Question

I just read a chapter in Janeway's Immunology on T cell development and thought about transplant rejection. But I couldn't figure out how the t cells develop which lead to the rejection.

So in direct allogenic antigen detection antigen-presenting cells from the donor present peptides on MHCI. The T cells from the receipient then detect directly that the MHC itself is foreign. The MHC here is not digested first.

Based on what I learned about T cell development I don't understand how these t cells develop which can detect foreign MHC directly. In t cell development there is positive and negative selection. T cells which present antigens on self-MHC are selected positively and t cells which interact too strongly with self-antigen die off. But this suggests only t cells develop which can only interact with self-MHC but not with foreign MHC. So where do these t cells come from?

Answer 1

T cells that can recognize foreign MHC molecules directly develop through clonal selection in the thymus. During negative selection, T cells that bind to self-antigens are killed by apoptosis, leaving only T cells that can bind to foreign antigens presented on MHC molecules. These T cells prevent attacks on one's own body tissues.

Step-by-step explanation:

T cells develop in the thymus through a process called clonal selection. During T cell development, the cells become double positives that express both CD4 and CD8 markers. These double positive T cells move from the cortex to the junction between the cortex and medulla, where negative selection takes place.

In negative selection, self-antigens are brought into the thymus by professional antigen-presenting cells. T cells that bind to these self-antigens are selected negatively and are killed by apoptosis. The only T cells that survive negative selection are those that can bind to foreign antigens presented on MHC molecules.

These T cells are able to recognize foreign MHC molecules directly because they have been positively selected during T cell development to recognize self-MHC molecules with foreign antigens presented on them. This helps prevent an attack on one's own body tissues.