Gaia, a 6 year old girl, is brought to the clinic by her parents because she’s been having diarrhea and abdominal cramps for the past few weeks.
When you ask about her clinical history, her parents tell you that Gaia was diagnosed with celiac disease a few years back; however, they point out that she's stopped consuming any food products that may contain gluten altogether.
You decide to first run stool tests, which reveal the presence of the parasite giardia lamblia.
In addition, Gaia’s parents tell you that she has a history of asthma and allergic rhinitis, so you also order an immunoglobulin test, which shows low IgA and increased IgE levels in her blood.
Next comes Joe, a 10 year old boy that’s brought to the clinic because he fell and broke his arm.
Upon physical examination, you notice a red, weeping rash on his scalp.
You also notice that there’s a skin abscess on his leg that lacks any surrounding warmth and redness.
Joe’s parents tell you that he develops abscesses like that all the time.
You order an immunoglobulin test for Joe too, which reveals increased IgE but normal IgA levels.
Based on the initial presentation, both cases seem to have some form of immunodeficiency, meaning that their immune system's ability to fight pathogens is compromised.
Immunodeficiencies can be classified according to the cell of the immune system that is defective, into B cell and T cell disorders, which respectively lead to a deficiency in humoral or antibody-mediated and cell-mediated immune responses.
Let’s begin with B cell disorders, starting with Bruton or X-linked agammaglobulinemia, or XLA for short.
This is caused by a mutation in the BTK gene, which is found on the X chromosome.
XLA is an X-linked recessive condition, so it almost exclusively manifests in biological males because they have only one X chromosome.