Antibody
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An antibody or immunoglobulin is a large Y-shaped protein used by the immune system to fight foreign objects like bacteria and viruses.
Each antibody recognizes a specific antigen unique to its target. This is because at the two tips of its "Y", have structures that act like locks. Every lock only has one key--in this case, its antigen. When the "key" is inserted into the "lock", the antibody attaches. When the antigen is on a microbe or an infected cell, this can "tag" the cell, marking it for attack by other parts of the immune system. Also, it can directly fight its target (i.e. blocking a part of the microbe that is essential for its invasion and survival).
The production of antibodies is the main function of the humoral immune system.
The terms "antibody" and "immunoglobulin" are often used for the antibody. They are found in the blood and tissue fluids as well as many secretions. Structurally, they are globulins.
Antibodies are made by plasma cells that are derived from the B cells of the immune system. B cells are activated when they bind to their specific antigen. This causes them to differentiate into plasma cells. In some cases, the interaction of the B cell with a T helper cell is also necessary.
