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USMLE Step 1MicrobiologyEP 07 of 14

Encapsulated organisms and the spleen: asplenia and conjugate vaccines

A missing spleen turns a minor illness into an emergency because encapsulated bacteria depend on it for clearance. Conjugate vaccines work by bringing in T cell help.

  • 5:08 video
  • 3 min read
  • 12 sources
  • Published 15 Nov 2026
Video lesson · 5:08 · Watch on YouTubeSeries: Microbiology · @medservatory
Chapters (14)
By the end of this lesson

Explain why asplenia predisposes to infection with encapsulated organisms and how conjugate vaccines protect.

The short version

  1. The classic encapsulated organisms are Streptococcus pneumoniae, Neisseria meningitidis and Haemophilus influenzae.
  2. The capsule resists phagocytosis, so clearance needs opsonins (antibody and complement) and the spleen.
  3. Polysaccharides are T-independent antigens: a fast but mostly IgM response with little class switching and little memory.
  4. Conjugate vaccines link the polysaccharide to a carrier protein, adding T cell help, class switching to IgG and memory.
  5. In asplenia, prevention rests on vaccines against pneumococcus, Haemophilus influenzae type b and meningococcus, with antibiotic prophylaxis as the other main preventive tool.

Encapsulated organisms and the capsule

A man had his spleen removed years ago after a car crash. On Friday he has a sore throat and chills; by Saturday he is in septic shock. The reason lies in the encapsulated organisms and in what the spleen does against them.

The classic encapsulated organisms are Streptococcus pneumoniae, Neisseria meningitidis and Haemophilus influenzae.[1][2]

The capsule is a sugar coat, a polysaccharide, and it resists phagocytosis.[3] Defence therefore needs opsonins: antibody and complement coat the capsule, and then phagocytes can engulf the bacterium.[3]

The spleen's two jobs against encapsulated bacteria

  • Filtering the blood. Red pulp macrophages remove opsonised bacteria from the circulation.[3][4]
  • Making IgM. The marginal zone holds IgM memory B cells, which answer polysaccharide antigens and make natural IgM.[2][4][7]

Polysaccharides are T-independent antigens: they activate B cells without help from T cells. That response is fast, but it makes mostly IgM, with little class switching and little memory.[4][5]

Asplenia and overwhelming post-splenectomy infection

Remove the spleen and both jobs go. Bacteria are no longer filtered from the blood efficiently, and the IgM response to capsules is lost.[2][4]

The result is overwhelming post-splenectomy infection, or OPSI, most often with pneumococcus, meningococcus or Haemophilus. It can strike decades after the operation.[1][2]

Reading the opening case
  1. Years after splenectomy: the filter and the marginal-zone IgM response are still missing.[1][2][4]
  2. Sore throat and chills on Friday: a minor infection that an intact spleen would normally help contain.[2]
  3. Septic shock on Saturday: an encapsulated organism reaches the blood and is not cleared, the picture of OPSI.[1][2]

Howell-Jolly bodies and functional asplenia

On the blood smear, a clue: Howell-Jolly bodies, which are nuclear remnants in red cells that a healthy spleen would remove.[8][12]

Functional asplenia counts too. In sickle cell disease the spleen is damaged until it no longer works, and those patients are managed as asplenic.[9][12]

Conjugate vaccines: why a carrier protein works

A plain polysaccharide vaccine is also T-independent. It gives a weak response in infants and builds no memory.[4][5]

A conjugate vaccine links the polysaccharide to a carrier protein.[5][6]

  1. The B cell binds the sugarIt swallows the whole conjugate.
  2. It shows the carrierPieces of the carrier protein appear on MHC class II.[5][6]
  3. A helper T cell helpsThe T cell recognises the peptide and helps the B cell.[5][6] The help comes through the same CD40 ligand signal taught in the hyper-IgM syndrome lesson.
  4. The response becomes T-dependentClass switching to IgG, affinity maturation and memory B cells follow.[5][6]

That is why conjugate vaccines work in babies, and why they still raise IgG in asplenic patients.[4][5][11]

Preventing infection in asplenia: vaccines and prophylaxis

Prevention rests on vaccines against pneumococcus, Haemophilus influenzae type b and meningococcus, plus antibiotic prophylaxis.[2][10]

Guidelines give the vaccines before the splenectomy, or about two weeks after it.[2]

Practice question

Original · USMLE style
Your turnQB-MS-S1-MICRO-014-01

A 29-year-old woman had a splenectomy last month for refractory immune thrombocytopenia and has not received any vaccines since. She now has fever, hypotension and confusion. Blood cultures grow gram-positive diplococci.

Which of the following best explains her susceptibility to this infection?
Pick one option. Your answers stay on this device.

Common questions

Which organisms are the classic encapsulated organisms?

Streptococcus pneumoniae, Neisseria meningitidis and Haemophilus influenzae. They are also the organisms most often responsible for overwhelming post-splenectomy infection.

Why does asplenia increase the risk of infection with encapsulated bacteria?

The capsule resists phagocytosis, so clearance needs opsonins and the spleen. Without a spleen, bacteria are not filtered from the blood efficiently and the IgM response to capsules is lost.

Why are polysaccharide antigens T-independent?

They activate B cells without help from T cells. The response is fast but mostly IgM, with little class switching and little memory.

How do conjugate vaccines work?

The polysaccharide is linked to a carrier protein. The B cell shows pieces of the carrier on MHC class II, a helper T cell helps it, and the response becomes T-dependent, with class switching to IgG, affinity maturation and memory B cells.

What does a Howell-Jolly body suggest?

Howell-Jolly bodies are nuclear remnants in red cells that a healthy spleen would remove, so they are a clue to an absent or poorly functioning spleen. Functional asplenia occurs in sickle cell disease.

Transcript

Read the full narration, by chapter613 words

0:00A missing spleen, a minor illness, an emergency

Years ago, a man had his spleen removed after a car crash. On Friday he has a sore throat and chills. By Saturday he is in septic shock. How can a missing organ turn a minor illness into an emergency? Today: why asplenia invites encapsulated bacteria, and how conjugate vaccines protect.

0:21Encapsulated organisms and the capsule

The classic encapsulated organisms are Streptococcus pneumoniae, Neisseria meningitidis and Haemophilus influenzae. The capsule is a sugar coat, a polysaccharide, and it resists phagocytosis.

0:33Opsonins: how phagocytes clear a capsule

So defence needs opsonins: antibody and complement coat the capsule, and then phagocytes can engulf the bacterium. The spleen does two jobs here.

0:44The spleen's two jobs: filtering and the marginal zone

First, it filters the blood: red pulp macrophages remove opsonised bacteria from the circulation. Second, its marginal zone holds IgM memory B cells, which answer polysaccharide antigens and make natural IgM. Polysaccharides are T-independent antigens.

1:03Polysaccharides are T-independent antigens

They activate B cells without help from T cells. That response is fast, but it makes mostly IgM, with little class switching and little memory. Remove the spleen and both jobs go.

1:17Without a spleen: OPSI

Bacteria are no longer filtered from the blood efficiently, and the IgM response to capsules is lost. The result is overwhelming post-splenectomy infection, or OPSI, most often with pneumococcus, meningococcus or Haemophilus. It can strike decades after the operation.

1:39Howell-Jolly bodies and functional asplenia

On the blood smear, a clue: Howell-Jolly bodies, nuclear remnants in red cells that a healthy spleen would remove. Functional asplenia counts too. In sickle cell disease the spleen is damaged until it no longer works, and those patients are managed as asplenic. Now the vaccines.

1:58Plain polysaccharide vaccines

A plain polysaccharide vaccine is also T-independent. It gives a weak response in infants and builds no memory.

2:08Conjugate vaccines: carrier protein and T cell help

A conjugate vaccine links the polysaccharide to a carrier protein. The B cell binds the sugar, swallows the whole conjugate, and shows pieces of the carrier on MHC class two. A helper T cell recognises that peptide and helps the B cell, through the same CD 40 ligand signal you met in hyper-IgM syndrome. The response becomes T-dependent: class switching to IgG, affinity maturation and memory B cells. That is why conjugate vaccines work in babies, and why they still raise IgG in asplenic patients. In practice, prevention rests on vaccines

2:45Prevention: vaccines and antibiotic prophylaxis

against pneumococcus, Haemophilus influenzae type b and meningococcus, plus antibiotic prophylaxis. Guidelines give the vaccines before the splenectomy, or about two weeks after it.

3:01Practice question

Your turn. A 29-year-old woman had a splenectomy last month for refractory immune thrombocytopenia and has not had any vaccines since. She now has fever, hypotension and confusion. Blood cultures grow gram-positive diplococci. Which of the following best explains her susceptibility to this infection? A, impaired splenic clearance of opsonised bacteria. B, absent oxidative burst in neutrophils. C, deficient killing of intracellular bacteria by macrophages. D, absent terminal complement components. E, impaired secretion of IgA at mucosal surfaces. Pause here and decide.

3:49Answer and explanation

B is out: a missing oxidative burst, as in chronic granulomatous disease, predisposes to catalase positive organisms, not to pneumococcus. C is out: killing of intracellular bacteria depends on T cell help and interferon gamma, which a splenectomy does not remove. D is out: missing terminal complement mainly invites recurrent Neisseria infection, and does not explain this picture. E is out: IgA deficiency affects mucosal surfaces, not the clearance of bacteria from the blood. So the answer is A. Without a spleen, opsonised encapsulated bacteria such as pneumococcus are not filtered from the blood.

4:34Recap

Three things to keep. One: the capsule resists phagocytosis, so clearance needs opsonins and the spleen. Two: polysaccharides are T-independent; conjugate vaccines add a carrier protein and T cell help. Three: asplenic patients need vaccines against pneumococcus, Haemophilus influenzae type b and meningococcus.

4:58Next: bacterial gene transfer

Next, the main gene-transfer routes between bacteria: transformation, transduction and conjugation.

Sources

Numbers in the text, e.g. [1], point here
  1. Fatal delayed-onset overwhelming post-splenectomy infection 26 years after splenectomy. Europe PMC PMC13384332. europepmc.org/article/PMC/PMC13384332
  2. Am J Hematol, splenectomy, vaccination and prophylaxis. Europe PMC PMC11966361. europepmc.org/article/PMC/PMC11966361
  3. Capsules, opsonins and red pulp macrophage clearance of pneumococci. Sci Adv. Europe PMC PMC11900858. europepmc.org/article/PMC/PMC11900858
  4. Systematic review of vaccination in asplenia. Europe PMC PMC13714245. europepmc.org/article/PMC/PMC13714245
  5. Capsular polysaccharide and conjugate vaccines, carrier effect. Europe PMC PMC13534131. europepmc.org/article/PMC/PMC13534131
  6. Carrier proteins and MHC-II presentation. Europe PMC PMC13120188. europepmc.org/article/PMC/PMC13120188
  7. Marginal-zone IgM memory B cells. Europe PMC PMC12031037. europepmc.org/article/PMC/PMC12031037
  8. Howell-Jolly bodies and hyposplenism. Europe PMC PMC12993292. europepmc.org/article/PMC/PMC12993292
  9. Sickle cell disease, functional asplenia. Europe PMC PMC13704711 and PMC11873697. europepmc.org/article/PMC/PMC13704711
  10. ACIP meningococcal vaccine recommendations 2020. Europe PMC PMC7527029. europepmc.org/article/PMC/PMC7527029
  11. Conjugate vaccine response in hyposplenism. Europe PMC PMC10891918. europepmc.org/article/PMC/PMC10891918
  12. Asplenia; Bacterial capsule; Howell-Jolly body. Wikipedia (background). en.wikipedia.org/wiki/Asplenia

The practice question is original and its patient is synthetic. Every fact is checked against the sources above in independent accuracy reviews. Always follow your local guidelines and your own laboratory’s ranges.

Image credits. Illustrations: Servier Medical Art (smart.servier.com), CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/), adapted (recoloured, outlined): spleen, IgM, immunoglobulin monomer (shown as IgG), baby. NIAID NIH BioArt Source (bioart.niaid.nih.gov), public domain, adapted (recoloured, outlined): macrophage, B cell with IgM receptors, red blood cell, vaccine vial. Original diagrams drawn in code: bacteria and capsules, the white-pulp schematic, sugar chains, MHC and T cell sketch, charts. No AI-generated teaching images.

Corrections. None so far. Spotted an error? Email info@medservatory.com with the lesson code MS-S1-MICRO-014.

For exam study only, not medical advice. Read the medical disclaimer.

In this series

Playlist on YouTube
← Previous · EP 06Staph vs strep: catalase, coagulase and haemolysis
  1. EP 01Gram-positive vs gram-negative cell walls5:39
  2. EP 02Acid-fast stain and the mycobacterial cell wall5:01
  3. EP 03Endotoxin vs exotoxin: lipid A, A-B toxins and toxoids5:46
  4. EP 04Bacterial toxins that raise cAMP: cholera, pertussis, anthrax5:54
  5. EP 05Clostridial toxins compared: tetanus, botulinum, alpha, C. difficile5:22
  6. EP 06Staph vs strep: catalase, coagulase and haemolysis5:23
  7. EP 07Encapsulated organisms and the spleen: asplenia and conjugate vaccines5:08
  8. EP 08Bacterial genetics: how bacteria swap genes5:13
  9. EP 09Beta-lactamase and antibiotic resistance: three ways bacteria win5:13
  10. EP 10DNA vs RNA virus replication: which enzyme each genome needs5:48
  11. EP 11HIV life cycle and drug targets, step by step5:39
  12. EP 12Hepatitis B serology: reading the panel5:17
  13. EP 13Antifungal drugs explained: mechanisms of action5:26
  14. EP 14Bacterial meningitis organisms by age and risk group5:19