MHC Class I Molecule Plus Sendai or Ovalbumin

Color coding: α-subunits; β-2 microglobulin; oxygen in H-bonded H₂O; acidic negatively charged; nitrogen atoms; polar noncharged atoms; basic positively charged atoms;
Lys₆₆, Arg₆₂, Arg₇₉, Arg₁₅₅; Glu₁₅₂; SEV-9 and OVA-8 peptides.

Like antibodies and T cell receptors, MHC molecules also bind antigens in the course of initiating specific immune responses. However, unlike these mono-specific antigen receptors, MHC molecules are capable of binding many different antigens with considerable structural variation. MHC molecules exhibit no idiotypic variability, like that associated with antibodies and T cell receptors, and yet an individual's limited number of MHC molecules must be able to bind sufficient numbers of different antigens to activate T_C and T_H lymphocytes against any invading organism. The multi-antigenic binding capacity of MHC molecules was an enigma until recently, but insights regarding this unusual property of MHC proteins have emerged from three-dimensional structural analyses of MHC-peptide complexes such as those featured in the animations below. These adjacent images compare the structures of different peptides bound to and "presented by" the same murine H-2K^b MHC class I molecule.

	Overview of MHC class I three-dimensional structure.
	Animation of MHC class I domain structure.
	Overview of MHC class II three-dimensional structure
	Animation of MHC class II domain structure.