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USMLE Step 1ImmunologyEP 10 of 14

Hyper IgM syndrome: CD40 ligand and class switching

High IgM, yet a deficiency. In its commonest form, the B cell is present but cannot switch class, because the T-cell signal through CD40 ligand fails.

  • 5:19 video
  • 3 min read
  • 12 sources
  • Published 15 Nov 2026
Video lesson · 5:19 · Watch on YouTubeSeries: Immunology · @medservatory
Chapters (12)
By the end of this lesson

Explain how defective CD40L–CD40 signalling blocks class switching in hyper-IgM syndrome and predict the immunoglobulin and infection profile.

The short version

  1. No CD40 ligand signal means no class switch, so IgM is normal or high and IgG, IgA and IgE are low.
  2. The commonest form is X-linked CD40 ligand deficiency, so boys are affected; a rarer autosomal recessive form lacks CD40 itself.
  3. It is a combined immunodeficiency: watch for Pneumocystis, Cryptosporidium and low neutrophils as well as sinus, ear and lung infections.
  4. AID deficiency gives a similar immunoglobulin pattern, but the fault is inside the B cell and lymph nodes show giant germinal centres.
  5. Flow cytometry on activated T cells usually shows no CD40 ligand; treatment is immunoglobulin replacement plus Pneumocystis prophylaxis.

The puzzle: a hyper antibody that is a deficiency

A four-year-old boy has Pneumocystis pneumonia. His blood test shows a very high immunoglobulin M, yet his IgG and IgA are almost absent. How can a hyper antibody be a deficiency?

That is hyper IgM syndrome. The IgM is not a sign of a strong immune response. It is the output of B cells that are stuck making their first antibody and cannot move on.[1][2]

CD40 ligand and CD40: how a B cell switches class

A B cell first makes IgM. To switch to IgG, IgA or IgE, it needs help from a T cell.[1][2]

An activated helper T cell displays CD40 ligand (CD40L), which binds CD40 on the B cell. With the right cytokines, that signal switches on the enzyme AID, and the B cell switches class and refines its antibody in a germinal centre.[1][3]

  1. T cell activatedThe helper T cell displays CD40 ligand.
  2. HandshakeCD40 ligand binds CD40 on the B cell.
  3. AID switched onWith the right cytokines, the signal switches on the enzyme AID.
  4. Class switchThe B cell switches class and refines its antibody in a germinal centre.

No CD40 ligand signal, no switch, no germinal centre.

Hyper IgM syndrome: the CD40 ligand block and the immunoglobulin profile

Hyper IgM syndrome is exactly that block. The commonest form is X-linked: a mutation in the gene for CD40 ligand, so boys are affected. A rarer autosomal recessive form lacks CD40 itself, and looks the same.[1][2]

The B cells are present, but stuck making IgM.[1]

So the B cell can still make antibody; what fails is the T-cell to B-cell handshake. Whether the gene lost is CD40 ligand on the T cell or CD40 on the B cell, the signal never arrives and the switch never starts.[1][3]

Immunoglobulins and findings in CD40 ligand deficiency
FindingPattern
IgMNormal or high
IgG, IgA, IgELow
B cellsPresent
Class-switched memory B cellsVery few
Lymph node germinal centresAbsent

The lymph nodes lack germinal centres, and class-switched memory B cells are very few.[1]

Low IgG means sinus, ear and lung infections, starting in infancy.[1][2]

Pneumocystis and Cryptosporidium: a combined immunodeficiency

CD40 ligand also lets T cells talk to monocytes and macrophages. So these boys also get Pneumocystis jirovecii pneumonia, and Cryptosporidium diarrhoea that can scar the bile ducts. Neutrophils can be low too.[1][2]

CD40 ligand deficiency is a combined immunodeficiency, not a pure antibody defect.

AID deficiency: the same immunoglobulin pattern, a different fault

A second cause, AID deficiency, also gives high IgM with low IgG, IgA and IgE, but the fault is inside the B cell, downstream of the T-cell signal.[1][3][5]

CD40 ligand deficiency compared with AID deficiency
FeatureCD40 ligand deficiencyAID deficiency
Where the fault liesT-cell signal (CD40 ligand)Inside the B cell, downstream of the T-cell signal
InfectionsSinopulmonary, plus Pneumocystis and CryptosporidiumMainly sinopulmonary bacteria; opportunistic infections rare
Lymph nodesNo germinal centresGiant germinal centres

Worked example: two boys with high IgM and low everything else

Two boys share the pattern: IgM high, the rest low. Boy one has Pneumocystis and low neutrophils. Boy two has only chest infections, with swollen lymph nodes full of giant germinal centres.

Telling the two apart
  1. Boy one: Pneumocystis and low neutrophils → CD40 ligand deficiency
  2. Boy two: chest infections only, giant germinal centres → AID deficiency

The test and the treatment

The test is flow cytometry on activated T cells, which usually shows no CD40 ligand.[1][4]

Treatment is immunoglobulin replacement, plus Pneumocystis prophylaxis. In X-linked disease, a stem-cell transplant can cure.[1][2]

Practice question

Original · USMLE style
Your turnQB-MS-S1-IMM-007-01

An 18-month-old boy is admitted with pneumonia, and bronchoalveolar lavage grows Pneumocystis jirovecii. He has had recurrent ear infections, and his mother's brother died of infection in childhood. Serum IgM is elevated, and serum IgG, IgA and IgE are markedly decreased. B-cell and T-cell counts are normal.

Which of the following is the most likely underlying defect?
Pick one option. Your answers stay on this device.

Common questions

What is hyper IgM syndrome?

It is a condition in which IgM is normal or high while IgG, IgA and IgE are low, because B cells cannot class-switch. The commonest cause is a failed CD40 ligand signal from the T cell.

Why is IgM high but IgG low in hyper IgM syndrome?

A B cell first makes IgM and needs CD40 ligand help from a T cell to switch to IgG, IgA or IgE. Without that signal the B cells stay stuck making IgM.

Which infections occur in CD40 ligand deficiency?

Sinus, ear and lung infections starting in infancy, plus Pneumocystis jirovecii pneumonia and Cryptosporidium diarrhoea. Neutrophils can be low too, so it is a combined immunodeficiency.

How does AID deficiency differ from CD40 ligand deficiency?

AID deficiency gives the same pattern of high IgM with low IgG, IgA and IgE, but the fault is inside the B cell. Infections are mainly sinopulmonary bacteria, opportunistic infections are rare, and lymph nodes show giant germinal centres.

How is X-linked hyper IgM syndrome diagnosed and treated?

Flow cytometry on activated T cells usually shows no CD40 ligand. Treatment is immunoglobulin replacement plus Pneumocystis prophylaxis, and a stem-cell transplant can cure X-linked disease.

Transcript

Read the full narration, by chapter633 words

0:00The puzzle: a hyper antibody that is a deficiency

A 4-year-old boy has Pneumocystis pneumonia. His blood test shows a very high immunoglobulin M. Yet his IgG and IgA are almost absent. How can a hyper antibody be a deficiency? Today: explain how a failed T-cell signal blocks class switching in hyper-IgM syndrome, and predict the immunoglobulin and infection profile.

0:24The CD40 ligand and CD40 handshake: how a B cell switches class

A B cell first makes IgM. To switch to IgG, IgA or IgE, it needs help from a T cell. An activated helper T cell displays CD40 ligand, which binds CD40 on the B cell. With the right cytokines, that signal switches on the enzyme AID, and the B cell switches class and refines its antibody in a germinal centre.

0:50No CD40L signal: the block (X-linked and autosomal recessive forms)

No CD40 signal, no switch, no germinal centre. Hyper-IgM syndrome is exactly that block. The commonest form is X-linked: a mutation in the gene for CD40 ligand, so boys are affected. A rarer autosomal recessive form lacks CD40 itself, and looks the same.

1:11The profile: IgM normal or high, IgG, IgA and IgE low

The B cells are present, but stuck making IgM. So the profile is IgM normal or high, with IgG, IgA and IgE low. Lymph nodes lack germinal centres, and class-switched memory B cells are very few.

1:29Chest infections, then Pneumocystis, Cryptosporidium and neutrophils: a combined immunodeficiency

Low IgG means sinus, ear and lung infections, starting in infancy. But CD40 ligand also lets T cells talk to monocytes and macrophages. So these boys also get Pneumocystis jirovecii pneumonia, and Cryptosporidium diarrhoea that can scar the bile ducts. Neutrophils can be low too. It is a combined immunodeficiency, not a pure antibody defect.

1:55AID deficiency: the fault is inside the B cell

A second cause, AID deficiency, also gives high IgM with low IgG, IgA and IgE, but the fault is inside the B cell, downstream of the T-cell signal. There, infections are mainly sinopulmonary bacteria, opportunistic infections are rare, and lymph nodes show giant germinal centres.

2:19Worked example: two boys, same IgM pattern

Worked example. Two boys share the pattern: IgM high, the rest low. Boy one has Pneumocystis and low neutrophils. Boy two has only chest infections, with swollen lymph nodes full of giant germinal centres. Boy one: CD40 ligand. Boy two: AID.

2:42The test and the treatment

The test: flow cytometry on activated T cells usually shows no CD40 ligand. The treatment: immunoglobulin replacement, plus Pneumocystis prophylaxis. In X-linked disease, a stem-cell transplant can cure.

2:58Your turn: a practice question

Your turn. An 18-month-old boy is admitted with pneumonia, and lavage grows Pneumocystis jirovecii. He has had recurrent ear infections, and his mother's brother died of infection in childhood. Serum IgM is elevated, and IgG, IgA and IgE are markedly decreased. B-cell and T-cell counts are normal. Which of the following is the most likely underlying defect? A, absent Bruton tyrosine kinase in B cells. B, absent CD40 ligand on activated T cells. C, absent NADPH oxidase in neutrophils. D, absent CD18 on leukocytes. E, failed development of T cells in the thymus. Pause here and decide.

3:53Ruling out A, C, D and E

A is out: lack of BTK leaves almost no B cells and low immunoglobulins of every class, including IgM. C is out: failed oxidative killing gives staphylococcal and Aspergillus infections, not a switching defect. D is out: lack of CD18 gives delayed cord separation and pus-free infections, with high neutrophil counts. E is out: failed T-cell development leaves low T cells, and his T cells are present.

4:24The answer: absent CD40 ligand on activated T cells

So the answer is B. His T cells are there but cannot signal through CD40 ligand, so B cells cannot switch, and the missing T-cell link to macrophages explains Pneumocystis. The uncle fits X-linked inheritance.

4:41Recap pocket card and next lesson

Three things to keep. One: no CD40 ligand signal, no class switch, so IgM is normal or high and the others are low. Two: it is a combined defect, so expect Pneumocystis, Cryptosporidium and low neutrophils as well as chest infections. Three: X-linked CD40 ligand is commonest; AID deficiency looks similar, but the fault is inside the B cell. Next, encapsulated organisms and the spleen, and why losing the spleen leaves you open to them.

Sources

Numbers in the text, e.g. [1], point here
  1. Class-switch recombination / hyper-IgM syndromes and PI3K defects. Front Immunol 2018. europepmc.org/article/PMC/PMC6168630
  2. X-linked hyper IgM syndrome: case report and review. BMC Pediatr 2022. europepmc.org/article/PMC/PMC8978371
  3. Deciphering B cell maturation dynamics in hyper-IgM syndromes. europepmc.org/article/PMC/PMC13630827
  4. CD40LG frameshift family report (CD40L expression by flow cytometry). europepmc.org/article/PMC/PMC13697288
  5. Revy P, et al. Activation-induced cytidine deaminase (AID) deficiency causes the autosomal recessive form of the Hyper-IgM syndrome (HIGM2). Cell 2000;102:565-575. pubmed.ncbi.nlm.nih.gov/11007475/
  6. Agammaglobulinemia: from X-linked to autosomal forms. europepmc.org/article/PMC/PMC8269404
  7. OpenStax Microbiology, 19.4 Immunodeficiency. openstax.org/books/microbiology/pages/19-4-immunodeficiency
  8. Staudacher O, von Bernuth H. Chronic granulomatous disease review. europepmc.org/article/PMC/PMC11239527
  9. Arnold DE, Heimall JR. Chronic granulomatous disease review. europepmc.org/article/PMC/PMC5709447
  10. LFA-1 and leukocyte adhesion deficiency type I review. europepmc.org/article/PMC/PMC8998723
  11. Leukocyte adhesion deficiency case report. europepmc.org/article/PMC/PMC10619948
  12. MSD Manual Professional Version. Severe combined immunodeficiency. msdmanuals.com/professional/immunology-allergic-disorders/immunodeficiency-disorders/severe-combined-immunodeficiency-scid

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, resized): immunoglobulins IgM, IgG, IgA and IgE, neutrophil, ear. NIAID NIH BioArt (public domain): lymph node, macrophage. Bioicons: paranasal sinus head by Servier (CC BY 3.0, https://creativecommons.org/licenses/by/3.0/). Health Icons (healthicons.org, CC0): child, lungs, intestine, liver, intravenous bag, medicine bottle, spleen. The B cell and T cell, the CD40L-CD40 handshake, AID gears, germinal centres, X chromosome, immunoglobulin bars, flow-cytometry plot, family tree, question card, pocket card and all handwriting are drawn in code (original). No AI-generated teaching images.

Corrections. None so far. Spotted an error? Email info@medservatory.com with the lesson code MS-S1-IMM-007.

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

In this series

Playlist on YouTube
← Previous · EP 09B-cell vs T-cell immunodeficiency: predict the infections
  1. EP 01MHC I vs MHC II: how antigen presentation works5:03
  2. EP 02T-cell activation: the two-signal model4:58
  3. EP 03Th1, Th2, Th17 and Treg: helper T-cell subsets5:13
  4. EP 04Antibody isotypes and class switching5:13
  5. EP 05Central vs peripheral tolerance: how lymphocytes learn self-tolerance5:26
  6. EP 06Complement system explained: three pathways, one ending5:24
  7. EP 07Complement deficiencies: C1q, C3, C5–C9 and C1 inhibitor5:15
  8. EP 08Hypersensitivity types I to IV: the four effectors5:20
  9. EP 09B-cell vs T-cell immunodeficiency: predict the infections5:24
  10. EP 10Hyper IgM syndrome: CD40 ligand and class switching5:19
  11. EP 11Chronic granulomatous disease: NADPH oxidase and the respiratory burst5:20
  12. EP 12Leukocyte adhesion deficiency: CD18 and neutrophil extravasation5:01
  13. EP 13Transplant rejection explained: hyperacute, acute, chronic and GVHD5:51
  14. EP 14Vaccine types explained: live, inactivated, conjugate and mRNA5:41