An antibody is a Y-shaped protein with each fork of the Y specifically shaped (a paratope) to lock onto a specific antigen (an epitope, equivalent to a key). This binding allows an antibody to tag a microbe or infected cell for other immune system actors to kill, or, alternately, neutralize the target, by blocking a part of a microbe essential for survival or invasion. Each B cell is programmed to make one, and only one, specific antibody.
Antibodies are manufactured by plasma cells derived from B lymphocytes that have transformed. T cells don’t make antibodies, but they help B cells do so.
A single B cell can produce more than 10 million antibodies in an hour and can make variations capable of tagging a remembered antigen that may have varied somewhat.
Manufactured antibodies are secreted: circulating in search of the antigen that triggered their formation. Antibodies work their way out of the bloodstream, settling in among other cells throughout the body, such as the digestive tract, where they tag swallowed germs. Antibodies are plentiful in saliva, and in mother’s milk, as well as other bodily fluid secretions.