Case 49: Contact Sensitivity to Poison Ivy

Question 1

what type of hypersensitivty reaction is poison ivy?

Answer 1

type IV = delayed hypersensitivity reaction

Question 2

what mediates type IV hypersensitivty reactions?

Answer 2

T cells only!

there is no antibody involvement

Question 3

what are the two classes of delayed hypersensitivty reactions?

Answer 3

1. damage is due to an inflammatory response and tissue destruction by TH1 cells
   and the macrophages they activate.
2. tissue damage is caused mainly by the direct action of antigen-specific
   cytotoxic CD8 T cells on target cells displaying the foreign antigen

some antigens may cause a combination of both types of reactions

Question 4

what are the steps in a delayed hypersensitivity reaction

Answer 4

1. antigen is injected into subcutaneous tissue and processed by local APCs
2. a TH1 effector cell recognizes antigen and releases cytokines that act on vascular endothelium
3. recruitment of phagocytes and plasma to site of antigen injection causes visible legion

this takes 24-72 hours

Question 5

why does a delayed hypersensitivtiy reaction take a little to kick in?

Answer 5

also the TH1 cells that activate are ones that have been sensitized by a previous exposure to the antigen!

because these
specific cells are rare, and because there is no inflammation to attract cells into the site, it
may take several hours for a T cell of the correct specificity to arrive.

Question 6

what causes poison ivy?

Answer 6

T-cell response to a chemical in the leaf called pentadecacatechol

on contact with the skin,
this small, highly reactive, lipid-like molecule penetrates the outer layers, and
binds covalently and nonspecifically to proteins on the surfaces of skin cells, in
which form it functions as a hapten

Question 7

what are Langerhans cells?

Answer 7

macrophages in the epidermis

Question 8

what does INFγ do?

Answer 8

actives macrophages