Question

Which of the following does not bind to Fc receptors? a. whole Ig b. fc c. heavy chain d. light chain

Answer 1

Recognition of foreign invader pathogens by the receptors present on the surface of the cells of the immune system is the basic mechanism of immune protection. Various types of receptors are found on the surface of different types of immune cells to elicit different mechanisms for immune protection.   

Fc receptors are one such class of receptors that are responsible for recognition/binding of various immune cells to the Fc portion of immunoglobulins (Ig). Hence, their name Fc receptors.   

Immunoglobulin is composed of four polypeptide chains two of which are light chains and two heavy chains.

Each immunoglobulin upon digestion with enzyme papain in the presence of reducing agent yields three fragments as a result of specific proteolytic hydrolysis.

Two of these fragments are antigen binding (Fab fragments) which comprise amino terminal ends of these four chains and have high amino acid variability. These are responsible for recognition and binding to the specific antigens. Hence, their name Fab (antigen binding). The third fragment is crystallisable. Hence, its name Fc (fragment crystallisable). The Fc fragment comprise major constant region portion of heavy chains.

Since light chains in immunoglobulin molecules are not part of their Fc portion, light chains cannot bind to Fc receptors on their own. While whole immunoglobulins, heavy chains and Fc parts can only bind to Fc receptors.

Most of the Fc receptors are glycoproteins that are found on the surface of various immune cells like macrophages, B lymphocytes, Natural Killer cells etc. They aid the immune process by promoting cytotoxic or phagocytic action by facilitating binding of immune cells carrying Fc receptors to the immunoglobulins that are already bound on the surface of pathogens or infected cells (antigens) through their other ends (Fab portions). This way Fc receptors promote antibody mediated cell phagocytosis or antibody dependent cell mediated cytotoxicity.