Spleen Anatomy and Function
Anatomical Structure
The spleen is organized into two functionally distinct compartments with different cellular compositions and vascular architectures 1:
White pulp: Contains T-lymphocytes organized in periarterial lymphoid sheaths (PALS) and B-lymphocytes in lymphoid follicles, responsible for immune surveillance and antibody production against blood-borne pathogens 2, 1
Red pulp: Functions as a sophisticated blood filter with narrow sinusoidal endothelial slits that trap and eliminate senescent, damaged, or abnormal erythrocytes through macrophage phagocytosis 2, 3
Marginal zone: Serves as the interface between white and red pulp, critical for initial pathogen detection 1
Primary Functions
The spleen serves as the body's largest secondary immune organ and blood filter, performing three essential roles 1, 4:
1. Immune Defense Function
- Acts as a phagocytic filter removing bacteria from the bloodstream, particularly poorly opsonized organisms that evade hepatic clearance 4
- Produces opsonizing antibodies critical for rapid bacterial removal in non-immune hosts 4
- Combines innate and adaptive immunity by releasing immediate innate responses while coordinating antigen-presenting cell interactions with T and B lymphocytes 3
2. Blood Filtration
- Removes cellular debris, bacteria, senescent erythrocytes, and mechanically damaged cells from circulation 2, 3
- Macrophages in the red pulp continuously filter blood passing through the organ 2
3. Hematopoietic Reserve
- Contains reserve populations of hematopoietic stem cells activated during stress when bone marrow cannot meet demand 5
- Releases platelets and neutrophils in response to bleeding or infection 5
- Provides extramedullary hematopoiesis on demand 2
Clinical Significance of Splenic Absence
Overwhelming Post-Splenectomy Infection (OPSI)
Asplenic patients face lifelong risk of fulminant sepsis with 30-70% mortality, requiring mandatory prophylactic measures 6:
- Causative organisms: Streptococcus pneumoniae (50% of cases), followed by Haemophilus influenzae type B and Neisseria meningitidis 6
- Highest risk period: First year post-splenectomy, especially in children under 5 years (>10% infection rate vs <1% in adults) 6
- Neonatal risk: Exceeds 30% 6
- Duration: Risk persists for more than 10 years and likely lifelong, with cases reported >20 years post-splenectomy 6
Mandatory Prophylaxis Protocol
All asplenic/hyposplenic patients require the following evidence-based interventions 6:
Vaccination Requirements (Grade 1A)
- Timing: Ideally 2 weeks before elective splenectomy; if post-traumatic, wait minimum 14 days post-operatively for optimal antibody response 6
- Exception: Vaccinate before discharge if patient leaving hospital before day 15 to prevent missed vaccination 6
- Required vaccines: Pneumococcal (23-valent polysaccharide), H. influenzae type B, and meningococcal vaccines 6
- Annual influenza vaccine: For all patients >6 months of age to reduce secondary bacterial infection risk 6
- Reimmunization: Consider after 2 years, especially in children <2 years who mount suboptimal initial responses 6
Antibiotic Prophylaxis
- Lifelong phenoxymethylpenicillin prophylaxis: Most critical during first 2 years post-splenectomy when infection risk peaks 6
- Emergency antibiotic supply: Patients must carry standby antibiotics for immediate self-administration with fever, malaise, chills, or constitutional symptoms when medical care not readily accessible 6
- Adults: Amoxicillin 3g loading dose, then 1g every 8 hours, OR Levofloxacin 500mg every 24 hours, OR Moxifloxacin 400mg every 24 hours 6
Additional Precautions
- Malaria prophylaxis: Strongly recommended for travelers (Grade 2C) 6
- Primary care notification: Community physicians must be informed of asplenic status to ensure appropriate care 6
- Patient education: Critical importance of seeking immediate medical attention for any febrile illness 6
Functional Hyposplenism
Patients with intact but dysfunctional spleens require identical prophylactic measures as asplenic patients 6:
Conditions Causing Splenic Dysfunction
- Sickle cell disease (HbSS, HbSC) 6
- Thalassemia major 6
- Essential thrombocythemia and lymphoproliferative diseases 6
- Celiac disease, inflammatory bowel disease, dermatitis herpetiformis 6
Diagnostic Markers
- Howell-Jolly bodies and Heinz bodies on peripheral blood smear indicate functional hyposplenism 6
Age-Related Considerations
Children under 5 years, particularly infants, face dramatically higher OPSI risk and mortality compared to adults 6:
- Pediatric infection rate >10% vs adult rate <1% 6
- Antibody levels decline more rapidly in children, necessitating earlier reimmunization (as early as 3 years post-initial vaccination) 6
- Children with sickle cell disease face especially elevated risk requiring aggressive prophylaxis 6
Common Pitfalls
- Never assume splenic preservation after partial splenectomy or autotransplantation provides adequate protection: Institute full prophylactic measures as for complete asplenia 6
- Do not delay vaccination beyond 14 days post-splenectomy: Antibody response is suboptimal before this timeframe 6
- Recognize that vaccine coverage is incomplete: Pneumococcal vaccine covers only 23 of 90 serotypes; meningococcal vaccine covers 5 of 6 serotypes 6
- Avoid complacency after initial years: Though highest risk is within first 2 years, life-threatening infections occur decades later 6