IgG antibodies are generally considered the smallest and most common type of antibody found in all body fluids. However, recent research indicates that even smaller antibody fragments exist.
Different Antibody Types and Sizes
Several sources cite IgG as the smallest antibody. However, this is a simplification. The size of an antibody can vary based on its structure and the specific type. While IgG is a relatively small antibody, research focuses on even smaller functional fragments.
-
IgG: These are the most abundant and generally considered the smallest complete antibody. They play a critical role in fighting infections and are the only antibodies that can cross the placenta to protect a developing fetus.
-
Nanobodies: Derived from camelid heavy-chain antibodies (HCAb), these are very small antigen-binding fragments. They are smaller than conventional antibodies. Their tiny size allows for better tissue penetration.
-
Ultralong CDR H3 antibodies: Recent research highlights bovine antibodies with ultralong CDR H3 regions. These antibodies have a unique structure and produce exceptionally small, functional antibody fragments, potentially even smaller than nanobodies. These are described as "knob domains" in some publications.
-
scFv (single-chain variable fragment): This is the smallest antibody fragment capable of binding antigens. It combines the variable heavy (VH) and variable light (VL) domains of an antibody linked by a peptide spacer.
The Smallest Functional Antibody Fragment
While IgG is often referred to as the smallest antibody, the smallest functional antibody fragment is currently under investigation. Recent research strongly suggests that ultralong CDR H3-based antibodies from bovine sources produce the smallest functional units. These fragments, referred to as "knob domains", maintain antigen-binding capabilities despite their extremely small size. Nanobodies remain strong contenders as exceptionally small functional antibody fragments.
In summary, while IgG is often cited as the smallest antibody, the title of "smallest functional antibody fragment" is a subject of ongoing research, with ultralong CDR H3-based antibody fragments and nanobodies emerging as strong candidates.
