Serological tests are in vitro tests designed to detect the presence of antibodies in serum (antibody detection tests), or the presence of antigen using antibodies as detection reagents (antigen detection tests). In some cases the mixing of antibody and antigen gives rise to a visible reaction, in other cases some other secondary mechanism is needed to detect whether a reaction has taken place.  The structure of antibodies and antigens and how they interact is addressed in a separate sub-module.  Some of the commonly used serological tests are briefly described in this sub-module. It is intended as an introduction to the subject for those that have little knowledge of serology.

Serology involves the detection of antibodies in serum, but in practice antibodies can be detected in various body fluids such as milk, cerebrospinal fluid, ascitic fluid, saliva, tears and mucus secretions. Serological tests are performed for various reasons, some of the most important of which are:

• The diagnosis of a recent infection with a particular micro-organism or recent exposure to specific antigens, for example drug residues in food animals
• Determination of the immune status of an individual animal or a group of animals
• Sero-epidemiological surveys
• Testing in-contact animals
• Screening animals before introduction into a household/farm/country
• Pre-mating screens
• Determination of the carrier status of an animal
• Detection of the spread of disease

The following topics will be covered in this sub-module:

• The value and shortcomings of serology as a diagnostic tool and aid in disease surveillance
• Different serological test procedures
• General requirements for setting up serological tests