Antibodies are proteins of 10–15 nm in length (a human hair is 60 000 nm in diameter). They have four main parts: two identical heavy chains and two identical light chains that are coded for by genes on different chromosomes (see Figure 1.2a). Each of these parts is made up of smaller building blocks known as domains.

The part of the antibody that recognises an antigen is made up of two domains, the variable domain of the light chain and the variable domain of the heavy chain. Collectively, these two domains form a unique 3D shape that allows the antibody to selectively bind to an antigen if it has a complementary shape. Together they are known as the Fv (fragment variable) region. 

Beneath the variable domains are further building blocks known as constant domains, one in the light chain and one in the heavy chain. These constant domains along with the Fv region make up the Fab (fragment antigen-binding) region. 

A hinge links the Fab region to another identical Fab region and then the other heavy chain domains, which are known as the Fc (fragment crystallisable) region. Once the antibody is bound to its target, the Fc region is recognised by innate immune cells, which then eliminate whatever the antibody is bound to, whether a virus, bacteria or something else.