Antibodies are specialised proteins that bind to antigens
Proteins are considered the workhorses of cells, and antibody proteins play no less of a central role in the body’s defence against disease. Antibodies, made by plasma B cells, are Y-shaped globular proteins called immunoglobulins (Ig).
‘Big Picture: Immune System’
Each type of antibody binds to a specific antigen. The ability to recognise a specific antigen comes from the diversity within the antibody structure. As for all proteins, antibody structure is determined by the sequence of different amino acids in the protein chain (the primary structure) and how it folds to form a 3D molecule (the tertiary structure).
There are different broad classes of immunoglobulin:
The classes are defined by the amino acid sequence in the ‘stalk’, or constant region, of the Y structure. IgA, for instance, is important at the sticky mucosal surfaces where many pathogens try to enter, like in the intestines. IgE in the blood binds to allergens and parasitic worms. And as well as differences in the stalk, there are differences in the amino acid sequences making up the ‘arms’, or variable regions, of the Y, which bind to antigens. These differences determine which pathogen an antibody is for.
Being able to recognise danger is one thing, but what does the antibody do about it? Lots, actually. Some, like IgA, can neutralise pathogens just by binding to them. Others act as labels for phagocytes, which recognise the stalks of antibodies stuck to the outer surfaces of invading pathogens and proceed to destroy them. Antibodies also trigger components of the non-specific (innate) immune system, including the complement system (see ‘Second-line defences’ for more).