# T-Cells **T-cells** (which you can remember because they mature in the "t"hymus) are are one of the [[lymphocytes]] of the [[adaptive immune system]]. They are responsible for **cell mediated** [[immunity]] or the eating up infected cells and/or pathogens outright. There are several different types of T-cells, but there are some things that they all have in common: T-cell receptors, CD molecules (either CD4 or CD8), and their ability to recognize [[antigen|epitopes]] as presented by other cells. - **T-cell receptors**, or TCRs, are molecules found on T-cells that can identify other molecules that are presented to them by other cells in the body. Unlike [[B-cells#B-Cell Receptor|B-cell receptors]], TCRs must be presented with an [[antigen#Epitopes|epitope]] on an [[MHC molecule]] molecule. It reads the whole molecule/antigen complex. - Both **CD4** and **CD8** molecules act to stabilize the bond between the [[#T-cell receptor]] and the [[MHC molecule]] molecule. Helper T-cells have CD4 molecules and cytotoxic T-cells have CD8. - T-cells have [[MHC molecule]] molecules in addition to the [[MHC molecule]] that all cells have. (Question: What does this mean? MCH II molecules?) Like [[B-cells]], they are born in the [[bone marrow]], but then they travel to the [[thymus]] to finish their maturation process. It's quite a brutal process where the T-cells are "tested" against foriegn antigens and are killed ([[apoptosis]]) if they do not recognize them, and self-antigens, and they are killed if they *do* recognize those. ## Types of T-Cells There are two major types of T-cells that fight for the immune system, *helper T-cells* and *cytotoxic T-cells*, and *regulatory T-cells* work to keep the immune at bay. - [[Helper T-Cells]] are T-cells whose job it is to go around and checks the molecules that dendritic cells have presented to them. - [[Cytotoxic T-Cells]] are T-cells whose job it is to go around and sample molecules that all human cells (who possess MHC I molecules) randomly take out of their own proteins. If the cytotoxic T-cell takes - [[Regulatory T-cells]] are T-cells that actually work to *suppress* the rest of the immune system. This can be beneficial in a couple ways, such as protecting our helpful bacteria, and also by preventing runaway immune reactions. ![[types of T cells.png]] ## Problems with T-Cells - [[type IV hypersensitivity]] has to do with t-cells attacking self cells. ___