The adherence together of small bodies in a ﬂuid. Thus, blood corpuscles agglutinate into heaps (rouleaux) when added to the serum of a person belonging to an incompatible blood group. Bacteria agglutinate into clumps and die when exposed to the presence of antibodies in the blood. This is important in regard to diagnosis of certain diseases due to bacteria. In typhoid fever (see ENTERIC FEVER), for example, the blood of an animal is immunised against typhoid bacilli by repeated injections of these.
The blood serum of the animal, known now as anti-typhoid serum, is issued to laboratories for use when bacilli are found in the excretions of a patient who is possibly suﬀering from typhoid fever. The bacilli are exposed to the action of a drop of the serum; if the serum shows the power of agglutinating these bacteria, this forms evidence that the bacteria in question are typhoid bacilli. The reaction may also be carried out in the contrary manner: that is to say, the serum from the blood of a patient who may be suﬀering from typhoid fever, but in whom the diagnosis is still doubtful, is added to a drop of ﬂuid containing typhoid bacilli; if these are agglutinated into clumps by the patient’s serum, the patient is then known to be suﬀering from typhoid fever. If they do not agglutinate, the symptoms are due to some other condition. This reaction for typhoid fever is known as the Widal reaction. Comparable agglutination reactions, using an appropriate serum, are used in the diagnosis of a number of diseases, including glandular fever (when it is known as the Paul-Bunnell reaction), typhus fever (when it is known as the Weil-Felix reaction), undulant fever, and Weil’s disease. (For more information about these diseases, see under separate entries.)