T Cells Recognize a Combination of Self and Nonself

There Are Two Major Types of MHC Proteins Class I and Class II 

There are two major classes of MHC proteins that have similar types of structures (fig. S3.16). Class I molecules contain three external domains, each about 90 residues in length a transmembrane region and a cytoplasmic domain. The third external domain is noncovalently associated with a small polypeptide known as the (32 microglobulin. Class II molecules are composed of two noncovalently associated polypeptide chains a and /3, whose overall structure resembles that of the class I complex. 

Several genes encode each type of MHC protein, and several alleles represent different MHC proteins for each of these genes. This gives rise to a tremendous variety of MHC proteins, with the consequence that it is extremely unlikely that any two members of the same species will possess the same assortment of MHC proteins. Thus, two humans are not likely to carry the same MHC proteins on their cell membranes unless they are identical twins. This is the reason why transplants between two individuals are almost 

The two classes of MHC proteins are displayed on different cell types. Class I MHC proteins are found on almost all nucleated cells, including killer T cells. Class II MHC proteins are found mainly on cells involved in the immune response, including antigen-presenting cells, B cells, and T helper cells, but not T killer cells. 

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Tolerance prevents the immune system from attacking self-antigens. To understand tolerance we must appreciate how T cells work. T cells recognize a combination of self and nonself. The cell surface antigens rec-... 

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