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Immunoglobulin Types and Their Functions

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Immunoglobulin

Immunoglobulin Types and Their Functions

Immunoglobulins were initially called neutralizing substances, and they were discovered in 1890 by two immunologists, Emil von Behring and Kitasato Shibasaburo. Immunoglobulins are special types of glycoproteins that play an important role in maintaining the body’s immune system. Also called antibodies, immunoglobulins are highly specific, and they are secreted in massive amounts by plasma cells in response to pathogenic antigens.

 

What Are the Functions of Immunoglobulins?

Immunoglobulins protect us against:

  • Bacteria 
  • Chemical substances
  • Viruses
  • Parasites
  • Allergens 
  • Synthetic substances 
  • Cancerous cells 
  • Fungus
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Plasma cells produce specific antibodies against a specific antigen. 

For instance, antibodies released by plasma cells against the COVID-19 virus will only bind to the COVID-19 virus and not any other virus or bacteria. This specificity of immunoglobulins makes the body’s immune system stronger and more active against multiple pathogenic invasions.

Immunoglobulins (Ig) primarily constitute 20% of blood plasma.

Body fluids that contain immunoglobulins:

  • Breast milk
  • Tears
  • Saliva
  • Gastric secretions
  • Mucosal secretions

The primary function of immunoglobulins is to elicit humoral immunity by binding to the foreign antigen. The antibody-mediated humoral immune response kills the invading microbes and prevents infections from spreading to other regions of the body.

This type of immune response is highly specific and complex. There are five different classes of immunoglobulins (IgG, IgM, IgA, IgD, and IgE) based on their functions, chemical structure, biological features, distribution, and target specificity.

 

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Any genetic abnormality during antibody processing, or any variation in the normal immunoglobulin levels in the body increases the risks of developing various immunological diseases such as primary immune deficiency diseases or autoimmune diseases (a condition in which immune cells attack the healthy cells/tissues of the body).

For the past decade, medical researchers have been using gammaglobulins (a major type of immunoglobulin) as immunotherapy for the treatment of multiple viral infections, bacterial infections, and autoimmune disorders associated with humoral immune deficiency.

 

Basic Structure of Immunoglobulins

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Immunoglobulins are Y-shaped globular proteins that are about 10nm in size and 150kDa in molecular weight.

All immunoglobulins consist of four common polypeptide subunits: 

  • Two identical heavy chains.
  • Two identical light chains.

The polypeptide chains are held together with a disulfide bridge and form a Y-shaped structure. 

This Y-shaped structure is divided into two regions:

  • The bottom part is called a constant region.
  • The upper, two-arm part is called a variable region.

As the name implies, the constant region does not change from one antibody to another, but the variable region does.

Furthermore, the variable region of the antibody binds to specific antigens and is called the fragment of antigen-binding (Fab) site. 

The region of the antigen that binds to the antigen-binding site is called an epitope.

The constant region is typically the tail of the antibody called the fragment of crystallization (Fc). It binds to the surface of immune cells or interacts with receptor proteins of the complement system.

All antibodies exhibit dual functionality: binding to viruses or bacteria and mediating biological immune responses such as complement system activation, opsonization, and neutralization of microbial toxins and viruses.

 

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Immunoglobulins Mode of Action

Immunoglobulins protect the body’s immune system by regulating the multiple cell destruction pathways, depending on the type of signals they received or the pathogens they interacted with. 

Immunoglobulins are generally present either on the surface of B-cells or circulating freely in the body fluids. The binding of the antibody to its specific antigen results in the formation of an antibody-antigen complex which elicits an immune response. 

The immunoglobulins kill off the foreign invaders through three different types of mechanisms: neutralization, opsonization, and complement activation.

Neutralization

The neutralization mechanism focuses on inactivating pathogens. When the antibody binds to the functional part of the virus or bacteria, it inactivates the virus’s ability to bind onto the surface of healthy cells. As a result, the biological effect of the toxins released by bacteria and viruses is neutralized.

Opsonization

In opsonization, antibodies recognize and label the infected or cancerous cells. The labeled cells are then digested and removed by phagocytic cells such as macrophages or cytotoxic T-cells. In short, the opsonization mechanism enhances phagocytosis.

Activation of the Complement System

The binding of immunoglobulins to pathogens leads to the activation of complement proteins. Upon activation, these proteins build a membrane attack complex (MAC). The MAC proteins then cause cell lysis by punching holes in the cell membrane of the bacteria. Only IgG and IgM antibodies can activate complement pathways.

immunoglobulin types and their functions

 

Immunoglobulin Types and Their Functions

The human body only makes five types of immunoglobulins: 

  • IgG
  • IgM
  • IgA
  • IgD
  • IgE

These immunoglobulins protect the body from multiple types of antigens. All antibodies are different based on their amino acid sequence in the constant region, structure (monomer, pentamer, and dimer), short-life in the blood, site, and immunological properties.

 

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Immunoglobulins G (IgG)

  • Immunoglobulin G (IgG) is the most common and abundant antibody present in the body. 
  • Blood plasma consists of 75-80% of IgG antibodies.
  • Of all antibodies, IgG has the longest lifespan of about 23 days.
  • IgG is the only antibody that can cross the placental barrier and provide passive immunity to a developing fetus.

IgG antibodies remember the pathogens that have previously entered the body and caused an infection. IgG also provides some immunity to infants when ingested through breast milk. 

The function of IgG is to enhance the phagocytosis of pathogens, neutralize bacterial or viral toxins, and trigger the activation of the complement system. 

IgG has four isotypes:

  • IgG1
  • IgG2
  • IgG3
  • IgG4

Immunoglobulin M (IgM)

Immunoglobulin M (IgM) is the first antibody that interacts with new bacteria that enter the body, and it initiates a primary immune response. 

IgM is also called a natural antibody because it serves as the first line of defense of the immune system and provides short-term protection. 

IgM has a gigantic pentamer structure above all other antibodies and consists of 10 antigen-binding sites, making them more effective than IgG in killing bacteria or viruses. 

The life span of the IgM antibody in our body is about five days, and it makes up 5% – 10% of the antibodies in blood plasma.

IgM also causes agglutination (formation of clumps) of bacteria when binding to its surface epitope. The IgM antibody is known as a potent agglutinin and is also found on the surface of naïve B-cells and red blood cells in its monomeric form.

Immunoglobulins A (IgA)

Immunoglobulin A (IgA) is the most common antibody after IgG. IgA is present in the blood, lymph, and other body secretions such as saliva, tears, and milk. It can also be found in the genital lining, respiratory tract, and intestine lining. 

IgA antibodies protect the body from bacterial growth and colonization. However, it is less stable than IgG and can be found in a lower quantity, accounting for about 10% – 15% of the total immunoglobulins in the blood. 

IgA is often called a secretory antibody because it has an attached secretory component that protects it from enzymatic digestion. Every day, a human secretes about 5 g to 15 g of secretory IgA into mucous secretions to prevent pathogenesis.

Immunoglobulins D (IgD)

Immunoglobulin D (IgD) makes up less than 0.5% of serum antibodies. IgD is found in lesser amounts in lymphatic fluids and blood. The function of IgD is still unknown, but it is present on the surface of immature B-cells as receptors, and it is also a part of the innate immune system. Researchers believe that IgD regulates the B-cells’ activation and differentiation into plasma cells.

 

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Immunoglobulins E (IgE)

Immunoglobulin E (IgE) makes up less than 0.01% of serum antibodies. Even though it is present in the lowest amount in the blood and has a shorter lifespan, it is still a potent antibody

IgE is the most effective antibody against parasitic infections, including helminths

A high level of IgE in the blood can sometimes cause hypersensitivity toward non-harmful substances such as pollen or dust particles. Recently, immunologists have begun formulating new anti-IgE antibodies for the treatment of allergy and asthma.

which type of immunoglobulin is present in tears, saliva, and breast milk?

Which Type of Immunoglobulin Is Present in Tears, Saliva, and Breast Milk?

Antibodies that can be found in breast milk are IgA, IgE, IgM, IgD, and IgG, with the highest levels of IgA found in colostrum.  Up to 50% of proteins in breast milk colostrum are due to the presence of IgA. IgA can also be found in tears and saliva.

IgA antibodies have two sub-classes: IgA1 and IgA2. Each has its own biological properties.

 

Diseases Related to Deficiency of Immunoglobulins

Immunologists believe that a person could have an immunoglobulin deficiency or non-functional immunoglobulins due to a defective immune system. This is a rare genetic condition known as primary immunodeficiency disease (PIDD). 

There are more than 300 types of primary immunodeficiency diseases that have already been reported by medical researchers. 

On the other hand, some secondary immunodeficiency disorders are caused by external environmental factors such as:

  • Severe malnutrition
  • HIV
  • Chemotherapeutic agents

A person suffering from primary immunodeficiency diseases is likely to have a weaker immune system and is more prone to infections and other severe immunological problems.

 

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Immunoglobulins Blood Test and Their Indications

If an individual is suffering from common infections and recurrent diseases at repeated intervals, there is a chance that he or she has a weakened immune system and has lost the immunity needed to fight off diseases.  

In these instances, medical professionals highly recommend an immunoglobulin blood test. 

An immunoglobulin blood test is conducted by taking a small blood sample from the patient’s body to estimate the level of specific antibodies such as IgA, IgG, and IgM.

Immunoglobulin tests help diagnose various health problems that are affecting the level of immunoglobulins in the body.

The antibody results are measured by comparing them with the normal immunoglobulin range. If the levels of IgA, IgG, and IgM are too high or low, there is an increased risk of severe health problems.

High IgG levels may indicate the presence of:

  • HIV/AIDS
  • Long-term hepatitis
  • Multiple sclerosis
  • Multiple myeloma

Low IgG levels may indicate the presence of:

  • X-linked agammaglobulinemia disease
  • Leukemia
  • Nephritic syndrome

The causes of IgG deficiency are still unknown, but deficiency could occur due to genetic defects.

High IgA levels indicate the presence of autoimmune disorders such as: 

  • Chronic hepatitis
  • Systemic lupus erythematosus (Sle)
  • Cirrhosis
  • Multiple myeloma
  • Rheumatoid arthritis

Low IgA levels elevate the risk of:

  • Autoimmune diseases
  • Enteropathy
  • Kidney damage

Some people are born with an IgA deficiency or a defective gene that produces too little IgA in the body.

High IgM levels may indicate the presence of:

  • Early viral hepatitis
  • Kidney damage
  • Rheumatoid arthritis

IgM serves as the first line of defense, and normally, increases in IgM levels in the blood indicate that the body has been exposed to new infections. 

People suffering from inherited immune diseases, leukemia, or multiple myeloma are more likely to have low levels of IgM in their blood. 

Common symptoms of IgA, IgM, and IgG deficiency:

  • Bronchitis
  • Pneumonia
  • Respiratory infections
  • Gastrointestinal infections
  • Frequent fever
  • Skin allergies
  • Sinus and lung infections
  • Unexplained weight loss
  • Diarrhea

Related conditions:

  • Kawasaki disease
  • Organ transplantation
  • Severe combined immunodeficiency disorders (SCID)
  • And many others

 Early symptoms and diseases can vary from person to person.

 

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Clinical Use Of ImmunoglobulinsClinical Use of Immunoglobulins

Immunoglobulins have always been recognized as the most effective immunotherapy to treat infectious and primary immunodeficiency disorders. 

Immunoglobulin replacement therapy was first introduced in 1952 to treat X-linked agammaglobulinemia (a genetically inherited condition). Over the past three decades, it has become the standard method for treating PIDD.

The primary goal of Ig replacement therapy is to provide strong immunity, stop invasive bacterial infections, and enhance the survival rate. Recent advancements in Ig replacement therapy have substantially improved the quality of life for patients with PI disorders.

 

Route of IVIG Administration

There are two methods for administering Ig replacement therapy: subcutaneously (SCIG) or intravenously (IVIG). 

The patient’s preference, previous medical history, IV access, cost, availability, and tolerability are all essential factors that are considered during the selection of the administration route.

Subcutaneous Immunoglobulin Therapy (SCIG)

SCIG injection is given into the subcutaneous layer of the skin. It is injected in small doses using a syringe at different sites of the body, such as the abdomen, thigh, or upper arm.  

The volume of SCIG injection is between 10 ml – 25 ml, in repeated intervals of once or twice a week.  

SCIG injections take between 1 to 2 hours and can be self-administrated at home.

Choosing the correct size and number of needles for this therapy is important to ensure a successful injection.

Intravenous Immunoglobulin Therapy (IVIG)

IVIG is directly administered through a vein in larger volumes and takes approximately 2 to 6 hours to complete. 

On average, repeated doses are given every 3 to 4 weeks. 

The dose and frequency of IVIG depend on the patient’s current body weight and disease state. The estimated starting dosage is 2 g/kg/month divided over 4 to 5 days.

 

Uses of Immunoglobulin Therapy

 Immunoglobulin therapy is used to treat various conditions, which include the following:

 

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Common and Adverse Side Effects

Common Side Effects:

  • Headache
  • Body ache
  • Nausea
  • Diarrhea
  • Muscle
  • Cramps

Adverse Side Effects:

  • Allergic Reactions
    • Hives
    • Swelling in mouth
    • Wheezing       
  • Kidney problems
  • Blood clots
  • Aseptic meningitis syndrome 
  • Hemolysis (destruction of red blood cells)
  • Transfusion-related acute lung injury

Side effects experienced by patients are often related to fast infusion rates and product temperatures.

To prevent or alleviate these side effects:

  • Patients should be fully hydrated before receiving an infusion.
  • The product should be given at room temperature.
  • Certain premedications such as diphenhydramine, acetaminophen, corticosteroids, or non-steroidal anti-inflammatory drugs should be used.
  • The IVIG infusion rate should be titrated up in 30-minute increments. 
  • A nurse should monitor vital signs.

 

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REFERENCES:

  1.     Schroeder Jr, H. W., & Cavacini, L. (2010). Structure and function of immunoglobulins. Journal of Allergy and Clinical Immunology, 125(2), S41-S52.
  2.     Janeway Jr, C. A., Travers, P., Walport, M., & Shlomchik, M. J. (2001). The structure of a typical antibody molecule. In Immunobiology: The Immune System in Health and Disease. 5th edition. Garland Science.
  3.     Vaillant, A. A. J. (2021). Immunoglobulin. StatPearls [Internet]. https://www.ncbi.nlm.nih.gov/books/NBK513460/.
  4.     Introduction to Immunoglobulins. Thermo Fisher Scientific – US. (n.d.). https://www.thermofisher.com/pk/en/home/life-science/antibodies/antibodies-learning-center/antibodies-resource-library/antibody-methods/introduction-immunoglobulins.html
  5.     Encyclopædia Britannica, inc. (n.d.). Classes of immunoglobulins. Encyclopædia Britannica. https://www.britannica.com/science/immune-system/Classes-of-immunoglobulins#ref215568.
  6.     Intravenous Immunoglobulin (IVIG). rheumatology.org. (n.d.). https://www.rheumatology.org/I-Am-A/Patient-Caregiver/Treatments/Intravenous-Immunoglobulin-IVIG.
  7.     Jolles, S., Sewell, W. A. C., & Misbah, S. A. (2005). Clinical uses of intravenous immunoglobulin. Clinical and experimental immunology, 142(1), 1.
  8.     Sriaroon, P., & Ballow, M. (2015). Immunoglobulin replacement therapy for primary immunodeficiency. Immunology and Allergy Clinics, 35(4), 713-730.
  9.     http://www.igliving.com/magazine/articles/IGL_2009-10_AR_Choosing-Needles-for-IVIG-and-SCIG.pdf

 

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