Immunoglobulins of different classes have different structures. Some immunoglobulins have long hinge regions between the Fab and Fc regions, whereas others lack the hinge region, as shown in the image below: The hinge region provides movement flexibility to the Fab regions. This is most likely to affect which of the following immunoglobulin properties?

Question

Immunoglobulins of different classes have different structures.  Some immunoglobulins have long hinge regions between the Fab and Fc regions, whereas others lack the hinge region, as shown in the image below:  The hinge region provides movement flexibility to the Fab regions.  This is most likely to affect which of the following immunoglobulin properties?

Quizplus · Accepted Answer

11ec160b_37b0_8816_9249_af2fa691618d_MD0001_00 Immunoglobulins are composed of a pair of heavy and light chains. Light chains have a variable and constant region; heavy chains have a variable region and 3 or 4 constant regions (CH1-4) depending on the immunoglobulin type. Immunoglobulins are generally further divided into 2 functional pieces as follows: The Fab fragment is composed of the entire light-chain and the heavy-chain variable and first constant region. This portion of the immunoglobulin uses the variable region of the light and heavy chain to bind specific antigenic epitopes. IgM has 10 Fab fragments; other immunoglobulins have 2 The Fc fragment is composed of the remaining constant regions (eg, CH2-4) of the heavy chain. The Fc fragment exerts the effector functions of the molecule, which may include activating the classical complement cascade (eg, CH2 on IgG) or binding the Fc receptor on phagocytic cells (eg, CH4 on IgM, CH3 on IgG) IgG, IgA, and IgD also have a hinge region between the Fab and the Fc fragments. The hinge region is rich in cysteine and proline amino acids, which provide significant flexibility to the immunoglobulin and allow greater movement of the Fab fragment arms. Fab fragments with long hinge regions are better able to reach multiple epitopes on an antigen; this increases immunoglobulin avidity because avidity is directly proportional to the number of Fab-antigen interactions. However, long hinge regions are more susceptible to destruction by bacterial proteases (eg, IgA protease). (Choice A) Affinity is the binding strength of the Fab variable region for a given antigenic epitope. Affinity is determined by the hypervariable amino acid sequence of the variable region; the hinge region does not play a role. (Choice B) IgM is a pentamer and IgA is usually dimeric. These multimers form due to disulfide bonds between the Fc chains of the molecule and an additional amino acid sequence called the J chain. Polymerization is not affected by the hinge region. (Choices D and E) The Fc portions of different immunoglobulins interact with different receptors on immune cells. IgE interacts with the Fc epsilon receptor on mast cells, IgG interacts with the Fc gamma receptor on phagocytes and natural killer cells, and IgA interacts with the Fc alpha receptor on neutrophils, macrophages, and eosinophils. The specificity of these interactions is mediated by the heavy-chain constant regions in the Fc portion of the immunoglobulins, not the hinge region. Educational objective: IgG, IgA, and IgD have a hinge region between the Fab fragment and the Fc fragment of the immunoglobulin. The hinge region increases flexibility of the Fab arms, which makes the antibody more likely to bind to more than one site on the antigen. This increases the avidity of the immunoglobulin but makes it more susceptible to destruction by bacterial proteases. __________ References: Structure and function of immunoglobulins. (http://www.ncbi.nlm.nih.gov/pubmed/20176268)

Immunoglobulins of Different Classes Have Different Structures