This section is from the book "A Manual Of Pathology", by Guthrie McConnell. Also available from Amazon: A Manual Of Pathology.
When bacteria or other cells are injected into the body there are formed within the serum definite substances, which when brought together with emulsions of the corresponding bacteria will cause the "clumping" or agglutination of the bacteria. If these are normally motile they will become less motile, and may lose that power entirely.
As the action is specific, it is commonly used for diagnostic purposes, especially in typhoid fever, when it is known as the "Widal reaction" (q. v.).