ANTIBODIES

Remember that the other term for antibody is 'immunoglobulin', and this name (often shortened to Ig) is what we will be using mainly throughout this section.

Immunoglobulins, as mentioned before, are the guided missiles of our immune defence system.

It is all a little like Star Wars!

Immunoglobulins are groups of molecules that are attached to the surfaces of plasma cells.

They consist of 2 heavy chains of molecules, and 2 light chains of molecules.

The light chains are called 'K' and 'L' ( or, if you're old enough to have the classical education, you can call them by their proper Greek letters of kappa and lambda!)

Having got that out of the way, we will not need to mention them again, because unlike the heavy chains they do not have a bearing on what we need to know at this level.

There are 5 heavy chain types called  'G', 'A', 'M', 'E', and 'D' ( or again, for all you classical scholars out there, they stand for the Greek letters gamma, alpha, mu, epsilon, and delta).

This is important to know (not the Greek names), because the type of heavy chain that the immunoglobulin is made up of, determines the way they work and what they do within our bodies.

Below is a diagram of a stylised immunoglobulin

The way in which immunoglobulins work is quite simple to explain.

As you can see, the immunoglobulin is 'Y'-shaped  with 2 ends called the Fc end and the Fab end ( not to be confused with the Fab Four- otherwise known as the Beatles for you who are not ageing hippies).

The Fc end is fixed and that is the end that is attached to the plasma cell.

Once the immunoglobulin is released from the plasma cell, the Fc end is the end which is attached to the killer cells and phagocytes.

The Fab end, however, is not fixed - rather it is variable and it can move around a hinge-like joint.

On each immunoglobulin molecule, there are two Fab arms. 

The very end of the Fab arms can be made up of many shapes (something like 100,000 different variations are possible), and these match up with the receptor molecules on the surfaces of micro-organisms.

Amongst this vast range of variations, it is hoped that there will be one to fit any antigen or invading micro-organism that we are likely to meet, so the idea is that an antibody will always be found to fit the receptor molecules on the surface of any antigen.

The antibody and the antigen fit together rather like a jigsaw.

If we get a good fit, then we will get a good immune response, which will go a long way to seeing off the invading micro-organisms.

If we only get a poor fit, then the immune system has got a real fight on its hands.

However, if we do not get any fit it all - then all I can say is 'HELP'!!

The process that has been described above is known as ANTIBODY-ANTIGEN BINDING.

The diagram below gives a very stylised and simplistic illustration of what we have described above as antibody-antigen binding.

So now it is time to return to the individual types of immunoglobulins.

Each of the 5 types of immunoglobulin have specific properties and roles to play in the defence of our bodies, so we will now look at them in turn.

1. Immunoglobulin G (IgG)

This is arguably the most important immunoglobulin.

It is certainly the most abundant immunoglobulin that we possess, because over 70% of our immunoglobulins are IgG

It also has the smallest molecular weight of all the immunoglobulins, so it can reach places that even a certain beverage cannot reach - Heineken for those too young to remember the iconic adverts - including the interstitial spaces (which are very small ones).

IgG plays a major part in our defence against infections, both infections of the blood and infections of tissues.

It is the major immunoglobulin that is involved in the secondary response (see the section on primary and secondary immune responses) to infections, and it has a long and active life.

Another advantage of IgG is that it is able to activate the complement system.

Of particular relevance to all who hope to become pregnant (that lets me out!) is the fact that it is small enough to cross the placental barrier, and so, when children are born they possess quite a store of their mother's IgG.

This is very handy as their own store of immunoglobulin is very, very small at birth.

The majority of these maternal antibodies disappear at about 4 months of age.

2. Immunoglobulin A (IgA)

 This is another very important immunoglobulin.

There are 2 types of immunoglobulin A, and they are found in 2 places in the body.

Serum IgA is found in body fluids, such as blood, whilst Secretory IgA (SIgA), as its name suggests, is found in the body's secretions.

In fact, secretory IgA is a major immunoglobulin of all the secretions that we produce.

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Consequently, IgA has a very important role to play in protecting us from infections that may attack those areas.

For example, IgA appears to be an essential part of our defence against us absorbing many pathogenic micro-organisms that may be in our food.

Due to the presence of secretory IgA in colostrum and breast milk, breast-feeding a baby is a useful way of giving the baby some protection against pathogenic micro-organisms in the gut before the baby's own immune system is capable of giving complete protection.

IgA is the last immunoglobulin to appear in the developing child - usually the full complement of IgA is not present until about the 12th year of life.

3. Immunoglobulin M (IgM)

This is the immunoglobulin which has the heaviest molecular weight.

This is because it is made up of 5 immunoglobulin molecules that are joined together to form a pentamer.

Therefore it is the largest immunoglobulin, and so can only be found in the large intravascular spaces (i.e. the blood and lymph vessels).

When a baby is born, IgM is the first immunoglobulin to be produced.

It is quite an effective first line of defence against bacteria, and is particularly important during the first days of a primary immune response.

It is not as effective as IgG, however, as it is slower to act during secondary responses.

It also has a shorter active life than IgG, but as a first line of defence, it serves us well.

4. Immunoglobulin E (IgE)

 We still do not know everything about this immunoglobulin, but we do know that levels of it in the body are higher in people who have an atopic disease (for example asthma or eczema), so this immunoglobulin is concerned in part with allergic reactions.

Normally, in a healthy person, it is found only in small amounts.

However, the levels are raised when we are infected by parasites, such as intestinal worms, so it is an important component of our defence against such parasites.

5. Immunoglobulin D (IgD)

We know even less about this immunoglobulin, apart from the fact is thought to be active against food antigens, so we will not discuss it any further - mainly because I have got nothing else to say about it!