IgG

Antibodies are proteins your immune system makes to recognise pathogens and render them harmless, and they come in a few types. IgM is the first, coarse response to something new; IgA guards the mucous membranes of nose, throat and gut; IgG is the refined, lasting form that remains after an infection or vaccination and makes up the largest part of all your antibodies. Each IgG antibody fits exactly one target, and your blood contains millions of different ones, each in a small amount. Total IgG counts them all together, in grams per litre. That is a different question from whether you are protected against one particular pathogen: for that there are separate tests that look for one kind of antibody, against hepatitis B, measles or the Epstein-Barr virus. Total IgG says whether your immune system as a whole can make enough antibodies, and whether it is not making far too many because an inflammation has been smouldering somewhere for months. Together with IgA and IgM it forms the pattern that says which branch of the immune system is moving. Your report prints IgG in g/L. Normal is 7 to 16 g/L in adults, and the three labs that measure it use exactly that band; below nineteen years the band is lower, 5.95 to 13.08 g/L, because the immune system is still building up. Within the band nothing is wrong, and where you sit in it is mainly disposition.

IgG is the number that answers whether your immune system can make enough antibodies. Anyone who goes through several pneumonias, sinus infections or ear infections every winter, or infections that keep coming back despite antibiotics, sometimes has an immune system that makes too little IgG, inborn or through medicines, and that is treatable with administered antibodies. Total IgG is then the first step. The second reason is the other side: an IgG that is too high, because the immune system has been running at full speed for months. That happens with a smouldering inflammation or infection, with autoimmune diseases in which the immune system attacks the body itself, and with a liver with scars, which clears antibodies from the gut less. Rarely one cell line in the bone marrow makes one kind of antibody in large quantities, and then a protein electrophoresis is the test that shows it. The honest limitation is that the number is coarse. It counts millions of different antibodies together and says nothing about which; a normal total does not rule out a deficiency of one kind, and a normal total does not say you are protected against a particular disease. Within the band where you sit is mainly disposition, and you answer whether it is right with IgA and IgM beside it and with your own course.

A low IgG means your immune system makes fewer antibodies or they are leaking away. From common to rare: how many antibodies your immune system naturally makes, because within and just below the band that is mainly disposition and without recurring infections it means nothing; medicines that suppress the immune system, such as prednisone, methotrexate and medicines after a transplant; rarely protein loss through the kidneys or the gut, where albumin falls too; and rarely an inborn or acquired antibody deficiency, with recurring infections of lungs, sinuses and ears as the hallmark, and then administration of antibodies is the treatment. A high IgG means the immune system has been running at full speed for a while. The most common explanation first: a lasting inflammation or infection, where CRP or ESR is usually raised too; autoimmune diseases such as lupus, rheumatoid arthritis or an autoimmune hepatitis; a liver disease with scars, which clears antibodies from the gut less, usually with abnormal liver values; and rarely a proliferation of antibody cells in the bone marrow, where the protein electrophoresis shows one sharp peak. Set IgG beside IgA and IgM, because the three together draw the pattern of which branch of the immune system is moving, beside CRP and ESR, which say whether an inflammation is at play, beside albumin and globulin, because IgG is the largest part of the globulin fraction, and beside the liver values with a high value.