There is a wide variety of diagnostic tests available to practitioners for infectious and immune-mediated diseases. Many of these tests detect antibodies, which are produced by the immune system in response to stimulation. However, misunderstandings and controversies sometimes arise when antibody tests are interpreted. Antigen tests would seem more straightforward but also may cause confusion. The following is a review of how the immune system reacts to potential pathogens along with information on the use (and misuse) of some of the diagnostic tests.

Immune system function

To protect against microbial invasion, animals have three basic defenses: physical barriers, innate immunity, and acquired immunity. Barriers include the skin, self-cleaning processes such as coughing and urination, and normal flora. Innate immunity is the second layer of defense, and is a quick response system for destroying invaders that are recognized as foreign. Inflammation, defensins, and lysozymes are features of innate immunity and are mediated by macrophages, dendritic cells, and neutrophils. Acquired (specific) immunity is the "smart" system that can recognize and remember invaders. It has a slower onset than the innate system but is effective for a longer period of time due to immunologic "memory". The acquired immune system can be divided into humoral (antibodies in body fluids that destroy pathogens) and cell-mediated (specialized immune cells that seek out and destroy other infected cells). Diagnostic immunology generally focuses on the antibody response to infection.

Lymphocytes originate from stem cells and are processed in primary lymphoid organs. Lymphocytes mature into cells with specific functions: B cells, T cells, and NK (natural killer) cells. B cells are found in the spleen, bone marrow, Peyer's patches (lymphoid organs in the wall of the small intestine), and mainly in lymph nodes. Very few B cells circulate in the bloodstream. Each B cell has a large number of identical receptors and can only bind and respond to a single antigen (Ag). When the receptors bind an antigen, the B cell produces antibodies (Ab) specifically to bind and help destroy the invader. Co-stimulation of B cells leads to maturation into plasma cells, which are simply B cells terminally differentiated into Ab "factories".

Antibodies are proteins secreted by the plasma cells. They are classified as immunoglobulins, and each plasma cell can manufacture and release up to 10,000 molecules per second. The lifespan of the cells varies from a few days to over a year, but even after an Ab-secreting plasma cell dies the antibodies persist until slowly catabolized. Long-lived plasma cells are a form of "memory" cells and are found in lymph nodes and the bone marrow. If the animal is exposed to the same Ag, a rapid secondary immune response occurs with a shorter lag period due to memory. This explains why antibodies can be detected many years after an infection or vaccination, and theoretically some memory cells persist for the life of the animal.

Antibodies are classified into 5 classes (isotypes): IgG, IgM, IgA, IgE, and IgD. IgG plays the major role and is found in the highest concentrations in blood. IgM has the second highest concentration and is secreted first (primary immune response) by plasma cells when Ag is recognized. IgA is secreted at epithelial surfaces and helps with local immunity in the respiratory, GI, urinary, and other body systems. IgE is primarily bound to mast cells and basophils and only circulates in very low concentrations. IgD has been identified in some but not all mammals. For diagnostic purposes, it is important to remember that IgM is secreted early in an immune response (days to weeks) and then wanes, while IgG is slower to be produced (usually several weeks) and persists much longer.