Immunoglobulins: Structure and Function

Antibodies!

Be able to describe the basic structure of immunoglobulins (Ig)

I wrote quite a bit about this in one of my posts for BIOC2001, which also explains how hybridomas are made. (Hybridomas are immortal cells bred to produce monoclonal- i.e. all of the same kind- antibodies.) However, there's a bit more detail that you'll need to know for this course. I'm not going to go over the really basic stuff in the BIOC2001 post because I can't be bothered typing that out again, so if you know absolutely nothing about antibodies, please start there first!

Aside from the stuff I wrote on the BIOC2001 post, you also need to know that the light chains have a V_L (variable) and C_L (constant) domain, and the heavy chain has one variable V_H domain plus 3 or 4 constant domains (named C_H1-C_H4). Oligosaccharides are often seen N-linked to the C_H2 domain, allowing for greater stability and improved interactions with Fc receptors. IgG, IgA and IgD also have a "hinge region" at the area where the arms converge, which allows for some flexibility.

In the BIOC2001 post I also mentioned the Ig-folds (immunoglobulin folds) that make up the structure of each of the domains of the immunoglobulin. Well, it turns out that not only immunoglobulins have these. The immunoglobulin superfamily is a large group of cell membrane proteins that have at least one Ig-fold domain, and include T-cell receptors, MHC molecules, CD4, CD8 and CD3.

Another thing I mentioned in the aforementioned post (thanks Pattwood for sparing me having to type all of this again!) was the "papain cleavage site," where papain can eat an antibody, resulting in two Fab fragments and an Fc fragment. Well, pepsin can also be used to cleave antibodies, but in a slightly different place, or rather places as pepsin cleaves in more than one place. Pepsin cleaves a bit lower down, so the Fab fragments remain stuck together and are known as F(ab')₂. It also chops up the Fc fragment into multiple smaller ones. This chopped up Fc fragment is known as pFc'.

Be able to describe the biological activities of Ig

The most important thing to know here is that there are parts of antibodies that bind to antigen, as well as an Fc (constant) region that interacts with Fc receptors, resulting in other biological effects. Here are the most important effector functions of antibodies:

1. Neutralisation: Antibodies bind to toxins or microbes, which prevents said toxins or microbes from binding to receptors on the cell surface. This, in turn, prevents toxins from having their toxic effects.
2. Opsonisation: Binding to a microbe might make it easier for a macrophage or other phagocytic cell to eat it up.
3. Activation of complement: Antibody-antigen complexes can activate the classical complement pathway, which I'll discuss in a later post. This also leads to clearing out the invader.
4. Antibody-dependent cell mediated cytotoxicity (ADCC): NK cells can bind to antibodies which have become bound to a target cell. Cross-linking of Fc receptors signals the NK cell to release cytotoxic granules, killing the infected cell.

Be able to describe classes of Ig

There are five main classes of Ig: IgG, IgA, IgM, IgD and IgE (in order of abundance). Some of these also have subclasses: IgG has IgG1, 2, 3 and 4 and IgA has IgA1 and 2. They are named after the constant region of the heavy chain: γ, α, μ, δ or ε. All five classes can possess either a κ or λ light chain, with the κ chain being more common in both mice and humans.

IgG

IgG is the most common antibody, comprising around 80% of antibodies in serum. Its major activity is in plasma and extracellular fluids. IgG binds to FcγR (Fc gamma receptors), which allows for activation of complement and the ADCC pathway. IgG is unique in that it is the only antibody that crosses the placenta, allowing the foetus to benefit from passive immunity.

IgA

IgA is often found in secretions, such as tears, saliva, breast milk, digestive juices and so on. When in secretions, it is found as a dimer and associates with two other proteins: J-chain, which helps to hold the dimer together, and secretory component, which masks the protease cleavage sites. (When found in serum, however, IgA exists as a monomer.) IgA mainly helps by neutralising stuff so that the gut can push out the toxins via peristalsis. Unlike IgG, IgA doesn't activate complement and is unable to cross the placenta.

Secretory IgA gets into gut secretions by passing through epithelial cells located in intestinal crypts. These cells face MALT on one side and the gut lumen on the other. IgA in the MALT can bind to pIgR (poly Ig-receptor) on the MALT side of the epithelial cells. pIgR binds IgA as it passes through the cell in a vesicle. Once the IgA makes it to the other side, pIgR is cleaved and becomes the secretory component that associates with IgA and protects it from proteolytic cleavage.

IgM

IgM exists as a pentamer in serum, but as a monomer when docked in the B-cell membrane. Like IgA, it also has a J-chain, but it does not have a secretory component. IgM is the "first" antibody in the sense that it's the first to appear in phylogeny, the first to appear in the immune response and the first type that B-cells make during their development (I'll write more on this later!). They are largely confined to the blood and are good at agglutinating stuff (like blood from an incompatible blood type) and activating complement.

IgM is slightly different structurally from IgG and IgA. While IgG and IgA have only three C_H domains as well as a hinge region, IgM lacks a hinge region, but has an extra C_H domain to make up for it. C_H2 is essentially the surrogate hinge-region for IgM. (IgE is also a bit like this, as you shall soon see.)

IgD

IgD is kind of a mystery antibody in that we're not really sure what it does. It mainly exists on B-cell membranes, but it's not expressed until a B-cell becomes mature. Like most antibodies, it has three C_H domains and a hinge region.

IgE

IgE, as mentioned before, has four C_H domains but lacks a hinge region. It is normally found in only very low levels in the serum, which is good, because they're infamous for being involved in allergic responses due to their interactions with mast cells and basophils. Activation of these cells causes degranulation (i.e. release of histamine-containing granules), which results in many of the effects of an allergic reaction.

Be able to describe antigenic determinants on immunoglobulins

Antibodies have antigenic determinants, which according to my quick Google search, is really just another name for epitope. Yup, antibodies can bind to other antibodies.

There are three antigenic determinants:

• Isotypes: Differences in heavy and light chains that separate the classes and subclasses from each other. For example, γ, α, μ, δ and ε heavy chains and κ and λ light chains.
• Allotypes: Allotypes have the same isotype, but have small variations in the constant regions.
• Idiotypes: Idiotypes also have the same isotype, but have variations in the variable regions, causing them to recognise different antigens.

And that's the first three weeks of content down! Yay!