Question

List the properties of antibodies that determine their effector function. What is the advantage to switching to an IgG isotope?

Answer 1

• Antiodies are glycoprotein molecules which recognise a particular epitope on an antigen,bind specifically to it and finally facilitate the clearance of that antigen.
• There are five types of mammalian Ig heavy chain denoted by the Greek letters: ?, ?, ?, ?, and ?. The type of heavy chain present defines the class of antibody; these chains are found in IgA, IgD, IgE, IgG, and IgM antibodies, respectively.
• In mammals there are two types of immunoglobulin light chain, which are called lambda (?) and kappa (?).
• Both the heavy chain and the light chain, have variable and constant regions.
• The arms of the Y shaped antibody contain the sites that can bind to antigens (in general, identical) and, therefore, recognize specific foreign objects. This region of the antibody is called the Fab fragment, while the base of the Y which plays a role in modulating immune cell activity is called the Fc region. These  Fc mediated effects are directed at effector cells or effector molecules.
• The F_c region is an insoluble fragment, which crystallizes in the cold. This is the region which determines the antibody's class effects. Since only the constant domains of the heavy chains make up the Fc region of an antibody, the classes of heavy chain in antibodies determine their class effects.
• Constant region domains are associated with various biological functions and the different classes of antibodies and thus the effector functions, are defined by the constant (C) regions of the immunoglobulin heavy chain.
• Isotype or class switching is a biological process occurring after the activation of B cells, which allows the cell to produce different classes of antibody (IgA, IgE, or IgG). Class switching allows different daughter cells from the same activated B cell to produce antibodies of different isotypes. Only the constant region of the antibody heavy chain changes during class switching; the variable regions, and hence the antigen specificity, remain unchanged. Thus the progeny of a single B cell can produce antibodies, all specific for the same antigen, but with the ability to produce the effector function appropriate for each antigen that is encountered.
• Thus in conclusion, engagement of a particular antibody with the Fc receptor on a particular cell triggers an effector function of that cell; these can be : phagocytes will phagocytose, mast cells and neutrophils will degranulate, natural killer cells will release cytokines and cytotoxic molecules; that will ultimately result in destruction of the invading microbe.
• IgG is the main serum immunoglobulin, consisting about 80% of the total. It is the only maternal immunoglobulin which is transported across the placenta, and provides natural passive immunity to the newborn. It participates in most immunological reactions like complement fixation, precipitation and neutralisation of toxins and viruses. It may be considered as a general purpose antibody. Also, IgG activates the classical pathway of the complement system, a cascade of immune protein production that results in pathogen elimination. Due to all these important properties, it is beneficial to switch to IgG antibody.