With all the discussion surrounding antibody production after receiving a COVID-19 vaccine, let’s talk about the structure of an antibody.
An antibody (immunoglobulin, or Ig) is a protein synthesized by plasma B cells targeting unique molecular structures called antigens (Ag) classically from bacteria and viruses (ie, regions on the SARS-CoV-2 spike protein). When these antigens are detected by the immune system, a series of adaptive response events generates antibodies targeting them for destruction.
The fragment crystallizable (Fc) region is composed of two heavy chains (H) that modulate immune system activity by interacting with surface receptors, complement, etc. These heavy chains also determine the antibody class:
- IgA (10-15%, dimer): GI tract, mucosal surfaces, breast milk – primary defense against inhaled/ingested pathogens
- IgD (< 1%, monomer): modulates humoral response, unclear role
- IgE (least prevalent, monomer): increases in allergic conditions, binds mast cells
- IgM (5%, pentamer): synthesized first in response to pathogen
- IgG (70-75%, monomer): most abundant, involved with neutralization of pathogens, crosses placenta
The fragment antigen-binding (Fab) regions bind to antigens. Each consist of a heavy chain (H) and light chain (L, either kappa or lambda). The variable (V) domain of the H and L chains create the paratope which binds an antigen at the antigen binding site.
In summary, an antibody consists of two heavy (H) and two light (L) chains. These are spread across constant (C) and variable (V) domains. Disulfide bonds (-SS-) hold parts of the antibody together.