U01.02.004 Spleen

The spleen is located in the left upper quadrant (LUQ) of the abdomen, anterolateral to the left kidney, and is protected by the 9th to 11th ribs.

Sinusoids: These are long, vascular channels found in the red pulp with a fenestrated “barrel hoop” basement membrane, allowing blood filtration.
White Pulp: The spleen contains T and B cells in the white pulp.
- T cells are located in the periarteriolar lymphatic sheath (PALS).
- B cells reside in follicles within the white pulp.
Marginal Zone: This is the region between the red and white pulp. It contains:
- Macrophages and specialized B cells.
- Antigen-presenting cells (APCs) that capture blood-borne antigens for immune recognition by lymphocytes.

Table: Structure of the Spleen

Spleen Component	Location	Function
Red Pulp	Surrounds sinusoids	Filters blood, stores red blood cells, and removes old/damaged cells.
White Pulp	Periarteriolar region	Hosts T and B cells for immune response.
Marginal Zone	Between red and white pulp	Contains macrophages and APCs that process antigens from the blood.
Sinusoids	It contains macrophages and APCs that process antigens from the blood.	Allows passage of blood cells, helping in filtration and immune surveillance.

Table: Structure of the Spleen

Immune Function of the Spleen

The spleen plays a critical role in removing encapsulated bacteria and supporting the body’s immune response through its macrophages and lymphocytes. The marginal zone is where antigen-presenting cells (APCs) capture and present blood-borne antigens to lymphocytes for immune recognition.

Common Pathologies of the Spleen

Splenic Dysfunction: Conditions such as a post-splenectomy state or sickle cell disease can lead to autosplenectomy (functional removal of the spleen).
- Consequences:
  - ↓ IgM levels.
  - ↓ Complement activation.
  - ↓ C3b opsonization (reduced ability to tag bacteria for destruction).
  - ↑ Susceptibility to encapsulated organisms (e.g., Streptococcus pneumoniae, Haemophilus influenzae, and Neisseria meningitidis).

Blood Findings Post-Splenectomy

Finding	Description
Howell-Jolly Bodies	Nuclear remnants in red blood cells, normally removed by the spleen.
Target Cells	Red blood cells with a bullseye appearance, due to membrane defects.
Thrombocytosis	Increased platelet count, due to the loss of spleen’s sequestration ability.
Lymphocytosis	Increased lymphocyte count, as they are no longer sequestered by the spleen.

Blood Findings Post-Splenectomy

Vaccination in Splenic Dysfunction

Patients who undergo splenectomy or have splenic dysfunction should be vaccinated against encapsulated organisms to reduce the risk of infection. Recommended vaccines include:

Pneumococcal vaccine (Streptococcus pneumoniae)
Haemophilus influenzae type B (Hib) vaccine
Meningococcal vaccine (Neisseria meningitidis)

Points to Remember

The spleen filters blood, removes old or damaged blood cells, and supports immune function.
The white pulp contains both T and B cells, while the marginal zone captures and processes blood-borne antigens.
Splenic dysfunction increases susceptibility to infections, particularly from encapsulated organisms.
Blood findings in post-splenectomy patients include Howell-Jolly bodies, target cells, thrombocytosis, and lymphocytosis.
Vaccination is crucial in splenectomized or splenic dysfunction patients to prevent infections by encapsulated bacteria.

Bibliography

Kumar, V., Abbas, A.K., Aster, J.C. (2014). Robbins & Cotran Pathologic Basis of Disease. Elsevier Saunders.
Goldsby, R.A., Kindt, T.J., Osborne, B.A., Kuby, J. (2003). Immunology. W. H. Freeman.
Thomas, S., and Silk, H. (2008). “Spleen: Anatomy, Function, and Clinical Aspects.” American Family Physician.

USMLE Step 1 Review Series

Curriculum

U01.02.004 Spleen

Table: Structure of the Spleen

Immune Function of the Spleen

Common Pathologies of the Spleen

Blood Findings Post-Splenectomy

Vaccination in Splenic Dysfunction

Points to Remember

Bibliography

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