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Immune system


General infections
Hypersensitivity reactions
Immune system organ disorders
Immune system pathology review



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USMLE® Step 1 style questions USMLE

1 questions

An 8-year-old girl is brought to her primary care physician's office for evaluation of unsteady gait. The parent states the patient’s gait has become “wobbly,” and she has been experiencing recurrent falls. The parent also reports it is difficult to understand her speech. Her development was normal until the age of two years, when she started developing these symptoms. Past medical history is significant for recurrent respiratory tract infections. Family history is unremarkable. Vitals are within normal limits. Physical examination demonstrates numerous superficial nests of distended blanchable capillaries on sun-exposed areas. The patient is unable to coordinate head and eye movements when shifting gaze. This patient is at increased risk of developing which of the following complications?

External References

With ataxia telangiectasia, ataxia refers to poor coordination and telangiectasia refers to dilated blood vessels, which are the two key symptoms of this disease.

Ataxia telangiectasia develops when a genetic mutation causes the lack of a protein called ataxia telangiectasia mutated serine-threonine kinase, or just ATM for short, which normally fixes up damaged DNA.

DNA of every cell gets damaged over and over again from various environmental factors like radiation and chemical toxins.

Now, one of the most severe types DNA damage is a double-strand break, where both strands of the DNA’s double helix are severed, damaging the genetic information that was stored there. To help with this sort of repair, there’s a protein called ATM.

ATM is primarily located in the cell's nucleus, and you can sort of think of ATM as like a manager of the DNA’s repair.

It’s protein kinase, which means that it uses it’s managing skills to activate other proteins through phosphorylation, which is the addition of a phosphoryl or -PO32− group.

So at the site of the double strand break, ATM phosphorylates proteins like the tumor suppressor protein p53, that stop the cell from reproducing.

ATM also phosphorylates additional proteins which will either fix the DNA or kill the faulty cell through a process of apoptosis - controlled cell death. This way we don't end up with a bunch of defective cells trying to reproduce.

That's the main role of ATM, but it also plays a role in the development of immune cells, especially T lymphocytes.

T lymphocytes need to be able to recognize a wide variety of antigens, and to do this, they purposefully create double strand breaks in their DNA during development. That way parts of their DNA can get rearranged, and code for new and unique antigen receptors.

Once again, ATM helps to fix these breaks - and that keeps the T-cells functioning normally.

In ataxia telangiectasia, there's an autosomal recessive mutation of the ATM gene.

This leads to a decrease in the amount of functioning ATM protein, and as a result, cells that undergo a double-strand break don't have sufficient ATM to repair the break.

As a result, some of the damaged cells might survive and continue to proliferate despite the damage, but most ultimately die off.

Now, if cells start dying off left-right-and-center that’s a big problem.

This can especially impact the nervous system.

When the cerebellum’s affected, it atrophies, and since the cerebellum coordinates movement, this degeneration is what causes the ataxia, or or difficulty with coordination and movement, in lots of individuals with the disease.

In many cases, the ataxia can lead to the muscles in the mouth and throat not coordinating, which can cause aspiration pneumonia. That’s where bits of food, liquid, and body secretions are able to enter the airways, and cause direct damage as well as infection in the lungs.


Ataxia-telangiectasia (A-T) also called Louis-Bar Syndrome, is an autosomal recessive condition characterized by progressive cerebellar atrophy and oculocutaneous telangiectasia (small dilated vessels under the skin). People with A-T often have high rates of cancer and immunodeficiency disorders. It is caused by a mutation of the ataxia telangiectasia mutated (ATM) gene, and it is characterized by poor muscle coordination or ataxia (often needing a wheelchair by the age of 10), dysarthria, and telangiectasia on the skin and eyes.

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