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Written by Tim Sheppard MBBS BSc. Last updated 9/11/10

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What is an antibody?

Antibodies are part of the acquired immune system, so they are specific to particular antigens. In fact, antibodies are what define humoral immunity, which is the part of the body's immunity which relies on antibodies.

They are proteins produced by those B-cells which have turned into plasma cells. The proteins are made up of two copies each of two different chains - so, two light chains and two heavy chains. Each light chain is connected to the heavy chain with a disulphide bond, and the two heavy chains are also connected similarly.

The antibody structure can be separated into two halves - the Fab region or fragment antigen binding, and Fc region or fragment crystallisable. The Fab region is the part of the protein which sticks to the antigen it's looking for, and this is the end which can have a highly variable structure; each B-cell produces a different antibody, and it is this end where it changes.

The Fc region is the part of the antibody which other cells bind to. B cells often have antibodies on their surface, acting like receptors; it is the Fc region which is bound in the B-cell, while the Fab region is exposed. Other cells such as natural killer cells, neutrophils and monocytes have Fc receptors.

What do antibodies do?

Antibodies are one of the body's greatest weapons against the threats it may receive. There are a whole range of functions which this clever little protein has, and remarkably its specific structure makes sure that it doesn't target everything, but - provided everything is working properly - only those things which are likely to pose a problem.

The first thing that an antibody can do is neutralise an enemy. Effectively it's like shooting the enemy in the leg so that it can no longer be effective. The Fab region described above binds to an antigen (e.g. on the surface of a virus) and it stops that antigen binding to a receptor or having any effect. If the antigens can't bind to anything, they're pretty useless and therefore the threat is gone. Eventually it will be found in by something like a macrophage and destroyed.

Another thing that an antibody can do is cause other cells to target the pathogen - it's like a flag, showing the white blood cells that this is an enemy. First of all the Fab region binds to the antigen. This leaves the Fc region (also described above) exposed, and enables those cells with Fc receptors to bind. When they do, they release perforin and granzymes to destroy cells which have an antibody on them - antibody dependent cell-mediated cytotoxicity. Antibodies therefore alert the immune system to the presence of a threat, and activate cells to destroy it.

So, the antibodies can neutralise and lead to destruction. There is another effect called opsonisation.

Phagocytic cells - those cells which 'eat' pathogens and destroy them - need something to trigger the phagocytosis. They need something to say 'Hi, I'm worth eating!' And there's various different things which can do this. Some things which are common to bacteria - which make it obvious that this is a bacteria kicking about - will stimulate cells to eat them. But when molecules from the body stick to the bacteria, this is called opsonisation, and it means that they are more easily eaten by the phagocytic cells.

Antibodies are not great at this, but they do help out - and they also stimulate other processes which cause a molecule called C3b (part of the complement pathways) to stick to the surface. This binds well with macrophages and neutrophils, and so makes the enemy readily removed.

What are antibody isotypes?

Antibodies come in a number of different forms - known as isotypes - which are classified according to differences in the heavy chain part of their structure. Humans have five types of antibody, namely IgA, IgD, IgE, IgM and IgG. They all hold the standard antibody shape, but they have differences in the way in which they work, and the part of the body where they work.

Antibodies are useful because they form such an important part of the immune system, but in fact they are particularly useful to the medical profession because each of these isotypes has a slightly different role. So, if the blood has a high concentration of one particular antibody isotype, this can give a great clue as to what's going on.

IgA is an immunoglobulin that's found wherever there's mucus, so particularly in the gut and the lungs, but also in the genitourinary tract. It's job here is to stop bacteria and other pathogens from making their home in the body - so if IgA is raised, it may suggest there there is an infection in the gut. It exists as a dimer, which means if floats around in a pair, connected to another IgA molecule.

IgD acts mainly as a receptor on the surface of B lymphocytes. It exists as a monomer - i.e. on its own.

IgE is also a monomer, and responds to things in the environment that the body is allergic to (i.e. allergens). If you have hay fever, then it will be your IgE that is responding to the pollen.

IgG is the big-guy, the one that faces most of the pathogens that try and make their way into the body and does its best to combat the invasion. Because of its dominant role, it's not so surprising that rogue antibodies tend to come from this camp. Autoimmunity, where the body makes antibodies against itself, is involved in a number of conditions, and such antibodies will be in the IgG isotype. This isotype is also a monomer.

IgM, which goes around with four buddies (in the rather scary pentamer!), is there to help out right at the beginning, in what is called the 'acute phase'. If there's an infection, this will probably be raised; it's thrown into the blood stream in order to eliminate pathogens as soon as possible, even when the IgG hasn't made it up to the level that it needs to.

Testing immunoglobulin levels isn't always necessary; sometimes it's clear what's going on from how a patient is feeling and what symptoms they have. But sometimes it can actually be very handy to have some idea of which particular antibody is raised, as it can show up what's going on in the blood. If there's a fight going on, the body is wonderfully designed to put forward the best men for the job, and as spectators who want to be assisting in any way they can, doctors have a great advantage of understanding the body's tactics.

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