What are the differences between immunoglobulin and antibody?

Immunoglobulin and antibody are two types of disease-fighting glycoproteins produced by most vertebrates in response to a specific antigen. There are significant differences between the two. 

Immunoglobulin

Immunoglobulin refers to any class of structurally-related proteins that function as antibodies and are found in the immune system serum and cells. It occurs on the surface of B cells. Immunoglobulin has a transmembrane domain that enables the molecule to be attached to the plasma membrane of B cells. There are five categories of Immunoglobulin - IgG, IgA, IgM, IgD, and IgE based on the type of heavy chain present in the molecule. Immunoglobulins have two major functions depending on the type of heavy chain in the molecule. Surface immunoglobulins help the B cell recognize the presence of a specific antigen in the body. They also stimulate the appropriate B cell activation. Secreted immunoglobulins assist in identifying and destroying invading pathogens such as viruses and bacteria.  

Antibody 

Antibody refers to a globin protein that is produced by B cells in response to a specific antigen. Antibody does not have transmembrane domains and occurs freely in circulation. The different immunoglobulin groups produce different antibodies.  Each antibody is specific to a particular pathogen. The main function of antibodies is to recognize and neutralize pathogens