Antibodies – an overview

In diagnostics, therapy or research – antibodies are used everywhere. But what actually are antibodies, and why do we need them?

 

What are antibodies?

Antibodies are proteins produced by the immune system when foreign substances such as bacteria, viruses or toxins enter the body. The body is able to render such antigens harmless through the formation of defence substances (antibodies) which can react to specific intruders (antigens). It is this specific binding ability which makes antibodies so important for diagnostics and research.

Why do we need antibodies?

Today, antibodies are used for numerous applications. Antibodies are required for numerous research processes, for example: western blots, radioimmunoassays or ELISAs. And immunohistochemical examinations, FACS and immunoprecipitation also use antibodies for detection purposes. Antibodies are also used in medical diagnostics and therapy in many different ways:

• in HIV testing
• in pregnancy testing (human chorionic gonadotropin tests)
• in the treatment of multiple sclerosis (natalizumab)
• in the treatment of osteoporosis (romosozumab)
  
• for migraine prophylaxis (fremanezumab)
  
  

What is the difference between monoclonal and polyclonal antibodies?

In a normal defensive reaction, numerous different antibodies (polyclonal antibodies) against the same antigen are formed at the same time. Polyclonal antibodies always work against the same antigen, but bind to many different epitopes. This property can be an advantage in some applications as the broad spectrum of binding can result in a stronger signal than with monoclonal antibodies. This multi-epitope specificity is an advantage, especially when detecting small amounts of protein.

On the other hand, it can be the case that some protein sequences are very similar and the antibody then binds non-specifically, which in turn leads to unwanted cross-reactions. Fortunately, this risk can be greatly reduced with affinity purification. In addition, polyclonal antibodies are generally cheaper.

By contrast, monoclonal antibodies are highly specific and only act against a single epitope. This results in significantly fewer unspecific bindings and fewer cross-reactions, which often makes them more sensitive to assays that require quantitation of protein levels. Monoclonal antibodies are more expensive to produce than polyclonal antibodies, but have the advantage that they can be used therapeutically.

Monoclonal antibodies are often obtained by immunising a mouse, and must first be optimised using genetic engineering so that the body does not reject them. The antibody’s "mouse components" are almost entirely replaced by human sequences and a humanised monoclonal antibody is generated. As a result, this no longer causes an immune reaction within the human body and is then mainly used in cancer therapy. And because the risk of side effects with monoclonal antibodies is usually much lower than with conventional drug medications, they are considered the future of modern medicine.

What are recombinant antibodies?

Recombinant antibodies are artificially produced using genetic engineering. They are considered a further development of polyclonal and monoclonal antibodies, because their production requires no animal testing. Recombinant antibodies are produced exclusively in vitro.

Dispensing with laboratory animals not only has welcome moral advantages, but also ensures greater repeatability and control. The procedure is then completely independent of animal health, immune status, and the efficiency of the immunisation programme. This eliminates the problem of different signal strengths in different production batches, making results more repeatable.

It is estimated less than 50% of commercially available antibodies used to date are actually specific. Recombinant antibodies, on the other hand, are sequence-defined and therefore much more repeatable. They can be adapted in size and shape and structurally changed, so any antibody format can be made available. In contrast to monoclonal or polyclonal antibodies, there are fewer constraints to artificial production, so recombinant antibodies can be created against toxic, pathogenic or endogenous substances.

So far, recombinant antibodies have been mainly used for research in structural biology, as nanobodies to stabilise a protein complex, and in standard methods such as western blotting, in flow cytometry and immunochemistry. Meanwhile, recombinant antibodies are also used therapeutically in the treatment of multiple sclerosis, asthma and Alzheimer's, and there are also ongoing studies seeking therapies against cancer, HIV and herpes simplex.

What does the future hold?

There are tens of thousands of monoclonal antibodies, and many more polyclonal antibodies, on the market today. Yet unfortunately, there are only a few thousand recombinant antibodies currently available. In recent years, however, the manufacturing process has been continuously optimised and recombinant antibodies can now be supplied at an affordable cost within just a few weeks. In the near future, significantly more recombinant antibodies will be available and the enormous potential of this technology can be then be exploited more intensively.

It seems recombinant antibodies really are the future. In 2014, Prof. Plückthun published an article in the specialist journal Nature in which he sums up the situation perfectly: "There is no real alternative to recombinant DNA technology."

Credentials: 
A. Bradbury, A. Plückthun and 110 co-signatories. Standardize antibodies used in research, Nature, February 4, 2014.
http://nature.com/articles/doi:10.1038/518027a