8536d_ch03_057-075 8/7/02 9: 18 AM Page 59 mac79 Mac 79: 45_BW: Goasby et al./ Immunology 5e Antigens CHAPTER 3 -Lys-Ala-His-Gly-Lys-Lys-Va Amino acid sequence a helix PRIMARY STRUCTURE SECONDARY STRUCTURE Monomeric polypeptide molecule meric protein molecule TERTIARY STRUCTURE QUATERNARY STRUCTURE FIGURE The four levels of protein organizational structure. ondary features to give the overall shape of the molecule or parts of The linear arrangement of amino acids constitutes the primary struc- it(domains) with specific functional properties. Quaternary struc- ture. Folding of parts of a polypeptide chain into regular structures ture results from the association of two or more polypeptide chains (e.g, a helices and B pleated sheets) generates the secondary struc- into a single polymeric protein molecule ture. Tertiary structure refers to the folding of regions between sec planted organ. The achievement and maintenance of ade- compounds such as glycolipids and some phospholipids can quate blood levels of this and other immunosuppressive be recognized by T-cell receptors when presented as com drugs is important to a successful outcome of transplanta- plexes with molecules that are very much like MHC mole- tion, and antibody-based immunoassays are routinely used cules. These lipid-presenting molecules are members of the to make these evaluations. The extraordinary sensitivity and CDI family(see Chapter 8)and are close structural relatives specificity of assays based on the use of anti-lipid antibodies of class I MHC molecules. The lipid molecules recognized by is illustrated by Table 3-2, which shows the specificity of an the CDl-T-cell receptor system all appear to share the com antibody raised against leukotriene CA. This antibody allows mon feature of a hydrophobic portion and a hydrophilic head the detection of as little as 16-32 picograms per ml of group. The hydrophobic portion is a long-chain fatty acid leukotriene CA. Because it has little or no reactivity with sim- alcohol and the hydrophilic head group is composed of highly ilar compounds, such as leukotriene Da or leukotriene E4,it olar groups that often contain carbohydrates. Recognition of can be used to assay leukotriene CA in samples that contain lipids is a part of the immune response to some pathogens, his compound and a variety of other structurally related and T cells that recognize lipids arising from Mycobacterium tuberculosis and Mycobacterium leprae, which respectively T cells recognize peptides derived from protein antigens cause tuberculosis and leprosy, have been isolated from hu when they are presented as peptide-MHC complexes. How- mans infected by these mycobacteria. More about the presen- ever,some lipids can also be recognized by T cells. Lipoidal tation of lipoidal antigens can be found in Chapter 8planted organ. The achievement and maintenance of ade￾quate blood levels of this and other immunosuppressive drugs is important to a successful outcome of transplanta￾tion, and antibody-based immunoassays are routinely used to make these evaluations. The extraordinary sensitivity and specificity of assays based on the use of anti-lipid antibodies is illustrated by Table 3-2, which shows the specificity of an antibody raised against leukotriene C4. This antibody allows the detection of as little as 16–32 picograms per ml of leukotriene C4. Because it has little or no reactivity with sim￾ilar compounds, such as leukotriene D4 or leukotriene E4, it can be used to assay leukotriene C4 in samples that contain this compound and a variety of other structurally related lipids. T cells recognize peptides derived from protein antigens when they are presented as peptide-MHC complexes. How￾ever, some lipids can also be recognized by T cells. Lipoidal compounds such as glycolipids and some phospholipids can be recognized by T-cell receptors when presented as com￾plexes with molecules that are very much like MHC mole￾cules. These lipid-presenting molecules are members of the CD1 family (see Chapter 8) and are close structural relatives of class I MHC molecules. The lipid molecules recognized by the CD1–T-cell receptor system all appear to share the com￾mon feature of a hydrophobic portion and a hydrophilic head group. The hydrophobic portion is a long-chain fatty acid or alcohol and the hydrophilic head group is composed of highly polar groups that often contain carbohydrates. Recognition of lipids is a part of the immune response to some pathogens, and T cells that recognize lipids arising from Mycobacterium tuberculosis and Mycobacterium leprae, which respectively cause tuberculosis and leprosy, have been isolated from hu￾mans infected by these mycobacteria. More about the presen￾tation of lipoidal antigens can be found in Chapter 8. Antigens CHAPTER 3 59 SECONDARY STRUCTURE TERTIARY STRUCTURE PRIMARY STRUCTURE α helix β pleated sheet Amino acid sequence of polypeptide chain Domain Monomeric polypeptide molecule QUATERNARY STRUCTURE Dimeric protein molecule —Lys—Ala—His—Gly—Lys—Lys—Val—Leu FIGURE 3-1 The four levels of protein organizational structure. The linear arrangement of amino acids constitutes the primary struc￾ture. Folding of parts of a polypeptide chain into regular structures (e.g., helices and  pleated sheets) generates the secondary struc￾ture. Tertiary structure refers to the folding of regions between sec￾ondary features to give the overall shape of the molecule or parts of it (domains) with specific functional properties. Quaternary struc￾ture results from the association of two or more polypeptide chains into a single polymeric protein molecule. 8536d_ch03_057-075 8/7/02 9:18 AM Page 59 mac79 Mac 79:45_BW:Goldsby et al. / Immunology 5e: