Besides conventional antibodies, llamas and other camelids also produce heavy chain-only antibodies (IgGs) also named single-chain Abs or heavy-chain Abs (HcAbs).
HcAbs – Full length immunoglobulins with a simpler structure
Immunoglobulins from camelids are sub-classified into IgG1, IgG2 and IgG3. Only IgG2 and IgG3 are single-chain Abs. Single-chain Antibodies are composed of only two heavy chains and no light chains or CH1 domains. IgG2 and IgG3 both bind the antigen through a unique domain, named VHH.
Compared to the +/- 150 kDa IgG1, heavy chain antibodies have a relatively smaller size of 90-100 kDa and a simpler structure. Accordingly they offer a much easier manipulation and a higher stability. HcAbs are able to bind antigens at high temperatures and under more stringent denaturing conditions.
VHH – The smallest naturally occuring antibody binding domain
It is usually produced by the phage display method which ensures the selection of the best VHH binding domain against a dedicated target. The generation of such small, single-domain antibody fragments (VHH ~15 kDa) has many advantages over traditional immunoglobulin fragments such as ScFv or Fabs.
Why use VHH Ab fragments?
Due to their small sizes, VHH binding domains can bind epitopes that are hidden. Hence the targeting precision is higher compared to normal antibodies
Because they are composed of a unique polypeptide chain, VHH binding domains are relatively easy to produce in lower eukaryotes up to very high amounts and purities.
VHH binding domains have a high tissue penetration and are cleared from circulation rapidly.
Some VHH binding domains can even cross the blood-brain barrier.
VHH binding domains are more stable than full length antibodies under various pH and temperature conditions and against proteases.
Hence, they keep their native folding and epitope binding
capacity under very diverse experimental conditions.