Antibodies (Ab), also known as an immunoglobulins (Ig), are  large Y-shape proteins produced by plasma cells that are used by the immune system to identify and neutralize foreign objects such as bacteria and viruses

A typical diagram of an antibody and its functional parts. image: Fvasconcellos /wikipedia

An antibody recognizes a unique part of the foreign target, called an antigen. Each tip of the "Y" of an antibody contains a paratope (a structure analogous to a lock) that is specific for one particular epitope (similarly analogous to a key) on an antigen, allowing these two structures to bind together with precision.


A molecular diagram of an antibody shows a more complex structure. image: National Library of Medicine/ wikimedia commons

Using this binding mechanism, an antibody can tag a microbe or an infected cell for attack by other parts of the immune system, or can neutralize its target directly (for example, by blocking a part of a microbe that is essential for its invasion and survival). The production of antibodies is the main function of the humoral immune system.