IgG is a monomeric immunoglobulin, built of two heavy chains γ and two light chains. Each molecule has two antigen binding sites. This is the most abundant immunoglobulin and is approximately equally distributed in blood and in tissue liquids. This is the only isotype that can pass through the placenta, thereby providing protection to the fetus in its first weeks of life before its own immune system has developed. It can bind to many kinds of pathogens, for example viruses, bacteria, and fungi, and protects the body against them by complement activation (classic pathway), opsonization for phagocytosis and neutralisation of their toxins. There are 4 subclasses: IgG1 (66%), IgG2 (23%), IgG3 (7%) and IgG4 (4%). -IgG1, IgG3 and IgG4 cross the placenta easily. -IgG3 is the most effective complement activator, followed by IgG1 and then IgG2. IgG4 does not activate complement. - IgG1 and IgG3 bind with high affinity to Fc receptors on phagocytic cells. IgG4 has intermediate affinity and IgG2 affinity is extremely low.
IgG (along with IgA below) gluten specific antibodies are one of the two main antibodies which are measured in patients with coeliac(gb)/celiac(am) disease.
When most people think about an antibody, the one they most commonly envision is IgG, which is the most abundant form (class) of antibody in the blood. IgG antibodies are considered the long-term or memory antibodies. For example, once we have been exposed to the measles virus, our body fights off the original infection over time, and further re-infection is prevented because the B-lymphocytes now have a memory of the original measles virus antigen, and have flexed their muscles a bit. For now on, your blood stream contains traces of anti-measles IgG antibody which serves to protect against re-infection.
IgG antibodies do not themselves kill invaders. Rather, they attach to the virus or bacteria and ‘tag’ it so as to give notice to the other circulating cells of the immune system (such as cytoxic T cells or macrophages) that there is an unwelcome guest in the body.
Structurally, it can be useful for use to imagine an IgG antibody as an adjustable monkey wrench. A monkey wrench is comprised of a variable portion (the jaws) and a constant portion (the handle). The benefit of an adjustable wrench is that you can vary the size of the gap between the two jaws of the wrench to accommodate a wide variety of different nut sizes. Like our money wrench, IgG is comprised of two portions, not surprisingly called the ‘variable’ and ‘constant’ regions. The variable region is adjusted to fit the particular shape of the antigen, and the constant portion (like our wrench handle) remains pretty much the same. B-lymphocytes vary the gap by rearranging different molecules to fit the shape of the antigen. Other cells attracted to the antibody (now attached to the invader) recognize the handle or constant portion of the antibody, and can attach to it.
IgG is one of the smaller class of antibodies, and is also the only class of antibody to pass the placental barrier. Therefore the mother’s IgG provides the only antibody protection for newborns until their own immune system is able to contribute to antibody production. Only blood type O individuals produce anti-A or anti-B in the IgG class. This is why occasionally a case of maternal-fetal incompatibility can result in a type O mother with an A or B fetus. The anti-A or anti-B IgG antibodies of the type O mother cross the placenta and react with the fetus.
Not well known is the fact that blood type O people not only carry anti-A and anti-B antibodies in their blood, they also carry a third antibody called anti-AB which is also an IgG class antibody and can react with either A or B antigens. Presumably this may have conveyed some benefit against organisms have both A and B characteristics. ()
It has been postulated that the near-similarity of the A and B antigens in man makes it impossible for individuals of blood types A or B to provide T-cell help to IgG producing B-lymphocytes. Thus, people of these blood types are unable to produce immune IgG antibody against each other. Since the type O antigen has no such similarity to A or B, it is not under this type of restriction. (
This is a minor point, for as we will see, the anti-blood type antibodies usually produced by type O (and exclusively produced by blood types A and B) are of an even more lethal design.