Serology is the scientific study of serum and other bodily fluids. The term usually refers to the diagnostic identification of antibodies in the serum. Such antibodies are typically formed in response to an infection (against a given microorganism), foreign proteins (in response to mismatched blood transfusion), or to one’s own proteins (autoimmune disease). Serological tests may be performed for diagnostic purposes when an infection is suspected, in rheumatic illnesses, and in many other situations, such as checking an individual’s blood type. Serology blood tests help to diagnose patients with certain immune deficiencies associated with the lack of antibodies. In such cases, tests for antibodies will be consistently negative.
Serum levels of the antibody IgM is used as early infection diagnostics, while IgG levels indicate immune status of infection. Low or absent IgA levels in the serum indicate immune deficiency. Subsequently, increased levels of specific IgE reactivity indicate allergy to the specific allergen Immunoglobulin G (IgG)
IgG antibodies are generated following class switching and maturation as part of the adaptive immune response. IgG is secreted as a monomer, small enough in size to allow easy perfusion into tissues. It is the only isotype with receptors to facilitate passage through the human placenta, thus providing immunity to the foetus. Along with IgA secreted in the breast milk, residual IgG absorbed through the placenta provides the neonate with protection before its own immune system develops. There are four IgG subclasses (IgG1, 2, 3 and 4) in humans, named in order of their abundance in serum (IgG1 being the most abundant). Clinically, measured IgG antibody levels are generally considered to be indicative of immune status to certain pathogens. A common example of this practice are titers drawn to demonstrate serologic immunity to measles, mumps, and rubella (MMR), hepatitis B virus, and varicella (chickenpox).
Immunoglobulin A (IgA)
IgA is an antibody that plays a critical role in the immune function of mucous membranes. IgA has two subclasses (IgA1 and IgA2) and can exist in a dimeric form called secretory IgA (sIgA). This IgA is produced by plasma cells in the lamina propria adjacent to mucosal surfaces. After binding to its intracellular receptor, it gets processed, leaving part of the receptor attached to the dimer before it is secreted. This “secretory component” is highly resistant to proteolysis inside the lumen of the gut. It later becomes part of the mucus layer to form a barrier to external threats. In its secretory form, IgA is the main immunoglobulin found in mucous secretions. This includes tears, saliva, colostrum and secretions from the genito-urinary tract, gastrointestinal tract, prostate and respiratory epithelium. It is also found in small amounts in the blood.
Decreased or absent IgA due to an inherited inability to produce IgA is termed selective IgA deficiency. The deficiency can lead to a clinically significant immunodeficiency when left untreated.
Immunoglobulin E (IgE)
Immunoglobulin E (IgE) is a class of antibody that has only been found in mammals. It plays an important role in the defence against parasites in type I hypersensitivity (allergic diseases), and in responses to allergens like anaphylactic drugs, bee stings, and antigen preparations used in desensitization immunotherapy. IgE synthesis pathway, and the IgE-mediated allergic/inflammatory pathways are all important targets in the intervention of pathological processes like allergy, asthma, and other IgE-mediated diseases.
Allergy diagnosis is most often done by reviewing a person’s medical history and finding a positive result for the presence of allergen specific IgE when conducting a skin or blood test. Specific IgE testing is the proven test for allergy detection.
Immunoglobulin M (IgM)
IgM is the first antibody to appear in response to initial infection as part of the adaptive immune response. IgM forms polymers where immunoglobulins are covalently linked together mostly as a pentamer. Because IgM is a large molecule, it cannot diffuse well, and is therefore primarily found in serum. This makes IgM useful as an early diagnostic serum marker for infectious disease.
Because IgM antibodies do not pass across the human placenta (only isotype IgG does), its presence in neonate’s serum indicates intrauterine infection (e.g. congenital rubella syndrome).