Type III hypersensitivity

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Type III hypersensitivity


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Type III hypersensitivity

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USMLE® Step 1 style questions USMLE

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A 35-year-old man is evaluated for a 6-hour history of fever, itchy rash, and generalized body aches. Ten days ago, he received a snake bite treatment in the emergency department. Physical examination is significant for fever, an urticarial rash, myalgia, and polyarthritis. Urine dipsticks show 2+ protein. Serum complement levels are reduced. Which is the most likely cause of this patient’s condition?  

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IgG antibodies p. 103

in type III hypersensitivity reactions p. 111

Type III hypersensitivity p. 111

C3 deficiency and p. 105

organ transplants p. 117

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Having a hypersensitivity means that someone’s immune system has reacted to something in a way that ends up damaging them, as opposed to protecting them.

There are four different hypersensitivities and the third type or type III hypersensitivity reaction happens when antigen-antibody complexes deposit in blood vessel walls, causing inflammation and tissue damage.

Alright so first off, type III hypersensitivity reactions are mediated by immune complexes. Immune complexes, aka antigen-antibody complexes are made of two parts—the antigen and the antibody.

Antibodies, sometimes called immunoglobulins, are produced by plasma cells, which are basically fully matured and differentiated B cells.

Initially these cells make IgM - which can be secreted or bound to the plasma cell surface where it acts as a B cell receptor.

When a B cell undergoes cross-linking of two surface bound IgMs, it then takes up the antigen and presents a piece of it to T helper cells via t cell receptor to the MHC- class II molecule presenting the piece of antigen, along with costimulatory molecule CD4.

The B cell’s CD40 also binds to the T cell’s CD40 ligand, and then the t cell releases cytokines, which results in b cell activation and class switching, or isotype switching, where it changes the type of antibodies it makes.

In type III hypersensitivity reactions, typically B cells will switch from making IgM to making IgG antibodies.


Type III hypersensitivity is a type of immune response in which antigen-antibody complexes accumulate in the tissues and cause inflammation and tissue damage. This type of hypersensitivity is also known as immune-complex-mediated hypersensitivity. Examples of Type III hypersensitivity reactions include systemic lupus erythematosus, rheumatoid arthritis, and serum sickness. Symptoms can vary depending on the tissues affected and may include joint pain and swelling, rashes, fever, and kidney damage. Treatment options may include removing the triggers, and medications like antihistamines, nonsteroidal anti-inflammatory drugs, and corticosteroids.


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