antigen presented by cells like macrophages.
Dendritic cells are another type of antigen-presenting cell found in the spleen. They are exceptionally efficient at capturing pathogens and their antigens from the blood. Once captured, they migrate to the T cell zones in the white pulp and present the antigen to T cells, initiating a powerful adaptive immune response.
How the Spleen Initiates an Immune Response
The spleen’s structure allows it to orchestrate an immune response against blood-borne pathogens. The process begins as blood enters the organ and its flow is deliberately slowed. This provides an opportunity for immune cells to screen the blood for foreign invaders.
As blood moves through the spleen, it encounters macrophages and dendritic cells stationed in the red pulp and the marginal zone. These cells act as sentinels, capturing circulating pathogens as part of the body’s innate immunity.
After capturing a pathogen, these antigen-presenting cells migrate into the T-cell rich zones of the white pulp. Here, they present specific antigenic fragments to the T cells. This interaction activates the specific T cells that recognize the antigen.
The activated helper T cells, in turn, help to activate B cells located in nearby follicles. This leads the B cells to multiply and differentiate into plasma cells, which function as antibody factories. These cells release large amounts of antibodies into the bloodstream to neutralize the pathogen.
Life Without a Spleen
While the body can compensate for the loss of a spleen, its absence, a condition known as asplenia, has immunological consequences. Individuals may lack a spleen from birth or have it surgically removed (a splenectomy). Without the spleen’s filtering capacity, the body’s ability to defend against certain types of infections is diminished.
The most serious risk for individuals without a spleen is overwhelming post-splenectomy infection (OPSI), a severe and rapidly progressing illness. This increased susceptibility is noted with encapsulated bacteria. These bacteria have a polysaccharide outer capsule that makes it difficult for immune cells to grab onto them, a process the spleen handles well.
Encapsulated bacteria that pose a threat include Streptococcus pneumoniae, Neisseria meningitidis, and Haemophilus influenzae type b. To mitigate these risks, individuals without a spleen are advised to follow specific preventative measures. These include vaccinations against these bacteria and, in some cases, taking daily antibiotics.