AssessmentsImmunodeficiencies: Clinical practice
USMLE® Step 1 style questions USMLE
USMLE® Step 2 style questions USMLE
A 12-year-old girl comes to the pediatrician’s office because of a recently diagnosed streptococcal infection. She has a history of repeated bouts of staphylococcal and streptococcal infections throughout her life, for which she had been prescribed antibiotics each time. Physical examination shows features similar to defective melanin synthesis. Which of the following immune deficiencies is most likely present in this patient?
Content Reviewers:Rishi Desai, MD, MPH
Immunodeficiencies can be classified into primary and secondary immunodeficiencies.
Primary immunodeficiencies are relatively rare, genetic and typically inherited defects in one or more of the elements of the immune system.
Secondary immunodeficiencies are much more common, acquired disorders that occur as a result of some extrinsic factor affecting the immune system.
Both primary and secondary immunodeficiencies can cause frequent or recurrent infections- specifically six or more new infections anywhere in the body within one year or four or more new ear infections within one year. Sometimes these infections turn out to be unusually severe.
Serious means that they cause persistent fever or confinement to bed for a week or more, are difficult to treat requiring two or more months of antibiotics with little effect or a need for IV antibiotics or hospitalization or cause unusual complications.
These include organ abscesses, non healing wounds, chronic diarrhea or failure to thrive meaning failure to gain weight or grow normally, persistent laboratory abnormalities such as leukocytosis or elevated ESR and CRP or persistent imaging abnormalities, such as bronchiectasis.
And then, it’s also concerning if the pathogens are opportunistic organisms, such as Candida albicans, nontuberculous Mycobacteria or Pneumocystis jiroveci, which usually don’t cause serious infections in individuals with a normal immune system.
Okay, now secondary immunodeficiencies usually occur well after infancy while most primary immunodeficiencies are inherited and present during the first few years.
Secondary immunodeficiencies can be caused by underlying diseases like HIV infection, malnutrition, diabetes mellitus, malignancy, splenectomy, for example due to sickle cell disease, trauma or previous surgery, ionizing radiation and immunosuppressive medications. Treatment is based on controlling the underlying disease or stopping the offending medication.
On the other hand, primary immunodeficiencies can be classified according to the element of the immune system that is defective.
The innate cellular defences are phagocytes, including neutrophils and macrophages, while the innate humoral defenses are complement proteins.
Antibody disorders are the most common type of primary immunodeficiencies. The main ones are Selective IgA deficiency, X-Linked Agammaglobulinemia or Bruton’s Agammaglobulinemia, and Hyper-IgM syndrome.
Selective IgA deficiency is the most common and most benign immunodeficiency, characterized by low levels of IgA, resulting in low protection against infections of the mucous membranes lining the respiratory and gastrointestinal tracts, with normal production of IgM, IgG, IgD and sometimes, increased IgE.
Most children with selective IgA deficiency have no symptoms, but sometimes there’s a tendency to develop recurrent or persistent sinopulmonary or gastrointestinal infections. But there’s also an increased frequency of atopy and asthma, as well as autoimmune diseases like rheumatoid arthritis. Finally, some individuals develop severe anaphylactic reactions, when they’re transfused with blood containing IgA, because the IgA is treated like a foreign antigen.
Diagnosis is based on an IgA level below 7 mg/dL with normal IgG and IgM in a child older than 4 years. A level above 7 mg/dL but below what’s expected for a given age, makes the diagnosis probable. Children should be monitored over time to see if IgA levels normalize.
X-linked agammaglobulinemia is an X-linked recessive genetic condition, meaning that it almost exclusively manifests in males. There’s a mutation that causes B cell maturation to stop, resulting in the absence of B cells and immunoglobulins. It typically appears after 6 months of age because that’s when the maternal immunoglobulins from the placenta start to disappear.
Usually the child develops recurrent or severe bacterial infections like acute and chronic pharyngitis, sinusitis, otitis media, bronchitis, and pneumonia. These are typically caused by encapsulated organisms like Streptococcus pneumoniae and Haemophilus influenzae, and less commonly, enteroviral infections like coxsackievirus, and finally intestinal parasites like giardia lamblia, all of which are usually neutralized by IgA in the respiratory and gut lining.
In general, an IgG level lower than 200 mg/dL, a total immunoglobulin level of IgG plus IgM plus IgA lower than 400 mg/dL or the complete absence of IgM or IgA after infancy are considered abnormal. Confirmation is done with flow cytometry showing CD19+ and CD20+ B-cell levels lower than 100 mg/dL. Additionally, we can measure IgG antibody titers to previously administered vaccine antigens to determine responsiveness to vaccination. These should be ideally checked four weeks after vaccination. Protein antigens such as tetanus, diphtheria, rubella, and mumps can be checked at all ages, while polysaccharide antigens such as Haemophilus influenzae and pneumococcus species can only be checked after two years of age.
Treatment for X-linked agammaglobulinemia includes lifelong monthly IVIG infusions, with a dose depending on the child’s weight and IgG blood count. This immunoglobulin is typically pooled across a lot of individuals and therefore provides a diverse set of antibodies, which gives people with X-linked agammaglobulinemia passive immunity and helps boost their immune system.
Next is hyper-IgM syndrome. This is an X- linked recessive disorder caused by a defect in CD40 ligand or CD40L which prevents class switching. This defect forces B cells to keep making IgM, which is the only form of antibody they produce before class switching.
Symptoms present after 6 months of age and include predisposition to fungal infections, like Pneumocystis jirovecii, which causes pneumonia, protozoa like Cryptosporidium, which causes chronic diarrhea and malabsorption, viruses like CMV which causes pneumonia and hepatitis, and encapsulated bacteria like Streptococcus pneumoniae causing otitis media, sinusitis, or pneumonia. Lab work shows an elevated IgM with reduced IgG, IgA and IgE levels. This is confirmed by flow cytometry showing a deficient expression of CD40L on CD4+ T lymphocytes. Treatment is of lifelong monthly IVIG infusions, with a dose depending on the child’s weight and IgG blood count.
Okay, next are T-cell deficiencies, the main one is DiGeorge syndrome or 22q11.2 deletion syndrome. This is a genetic condition where the q11.2 portion of DNA on chromosome 22 is deleted, which can cause developmental midline defects. These include midline brain defects, affecting the hypothalamus and pituitary gland, facial abnormalities, such as cleft palate, a long face, small teeth, or broad nose, parathyroid gland hypoplasia, causing hypocalcemia, hypoplasia or dysfunction of the thymus, congenital heart defects, in particular truncus arteriosus and tetralogy of Fallot as well as mental health conditions like schizophrenia.
Let’s focus on thymic hypoplasia. T cells are produced in the bone marrow but mature in the thymus. If someone has thymic dysfunction, the T cells don’t mature, and so these people often have a deficiency in mature T cells.
Complete DiGeorge syndrome, though, where thymic dysfunction is severe, it can be fatal within the first year of life. Often within 6 months of age, an infant begins having recurrent or severe infections from common viruses like Varicella zoster virus, or opportunistic fungi, like Candida albicans and Pneumocystis jiroveci, and bacteria like nontuberculous Mycobacteria. Screening begins with a CBC with differential.
A lymphocyte count below 1500 cells/uL in children over five years or less than 2500 cells/uL in younger children is considered abnormal. After that, the next step is determining the function of T-cells. This can be done both in vivo, meaning on the child or in vitro, meaning in the lab.
In vivo studies include delayed hypersensitivity skin tests to antigens like Candida, mumps, tetanus or tuberculin that most people have been exposed to at one time in their lives. The process involves injecting a few antigen drops under the skin, usually on the forearm. The site is then observed for 72 hours for a raised, red area, signifying normally functioning T-cells.
In vitro studies measure the proliferation of peripheral blood T cells in response to certain stimuli, such as mitogens like phytohemagglutinin or PHA and concanavalin A or ConA. The way this is done is that the child's purified peripheral blood lymphocytes are incubated in sterile media together with the mitogens for three to six days. A control tube with cells and media alone is also incubated. Radioactive thymidine is also added to the cultures, so that the dividing lymphocytes incorporate the thymidine into their DNA. We can then quantify the extent of proliferation by measuring the radioactivity taken up by cells. The test is then interpreted by comparing the proliferation of the lymphocytes in the test tube with normal control lymphocytes.
If there’s no response in either of these functional studies, we can then proceed to flow cytometry for lymphocyte subset analysis in order to determine the number and percentage of CD4+ and CD8+ T cells. Diagnosis is confirmed with fluorescence in situ hybridization or FISH for short, which is a type of genetic testing able to detect the 22q11.2 deletion. A chest x-ray, in some cases, also reveals a hypoplastic thymus.
Severe combined immunodeficiency or SCID for short, is the most severe form of primary immunodeficiencies caused by a variety of gene mutations, affecting the development of functional B and T cells. In fact, the immune system is so dysfunctional that it is considered almost completely absent. It usually presents in infancy as an extreme susceptibility to all kinds of infections, meaning bacterial, viral, and fungal infections - including opportunistic organisms like Pneumocystis jirovecii. There’s often chronic diarrhea as well as a failure to thrive.
Diagnosis of SCID requires an absolute lymphocyte count lower than 2500 cells/cubic millimeter, with T cells making up less than 20 percent of the total lymphocytes in flow cytometry, and a response to mitogens that’s less than 10 percent of the control response.
If left untreated, these infants have a high mortality rate in the first year, so hematopoietic stem cell transplantation is recommended before the age of 3 months. Until then, these infants are often kept in a sterile environment so that they are protected from pathogens. In addition, intravenous IgG infusions can be given about once a month, and antibiotic prophylaxis for Pneumocystis jirovecii infections can be given with trimethoprim sulfamethoxazole.
Ataxia telangiectasia is an autosomal recessive disease caused by a mutation in the so- called “Ataxia Telangiectasia Mutated protein” or ATM protein, which normally fixes damaged DNA. This causes cells throughout the body to accumulate mutations and die. This results in ataxia or poor coordination of movements, typically seen by age 2, telangiectasia or small dilated blood vessels visible especially on the white of the eyes by the age of 5 years, and telangiectasias that form in the skin by age 7. There’s also an increased incidence of cancer, especially leukemia and lymphoma. About two thirds also have low IgG, IgA, and IgE levels along with low levels of T-cells, causing recurrent infections especially of the sinopulmonary tract.
Diagnosis is made by detecting low levels of the ATM protein in cultured blood cells.
Individuals are given prophylactic antibiotics and regular IVIG infusions.
Wiskott–Aldrich syndrome is an X- linked recessive syndrome caused by a mutation in the so- called “Wiskott-Aldrich syndrome protein” or WASp. Lack of this protein affects the cytoskeleton of hematopoietic cells. As a result, T-cells and B-cells aren’t able to form an immunological synapse, resulting in an impaired immune response. For unclear reasons, this leads to an increase in IgA and IgE antibodies, and normal or decreased levels of IgM and IgG antibodies. Regulatory T-cells are also dysfunctional, making autoimmune diseases and cancers more likely.
The infections are classically due to encapsulated bacteria, such as Streptococcus pneumoniae, Haemophilus influenzae and Neisseria meningitidis, fungi, such as Pneumocystis jirovecii and Candida albicans, and viruses, such as Molluscum contagiosum, Varicella zoster virus, and cytomegalovirus. Individuals are also more prone to developing autoimmune conditions, such as idiopathic thrombocytopenic purpura, and cancers, like leukemia and lymphoma.