Antibody Structure

Antibodies are Y-shaped proteins consisting of four polypeptide chains: two identical heavy chains with approximately 440 amino acids each, and two identical light chains with roughly 220 amino acids each. These chains are held together by covalent disulfide bonds and noncovalent bonds. Most antibodies also carry sugar residues through a process called glycosylation.

The variable regions at the tips of antibody arms contain 110-130 highly variable amino acids that selectively bind antigens, allowing recognition of nearly unlimited antigen types. In contrast, the constant region, or Fc region, determines the antibody's specific immunological mechanism of action and enables interaction with Fc receptors on immune cells like macrophages and mast cells.

Antibody crosslinking occurs when one arm of an antibody binds to one antigen while the other arm binds to a second identical antigen. The flexible hinge region allows variable distances between binding sites, enabling formation of large lattices of crosslinked antigens. These lattices are engulfed more quickly and easily by macrophages, removing larger amounts of antigen at once.

IgD, IgE, and IgG each have a single Y-shaped structure with a valency of two antigen binding sites. IgA can occur as a monomer or dimer, giving it a valency of four as a dimer. IgM typically occurs as a pentamer, providing a valency of ten. These structural differences affect their distribution and immune functions throughout the body.

During an allergic reaction, IgE binds to its cognate antigen, and the Fc region of IgE then binds to mast cells and basophils. This interaction triggers the release of histamines and interleukins, causing allergic symptoms such as sneezing and itching. IgE is challenging to study due to its low levels in body fluids but is primarily known for mediating allergic responses.

IgG antibodies are the most abundant in blood and contribute to pathogen elimination through multiple mechanisms. They opsonize pathogens to trigger phagocytosis by macrophages or neutrophils, and the complement system enhances this activity. IgG is also the only antibody class that crosses the placenta, providing passive immunity to fetuses and infants through breast milk.

IgA protects mucosal surfaces including the gastrointestinal, respiratory, and urogenital tracts by neutralizing bacteria and preventing their movement across epithelia. IgA is secreted into mucus, tears, saliva, and colostrum as a dimer, providing immune defense at body surfaces. As a monomer in body fluids, IgA continues to offer localized protection against pathogens.