Hyper IgM syndrome

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Hyper IgM syndrome


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High Yield Notes

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Hyper IgM syndrome

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USMLE® Step 1 style questions USMLE

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A 2-year-old boy is brought to his primary care physician’s office for evaluation of recurrent pulmonary infections and failure to thrive. Since birth, the parents report the patient has had two episodes of pneumococcal pneumonia and five episodes of otitis media. He recently recovered from a diarrheal illness caused by Cryptosporidium parvum. His vaccinations are up to date, and he does not take any medications. Family history is significant for similar symptoms in the patient’s brother, who died from pneumococcal pneumonia last year. Temperature is 37.2 C (99 F), pulse is 80/min, respirations are 20/min and blood pressure is 90/55 mmHg. Physical examination reveals a child who is small for his age. Laboratory results are shown below.

 Laboratory value  Result 
 Complete blood count 
 Hemoglobin  12  g/dL 
 Leukocyte count  10,100 /mm3 
 Platelet count  200,000/mm3 
 IgG  250 mg/dL 
 IgA  24 mg/dL 
 IgM  700 mg/dL 
 IgE  undetectable 
 CD4/CD8 ratio  2 (normal, 1-4) 

This patient’s condition is most likely caused by a failure of which of the following processes?

External References

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Cryptosporidium spp. p. 152

hyper-IgM syndrome and p. 115

Cytomegalovirus (CMV)

hyper-IgM syndrome and p. 115

Hyper-IgM syndrome p. 115

IgA antibodies p. 103

hyper-IgM syndrome p. 115

IgE antibodies p. 103

hyper-IgM syndrome p. 115

IgG antibodies p. 103

hyper-IgM syndrome p. 115

IgM antibodies p. 103

hyper-IgM syndrome p. 115

X-linked recessive disorders

hyper-IgM syndrome p. 115

External Links


Hyper IgM syndrome is a problem where B cells are unable to undergo antibody class-switching, meaning that they can produce IgM antibodies, or immunoglobulins, but struggle to produce other types of antibodies, and that leaves individuals at risk for certain infections.

Let’s take a look at how B cells end up secreting different types of antibodies. Each B cell is born in the bone marrow from a stem cell and develops its own B cell receptor, which sits on the cell surface. The B cell receptor consists of two parts - a protein called CD79 that communicates with the rest of the cell and a membrane bound IgM or IgD antibody that can bind to an antigen. An antigen is any substance recognized by that particular antibody.

Each antibody has two identical light chains and two identical heavy chains that combine into a Y shape. So this Y-shaped antibody’s got two arms with identical tips, which is called the variable region. This variable region contains an antigen binding domain that’s unique to that antibody.

Below the variable region, or toward the point where the arms meet, is the constant region where every member of an antibody class is identical – so all IgM antibodies have the same constant region, but IgM and IgA constant regions are different.

And there are five classes of antibodies in total: IgM, IgG, IgA, IgE, and IgD class antibodies. And each antibody class has a slightly different job. For example, IgMs are part of B cell receptors, and are the first free-floating antibodies produced in an immune response. They’re secreted as a pentamer, meaning there are five antibodies connected together, which provides many binding sites for grabbing antigens and taking them out of the blood. Each antibody has complement protein binding sites on the heavy chains, so these IgM pentamers are also great at activating complement proteins, which help destroy and remove pathogens.


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  2. "Harrison's Principles of Internal Medicine, Twentieth Edition (Vol.1 & Vol.2)" McGraw-Hill Education / Medical (2018)
  3. "Pathophysiology of Disease: An Introduction to Clinical Medicine 8E" McGraw-Hill Education / Medical (2018)
  4. "CURRENT Medical Diagnosis and Treatment 2020" McGraw-Hill Education / Medical (2019)
  5. "Comprehensive review of autoantibodies in patients with hyper-IgM syndrome" Cellular & Molecular Immunology (2018)
  6. "Primary B-cell immunodeficiencies" Human Immunology (2019)

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