Learning Objectives
Master the structural and functional anatomy of the Spleen. Distinguish between White Pulp and Red Pulp, understand the clinical consequences of Asplenia, and identify the specific hematologic findings seen Post-splenectomy for the USMLE Step 1.
1. Anatomical Location
The spleen is located in the Left Upper Quadrant (LUQ) of the abdomen, lateral to the left kidney and protected by the 9th–11th ribs.
| Landmark | Anatomical Relationship |
|---|---|
| Abdominal Quadrant | Left Upper Quadrant (LUQ). |
| Bony Protection | Posterior to the 9th, 10th, and 11th ribs. |
| Adjacent Organs | Anterolateral to the left kidney and tail of the pancreas. |
2. Splenic Microanatomy
The spleen is divided into two main functional components: the White Pulp (immune) and the Red Pulp (filtration).
| Zone | Cell Types & Contents | Function |
|---|---|---|
| PALS | T cells | Periarteriolar Lymphatic Sheath: surrounds the central artery. |
| Follicle | B cells | Located within the white pulp; site of B-cell activation. |
| Marginal Zone | Macrophages and specialized B cells | Interface between red and white pulp. Site where APCs capture blood-borne antigens. |
| Red Pulp | RBCs, Sinusoids, Splenic Cords | Filtration: Macrophages remove old/damaged RBCs and “pitting” of inclusions. |
3. Splenic Dysfunction & Asplenia
Splenic dysfunction occurs following surgical Splenectomy or Autosplenectomy (commonly seen in Sickle Cell Disease).
| Step | Immunological Consequence |
|---|---|
| 1. Loss of Function | Decreased IgM production and splenic filtration. |
| 2. Opsonization Fail | Decreased Complement activation and C3b opsonization. |
| 3. Infection Risk | $\uparrow$ Susceptibility to Encapsulated Organisms. |
| 4. Prevention | Vaccination against Pneumococci, Meningococci, and Hib. |
4. Post-splenectomy Blood Findings
Without the “filtering” and “sequestration” functions of the spleen, specific changes appear on the peripheral blood smear.
| Finding | Mechanism |
|---|---|
| Howell-Jolly Bodies | Nuclear remnants (DNA) in RBCs that the spleen normally removes (pitting). |
| Target Cells | Excess membrane relative to volume (the spleen usually “grooms” the membrane). |
| Thrombocytosis | Loss of the splenic pool; the spleen normally sequesters  of the body’s platelets. |
| Lymphocytosis | Loss of sequestration of lymphocytes in the white pulp. |
Activity:
High-Yield Clinical Pearls:
- PALS vs. Follicle: In the spleen, the PALS (T cells) is the equivalent of the lymph node paracortex, and the Follicle (B cells) is the same as the lymph node cortex.
- Opsonization: Splenic macrophages are uniquely suited to clear bacteria that have been opsonized (tagged) by C3b or IgG.
- Surgical Emergency: In trauma (e.g., car accident), LUQ pain and referred pain to the left shoulder (Kehr sign) indicate splenic rupture and hemoperitoneum.