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Immunodeficiency: DiGeorge Syndrome, SCID, IgA Deficiency, MPO Deficiency & CGD

SKIP AHEAD:
1:12 – Immunodeficiency Keywords
2:08 – Microbe susceptibilities created by specific deficiencies
5:12 – X-Linked (Bruton’s) Agammaglobinemia
6:43 – Selective IgA Deficiency
7:44 – DiGeorge Syndrome
9:19 – Severe Combines Immunodeficiency (SCID)
10:41 – Chronic Granulomatous Disease (CGD)
13:38 – Myeloperoxidase (MPO) Deficiency

“Recurrent” is often the key word when it comes to identifying an immunodeficiency in a question stem. Anybody can get an isolated infection, but if a patient is having frequent infections that is a sign that their immune system is not functioning properly. Immunodeficient people also tend to get infections in various different locations and have infections that persist despite proper treatment. Failure to thrive (an infant that is very small for their age) is also common in immunodeficiency as these disorders are inherited and often appear during infancy. Sometimes question stems will hint at a family history of recurrent infections which points towards the genetic nature of most immunodeficiencies.

X-linked (Bruton’s) Agammaglobinemia is a rare mutation in Bruton Tyrosine Kinase which is important for the signal transduction that results in B-Cell maturation. Because maturation is inhibited only immature B-Cell Precursors are present. There is a resulting lack of germinal centers in the lymph node, because that is where B-Cells normally differentiate & proliferate. Very low levels of circulating B-Cells are present as well as almost no antibody of any class. The lack of antibody manifests primarily as a susceptibility to encapsulated bacteria that results in pneumonia, otitis media and cellulitis in early childhood. The defect is not apparent until about 6 months of age as infants are protected for the first 6 months by their mother’s antibodies. It is an X-linked disorder and therefore is much more common in boys.

(Selective) IgA Deficiency is a common immune deficiency that results in low levels of IgA and normal levels of all other isotypes. It is inherited, but the cause is not well understood. This disorder is usually asymptomatic and many people don’t even realize they have it. IgA deficiency is often not diagnosed until a patient receives a blood transfusion and has an anaphylactic reaction to IgA in the donated blood. A small percentage of IgA deficient individuals have recurrent mucosal infections (ears, GI, respiratory…) such as Giardia infections.

DiGeorge Syndrome is an abnormal development of the 3rd and 4th pharyngeal pouches that causes an absence of the thymus and parathyroid glands. Absence of the thymus means T-Cells cannot mature and absence of the parathyroid glands cause low levels of parathyroid hormone. This congenital hypoparathyroidism leads to low calcium levels and twitching/spasms. The low levels of T-Cells leads to susceptibility for viral and candida infections. The keywords here are absent thymic shadow on chest x-ray. DiGeorge Syndrome is also associated with Truncus Arteriosus and facial abnormalities such as a cleft palate and low set ears.

Severe Combined Immunodeficiency (SCID) is a T-Cell deficiency COMBINED with a B-Cell deficiency. This can either be a defect in the IL-2 receptor or dysfunction the Adenosine Deaminase enzyme (adenosine builds up and is toxic to B & T Cells). There is an absent thymic shadow on the chest x-ray similar to DiGeorge, low levels of T-Cells and low levels of immunoglobulin of all isotypes.

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