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What is CAR?CAR is shorted for Chimeric Antigen Receptor. If you want to know what is CAR, you shall learn from how acquired immunity identify the antigen. Acquired immunity is composed of humoral immunity and cellular immunity. Humoral immunity mainly is the antibody secreted by B cells. The antibody is a “Y” shaped small protein which is like a man opens his arms. The end of the “Y”, which is also the location of the hands, is the site where the antibody specifically binds to the antigen. Cellular immunity is mainly composed of CD4 T cells and CD8 T cells. CD8 T cells are responsible for killing the abnormal cells called Cytotoxic T Lymphocyte. CTL identify the abnormal cells with its MHC I receptor by T Cell Receptor. Almost all cells express MHC I which is like the “Review mechanism” of cells. The cells randomly pick out some proteins in the cell and break them down into 10 amino acid short peptide chains to combine with MHC I receptor to present to the surface of the cell membrane for CTL Review. If the cells are abnormal, for example, being infected with the virus or cancerization occurred, they will generate non – self – contained proteins. Once there were MHC I receptors of non – self – contained proteins recognized by some CTL through TCR recognition，the CTL attack will be triggered.What is the structure of TCR?The reason why all the antibody and CTL can recognize abnormal cells is that they can identify by different mechanisms. Antibody may combine with any protein, besides MHC I receptor, in the way of recognizing by the spatial structure of proteins. And CTL identifies them in the way of recognizing by the polypeptide chain transferred by MHC I which is mainly amino acid sequence recognition. Antibody cannot go through the cell membrane, so identify by the antigen on the surface of the cell is the only way. Although MHC I is also on the surface of the cells, the polypeptide chain transferred may be original from any proteins. That’s why CTL can identify the antigen in the cell.What is CAR T?Antibodies can bind to proteins on the surface of cancer cells, such as Rituximab and Trotozumab which are all applied on the treatment for cancer successfully. These mAbs inhibit the proliferation of cancer cells by binding to receptors on the surface of cancer cells. However, sometimes we want to go further that not only binding to the receptors of cancer cells but to kill those cancer cells all. But antibodies themselves are not able to kill cells meanwhile the CTL is able to clean off cancer cells in the way of receptor combing with TCR and MHC I.