Antibody - IgD

Immunoglobulin D - IgD - this model of IgD depicts the Fab arms reaching laterally away from the Fc region. It is a surface representation, based on backbone atoms only. IgD is an antibody isotype that makes up about 1% of proteins in the plasma membranes of mature B-lymphocytes where it is usually coexpressed with another cell surface antibody called IgM. IgD's function has always been a puzzle in immunology since its discovery in 1964. IgD is present in species from cartilaginous fish to human (probably with the exception of birds). This nearly ubiquitous appearance in species with an adaptive immune system demonstrates that IgD is as ancient as IgM and suggests the notion that IgD has important immunological functions. In B cells, IgD's function is to signal the B cells to be activated. By being activated, they are ready to take part in the defense of the body in the immune system. [information from wikipedia: http://en.wikipedia.org/wiki/Immunoglobulin_D]