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https://www.patreon.com/LYMED Welcome to LY Med, where I go over everything you need to know for the USMLE STEP 1, with new videos every day.

Follow along with First Aid, or with my notes which can be found here: https://www.dropbox.com/sh/dylkj0bomn3dy7r/AADkMDWMstA0GCcTivBlId0Va?dl=0

This video will be on B cell and T cell disorders. Now just to preface it, know that T cells are seen more in viral and fungi infections. So in more viral and fungal infections, think of T cell disorders. Now that’s out of the way, let’s talk about B cell disorders.
1) X linked agammaglobulinemia: this is a mutation in bruton tyrosine kinase, which is needed to mature B cells and make plasma cells. Without this, there are no plasma cells and no immunoglobulins! This will show up at 6 months due to the loss of maternal antibodies.
2) Selective IgA deficiency: with a loss of IgA, you have more mucosal and respiratory and GIT infections. It is also linked to atopic diseases and allergic blood transfusion reaction.
3) Common variable immunodeficiency: this is a multifactorial disorder in class switching. This leads to variable decrease in immunoglobulin. Done with B cell disorderse, let’s discuss T cell disorders.

1) 22q11.2 deletion syndrome: Also known as DiGeorge Syndrome. This is the failure of the third and fourth pharyngeal pouch to develop, which lead to a loss of the thymus. Without a thymus, there is no T cells! On chest x-ray (CXR), there is a loss of thymic shadow. Now these pouches also make your parathyroid. Without your parathyroid and PTH, you have low blood calcium. Lastly, systemic manifestations include cardiac abnormalities and abnormal facies.
2) IL-12 receptor deficiency: without functional IL-12, you lower TH1 and IFN-Y. This leads to a decrease in T cells and macrophage recruitment which leads to infectious of mycobacterium and fungi. The first sign is after administration of TB vaccine.
3) Hyper IgE syndrome: this is a mutation in STAT3 which controls cytokines and growth factors. Some cytokines it can’t release will be IL-12! Therefore there are similar signs, however, without other cytokines you have cold abscesses. Also with low IL-12, you have an increase in TH2 which leads to IgE and eosinophils and atopic diseases like eczema. That’s the cytokine part, how about the growth factors? With abnormal growth factors you have coarse facies, and retained primary teeth.
4) Our last one is chronic mucocutaneous candida: this is just candida over your skin and mucus membranes.

Now let’s combine them and talk about combined B cell and T cell disorders!
1) SCID – severe combined immunodeficiency: there are many ways to get SCID, but the most common is IL2 receptor deficiency. Another cause of SCID is adenosine deaminase deficiency which is an enzyme used in your purine salvage pathway. Due to the mitotic activity of T/B cells, a deficiency in this leads to decreased immune cells. Deficiency leads to loss of follicles, thymus or T-cell receptor excision circles.
2) Ataxia telangiectasia: this is due to an ATM mutation which normally repairs dsDNA breaks. Some signs include ataxia and telangiectasia! Labs show increased alpha fetoprotein.
3) Hyper IgM: this is a XLR disorder of your CD40. This leads to an inability to class switch. Therefore your default IgM will increase.
4) Wiskott aldrich: this is due to a mutation in WASP gene which helps regulate the cytoskeleton and receptors. You can see increased igE and IgA which can lead to eczema. Also your platelets can be dysfunctional leading to thrombocytopenia and bleeding.

Our last topic will be on phagocyte disorders! Let’s finish up with chediak higashi syndrome. This is due to defective LYST gene which leads to microtubule dysfunction. This leads to dysfunctional phagolysosomes. On blood smear, there are massive granules. The dead giveaway is albinism, due to loss of transport of melanin.
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