1. Antigens are molecular structured, present at the outside of a pathogen that can be bound to by an antigen-specific antibody or B cell antigen receptor.
2. Most antigens have a large molecular weight. And are usually composed of proteins, peptides (amino acid chains) and polysaccharides (chains of monosaccharides/simple sugars) but lipids and nucleic acids become antigens only when combined with proteins and polysaccharides.
3. Antigens are “targeted” by antibodies which means each antibody is specifically produced by the immune system to match an antigen after cells in the immune system come into contact with it.