Type IV Hypersensitivity Flashcards
What is type IV hypersensitivity also known as?
Delayed-type or T cell-mediated hypersensitivity.
How does type IV hypersensitivity differ from the other types?
While the other types of sensitivity are caused by antibodies reacting with antigens, this is primarily initiated by T cells.
What must first happen before someone can have a T4 hypersensitivity reaction?
They must be first exposed and sensitised to the antigen. It needs to be presented to naïve T cells by antigen-presenting dendritic cells, resulting in the generation of antigen-specific memory T cells. This process takes several weeks.
What happens on subsequent exposure to the antigen?
Memory T cells respond by promoting inflammation at the site of exposure.
Why is T4 hypersensitivity also known as the “delayed-type”?
Because the memory T cell response (which requires recruitment and expansion) is slightly slower than antibody-mediated memory, there is often a delay between exposure and response, with peak responses often seen 2-3 days after inflammation.
How does poison ivy cause a T4 hypersensitivity reaction?
A small molecule called urushiol acts as a hapten (and binds to proteins in the skin) and drives a T helper type 1 response (but due to its small nature rarely results in antibody production).
On re-exposure, these memory cells produce cytokines such as IFN-gamma which promote the pro-inflammatory activation of macrophages resulting in swelling and oedema, and the formation of blister like lesions.
What are some other contact antigens can drive (type 1) T helper based inflammation?
Nickel salts, hair dyes, the measels virus and Mycobacterium tuberculosis (this is what the BCG skin test is based on).
Can only type 1 T helper memory cells cause a T4 hypersensitivity reaction?
No, any T memory cell is capable of driving an immune overreaction if they put their mind to it 💯🙏😤.
In asthma, allergens can cause an overreaction of type 2 T helper cells (which produce soluble mediators that promote bronchoconstriction).
CD8 T cells can lead to inflammation and rejection of a tissue graft by directly killing transplanted cells. Where did we also see a hypersensitivity reaction that was as a result of transplantation? What is the importance of this?
Type II hypersensitivity. It is important to realise that multiple hypersensitivity reactions may be going on concurrently.
