Immunology and AllergyTier 2Topic

Spleen

Structure and function

Largest lymphoid organ. Two functional compartments

	Structure	Function
Red pulp	Cords of Billroth + sinusoids	Filtration - removes senescent RBCs, Howell-Jolly bodies, opsonised particles. Reservoir of ~1/3 of platelets
White pulp	PALS (T cells), follicles (B cells), marginal zone	Immunity - antigen presentation, T-independent antibody response to polysaccharide capsules, IgM memory B cells

Why encapsulated organisms: capsular polysaccharide is a T-independent antigen handled almost exclusively by splenic marginal-zone B cells
Also: extramedullary haematopoiesis (fetal, or when marrow fails)

Key numbers

Normal spleen 12 cm x 7 cm; not palpable until roughly doubled
Palpable in ~3% of healthy young adults
Post-splenectomy sepsis: lifetime risk ~5%, highest in the first 2 years and in children

Splenomegaly by size - the classic exam frame

Size	Causes
Massive (crosses midline / to pelvis)	Myeloproliferative - myelofibrosis, CML; malaria, kala-azar (visceral leishmaniasis), Gaucher disease; occasionally CLL, hairy cell leukaemia
Moderate	Portal hypertension from chronic liver disease; lymphoproliferative - Hodgkin lymphoma, CLL; haemolytic anaemias, storage disease
Just palpable	Infectious mononucleosis, infective hepatitis, subacute infective endocarditis, sarcoidosis, ITP, SLE, RA (Felty), early portal hypertension, lymphoproliferative disease

Mechanistic grouping

Congestive - portal hypertension, splenic/portal vein thrombosis, right heart failure
Infiltrative - myeloproliferative, lymphoproliferative, amyloid, storage disease
Work hypertrophy - haemolysis (spherocytosis, thalassaemia), chronic infection
Immune hyperplasia - EBV, endocarditis, SLE, sarcoid, Felty

Hyposplenism

Surgical splenectomy, trauma
Functional: sickle cell (autosplenectomy), coeliac disease, IBD, amyloid, SLE, HSCT, splenic irradiation, alcoholic liver disease

Examining the spleen

Palpate from the right iliac fossa toward the left costal margin on inspiration
Discriminate from kidney: cannot get above it, moves with respiration toward RIF, notched medial border, dull to percussion, not ballotable
Percuss Traube space and Castell point - more sensitive than palpation
Ultrasound if in doubt - examination misses up to half of enlarged spleens

Workup of splenomegaly

FBE + film (blasts, lymphocytosis, tear drops, atypical lymphocytes, parasites), reticulocytes, LDH, haptoglobin
LFT, coagulation, viral hepatitis serology, EBV/CMV, HIV
Blood cultures + echo if endocarditis suspected; malaria films/RDT with travel history
SPEP, immunoglobulins, ANA, ferritin
Imaging: US then CT; JAK2/BCR-ABL if myeloproliferative suspected; bone marrow

Hyposplenism on the film

Howell-Jolly bodies (the key sign), target cells, acanthocytes, Pappenheimer bodies, thrombocytosis, lymphocytosis

Splenomegaly - management

Splenomegaly

Treat the cause. Splenectomy is rarely the answer
Advise against contact sport while the spleen is enlarged (EBV - avoid for at least 4 weeks and until resolved)
Hypersplenism with symptomatic cytopenias -> splenectomy, splenic artery embolisation, or ruxolitinib in myelofibrosis

Asplenia and hyposplenism - lifelong management

### A. Vaccination

Give at least 2 weeks before elective splenectomy, or from 7 days after emergency splenectomy
Pneumococcal (13/15/20-valent conjugate then 23-valent polysaccharide, with boosters)
Meningococcal ACWY and B
Haemophilus influenzae type b
Annual influenza + COVID-19

### B. Antibiotic prophylaxis

Phenoxymethylpenicillin 250 mg bd (or amoxicillin); roxithromycin if penicillin-allergic
For at least 3 years post-splenectomy and until age 18; lifelong if additional immunocompromise or prior invasive pneumococcal disease
Standby antibiotic supply (amoxicillin-clavulanate) - take immediately with any fever and seek urgent review

### C. Education - the part that actually saves lives

Any fever -> emergency department, blood cultures, IV ceftriaxone without waiting
MedicAlert bracelet, written asplenia card, entry on the Australian Spleen Registry
Travel: malaria prophylaxis is essential (severe falciparum and babesiosis in asplenia)
Animal bites -> immediate prophylaxis for Capnocytophaga canimorsus

Infection risks

Encapsulated organisms: S. pneumoniae (>50% of OPSI), N. meningitidis, H. influenzae type b
Capnocytophaga canimorsus (dog bite), Babesia, severe P. falciparum
Coeliac disease, IBD, sickle cell - functional hyposplenism
Portal hypertension, splenic vein thrombosis
Post-splenectomy: thrombocytosis, portal/splenic vein thrombosis, increased long-term VTE and pulmonary hypertension risk

Overwhelming post-splenectomy infection

Overwhelming post-splenectomy infection (OPSI)
- Lifetime risk ~5%; greatest in the first 2 years and in children under 5
- Fulminant: prodrome -> septic shock and DIC within 12-48 hours
- Mortality 40-70% despite treatment
- Risk is lifelong - never reassure a patient that they are past it
Splenectomy for haematological disease carries higher infection risk than for trauma
Thrombotic risk persists long term

Monitor

Vaccination currency - pneumococcal and meningococcal boosters
Film for Howell-Jolly bodies to confirm functional hyposplenism
Platelet count post-splenectomy

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