Immunoglobulins are proteins, made up of chains of polypeptides, strings of the basic units known as amino acids. The sections that make up the tips of the Y's arms vary greatly from one antibody to another, creating a pocket uniquely shaped to enfold a specific antigen. The stem of the Y serves to link the antibody to other participants in the immune defenses. IgM, which usually combines in star-shaped clusters, tends to remain in the bloodstream, where it is very effective in killing bacteria. IgD is almost exclusively found inserted into the membranes of B cells, where it somehow regulates the cell's activation.
Antibodies can work in several ways, depending on the nature of the antigen .