Final answer:
An antibody has two Fab regions, which are the 'arms' of the Y-shaped molecule. These regions consist of variable regions determining specificity to antigens and are essential for immune response functions.
Step-by-step explanation:
An antibody molecule is Y-shaped and contains two Fab regions that represent the 'arms' of the Y. Each Fab region consists of a variable region at its far end which is crucial for the binding to antigens. The structural and chemical characteristics of the variable region depend on its unique amino acid sequence, which determines the specific three-dimensional shape (epitope) the Fab region can bind to. Even though the epitope specificity of the Fab regions is identical for both arms of an individual antibody, among different antibodies, the Fab regions manifest a high degree of variability. These regions are vital for the immune response, specifically in the neutralization of pathogens, agglutination, and antibody-dependent cell-mediated cytotoxicity.