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IMMUNE TISSUES

Spleen

Filtering Blood. Activating Immunity.

The spleen is a unique immune organ that filters the blood, removes old or damaged cells, detects blood-borne pathogens, and initiates immune responses. It acts as a bridge between innate and adaptive immunity, helping the body respond quickly to infection while maintaining blood homeostasis.

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What Is the Spleen?

The spleen is a fist-sized organ located in the upper left abdomen, behind the stomach and beneath the diaphragm. It is highly vascularized and continuously filters blood flowing from the circulatory system.

The spleen contains immune cells, including B cells, T cells, dendritic cells, and macrophages, that detect and respond to blood-borne pathogens such as bacteria, viruses, and parasites. It also removes old or damaged red blood cells and platelets, helping to maintain healthy blood circulation.

The spleen is a key site for the generation of antibody responses to circulating pathogens and plays an important role in immune surveillance.

Spleen at a glance

Type of tissueSecondary lymphoid organ
Main roleFilter blood, initiate immune responses
Where it isUpper left abdomen, behind the stomach, beneath the diaphragm
LifespanFunctional throughout life (may show age-related changes)

Key Functions of the Spleen

The spleen performs several essential functions that help protect the body and maintain blood health.

1. Filter blood and remove old cells

Removes old, damaged, or abnormal red blood cells, platelets, and cellular debris from circulation.

2. Detect pathogens in the blood

Captures blood-borne pathogens (e.g., bacteria, viruses, parasites) through resident macrophages and dendritic cells.

3. Activate B and T cells

Supports activation, proliferation, and differentiation of B cells (producing antibodies) and T cells in the white pulp.

4. Maintain blood homeostasis

Helps regulate blood cell numbers, recycles iron from degraded hemoglobin, and stores platelets that can be released when needed.

Anatomy and Structure

The spleen is organized into distinct regions, each with specialized functions.

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  1. 1
    Capsule

    The protective outer layer of dense connective tissue.

  2. 2
    Trabeculae

    Connective-tissue partitions that carry blood vessels into the spleen.

  3. 3
    Red pulp

    Filters blood and removes aging or damaged red blood cells.

  4. 4
    White pulp

    Immune tissue containing T-cell zones and B-cell follicles.

  5. 5
    B-cell follicle

    A B-cell region; an activated follicle can contain a germinal center.

  6. 6
    Arterial branch

    Branches of the splenic artery carry blood into the organ and supply smaller arterioles.

  7. 7
    Follicle boundary

    The peripheral region around a follicle, where specialized marginal-zone cells help detect blood-borne material.

Red pulpFilters blood; clears old cells
White pulpLymphoid tissue for immune responses
TrabeculaeConnective tissue with blood vessels
Arterial branchCarries blood into the spleen
CapsuleProtective connective tissue
B-cell follicleB-cell region; pale germinal center
Follicle boundaryPeripheral region around a follicle

Spleen and Aging

A resilient immune system today
for healthier tomorrows.

With age, several changes can occur in the spleen:

  • Reduced size and changes in tissue architecture.
  • Altered immune cell composition and reduced B cell responses.
  • Decreased ability to clear damaged cells and pathogens.
  • Changes in blood flow and screening efficiency.
  • These changes can contribute to immunosenescence and an increased susceptibility to infections in older adults.

Spleen in Health and Disease

InfectionsParticularly important for defense against encapsulated bacteria (e.g., Streptococcus pneumoniae, Haemophilus influenzae, Neisseria meningitidis).
Asplenia or splenic dysfunctionIncreases the risk of severe infections. Vaccination and preventive antibiotics may be recommended.
Blood disordersInvolved in the clearance of abnormal blood cells (e.g., in hemolytic anemia, thalassemia).
CancerCan be a site of lymphoma, leukemia, and metastases from other cancers.
Autoimmune diseasesMay contribute to autoantibody production (e.g., in systemic lupus erythematosus).

The good news

Healthy lifestyle choices — including regular physical activity, a balanced diet, adequate sleep and stress management — can support spleen function at any age.

Learn more about aging & immunity   →

Selected sources and references

Artwork illustrates the main mechanisms schematically; molecular sizes, colors and distances are not literal.