Introduction to Serology Markers

Georgina Grimsey

Nov 22, 2019

What Are Serology Markers?

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), against other foreign proteins (in response,
for example, to a mismatched blood transfusion), or to one’s own proteins (in
instances of 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, and 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),
among others.

Immunoglobulin
A (IgA)

IgA is an antibody which 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 sIgA 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 when it 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, including
tears, saliva, colostrum and secretions from the genito-urinary tract,
gastrointestinal tract, prostate and respiratory epithelium. It is also found
in small amounts in blood.

Decreased or absent IgA due to an inherited inability to
produce IgA is termed selective IgA deficiency and can produce a clinically
significant immunodeficiency.

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, its 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.

Diagnosis of allergy 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
(exposure to antigen) 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).

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