Learning Objective: Describe the anatomical structure, circulation, immune organization, and clinical relevance of the spleen, including the consequences of splenic dysfunction.
Overview
- Location: Left upper quadrant (LUQ), anterolateral to the left kidney
- Protection: Enclosed by 9th–11th ribs
- Type: Secondary lymphoid organ
- Main Functions:
- Filters aged/damaged RBCs
- Stores platelets
- Mounts immune response to blood-borne antigens
Structural Organization
| Component |
Description |
| Capsule |
Dense fibrous connective tissue enclosing the spleen; sends trabeculae inward |
| Trabeculae |
Connective tissue extensions supporting blood vessels and lymphatics |
| Reticular Framework |
Meshwork of reticular fibers forming internal structure |
| Artery & Vein |
Splenic artery → trabecular arteries → central arterioles → sinusoids → pulp veins → splenic vein |
| Circulation Types |
Open circulation (blood percolates into red pulp before re-entering sinusoids) and Closed circulation (direct connection to sinusoids) |
Microscopic Zones
A. White Pulp (WBC Zone)
| Subzone |
Cell Type |
Function |
| Periarteriolar Lymphoid Sheath (PALS) |
T cells |
Surrounds central arteriole; site of T-cell activation |
| Follicle |
B cells |
Site of B-cell activation and proliferation |
| Mantle Zone |
Small B cells |
Surrounds germinal center |
| Germinal Center |
Activated B cells |
Site of isotype switching and affinity maturation |
| Marginal Zone |
Macrophages + Specialized B cells |
Interface between white and red pulp; captures antigens from blood |
B. Red Pulp (RBC Zone)
| Structure |
Function |
| Splenic Sinusoids |
Lined by discontinuous endothelium; filters aged/damaged RBCs |
| Cords of Billroth |
Contain macrophages that remove defective RBCs and pathogens |
Clinical Correlation
Splenic Dysfunction / Asplenia
- Causes:
- Postsplenectomy
- Autosplenectomy in sickle cell disease
- Immune Effects:
- ↓ IgM → ↓ complement activation → ↓ C3b opsonization → ↑ susceptibility to encapsulated bacteria
Encapsulated Organisms to Vaccinate Against (Most to Least Common):
- Streptococcus pneumoniae
- Neisseria meningitidis
- Haemophilus influenzae type b (Hib)
Postsplenectomy Blood Findings
| Finding |
Explanation |
| Howell–Jolly bodies |
Nuclear remnants in RBCs |
| Target cells |
RBCs with excess membrane |
| Thrombocytosis |
Loss of platelet sequestration |
| Lymphocytosis |
Loss of lymphocyte storage |
Key Takeaways
- White pulp = immune surveillance (T and B cells)
- Red pulp = blood filtration and RBC recycling
- Marginal zone = key site for antigen capture
- Asplenia = risk for sepsis from encapsulated bacteria
- Always vaccinate postsplenectomy patients against S. pneumoniae, N. meningitidis, and Hib
Activity:
