A glycoprotein formed by immune cells (B-cells) that specifically recognize certain molecules (antigens) to neutralize pathogens such as bacteria and viruses. They exist in pentameric form (IgM), dimeric form (IgA) or single form (IgG, IgE, IgD), which consist of Y-shaped units that each have two antigen-binding pockets on one side and a region recognized by immune cells on the opposite. The tips of the “Y” can recognize specific antigens and lead to a successful immune response, while the bottom of the “y” regulates immune cell responses. Also known as immunoglobulin.