Immunoglobulins, also known as antibodies, are heterodimeric proteins made up of two heavy (H) chains and two light (L) chains. They can be classified into five primary isotypes (IgG, IgA, IgD, IgE, and IgM) determined by the constant domains in their H chains, which give each isotype their different characteristics.
Immunoglobulin G is the most abundant subtype, being present in the largest concentrations at 75-80% of IgGs in blood and tissue fluids. IgG makes direct impacts on the immune response with high neutralization and opsonization activities towards toxins and viruses, acting as the first line of defense after memory B cells trigger a response. They are responsible for both existing and acquired immunity against previously encountered antigens. Once IgG binds to antigens, the Fc receptor can bind to macrophages, which can carry out phagocytosis on the intruder.1,2
Uniquely, IgG can also cross the placenta and impart immunity to the fetus. This occurs through transcytosis, in which the IgG is endocytosed and shuttled across the cytoplasm to be secreted towards the basal cell membrane of syncytiotrophoblasts. Once there, the complex is released into fetal circulation.
IgG can be further classified into: IgG1, IgG2, IgG3, and IgG4. All four subtypes exhibit different functional activities. For example, with a HIV patient, IgG3 antibodies have been demonstrated to be more effective at neutralizing HIV over IgG1, likely because of differing antibody flexibility affecting access and how changes are induced in the virus’ oligomer structure.3
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References:
Schroeder, H. W., & Cavacini, L. (2010). Structure and Function of Immunoglobulins. The Journal of Allergy and Clinical Immunology, 125(2 0 2), S41. https://doi.org/10.1016/J.JACI.2009.09.046
Ciobanu, A. M., Dumitru, A. E., Gica, N., Botezatu, R., Peltecu, G., & Panaitescu, A. M. (2020). Benefits and Risks of IgG Transplacental Transfer. Diagnostics, 10(8). https://doi.org/10.3390/DIAGNOSTICS10080583
Antibodies are versatile molecules that perform a range of effector functions, many of which engage different arms of the immune system. Their modes of action extend beyond simple antigen binding, enabling the activation of various immune mechanisms that lead to pathogen neutralization and clearance. These functions include blocking molecular interactions, activating the complement system, and linking the humoral immune response to cellular immune responses via Fc receptor engagement.
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In addition to isotypes and subtypes, antibodies exhibit genetic variation known as allotypes, which are polymorphic epitopes on immunoglobulins. These allotypic differences arise from allelic variations in immunoglobulin genes, causing certain antibody subtypes to differ between individuals or ethnic groups. The presence of these polymorphic forms can influence immune responses, particularly when an individual is exposed to a non-self allotype, potentially triggering an anti-allotype immune reaction.
In mammals, antibodies are classified into five major isotypes: IgA, IgD, IgE, IgG, and IgM. Each isotype is defined by the heavy chain it contains: alpha (IgA), delta (IgD), epsilon (IgE), gamma (IgG), or mu (IgM). These structural differences in the heavy chains determine the antibody's function, tissue localization, and role in the immune response. Furthermore, antibody light chains fall into two classes—kappa and lambda—with kappa being more common, though both exhibit similar functions despite differences in sequence.
