Immunoglobulins - Antibodies

Introduction to Immunoglobulins: Antibodies

What are?Although the term may sound strange immunoglobulins (Ig) is nothing more than a synonym for antibodies.

These are, essentially, globular proteins involved in immune response: their function consists in binding and neutralizing substances potentially harmful to the organism (called antigens) and counteract the attack of microorganisms considered foreign.

Immunoglobulins are sometimes called gamma globulins due to their particular Y-shaped conformation.

What are Immunoglobulins (Ig)?

Immunoglobulins are globular proteins antibody activityvery important for the correct functioning of our immune system.

These are made up of four polypeptide subunits (two heavy chains and two light chains), linked together through a disulfide bond. The heavy chains are of five main types which correspond to as many classes of Ig (A, D, E, G and M), some of which (A and G) can be divided into subclasses.

Immunoglobulins are produced by B lymphocytes in response to one antigenic stimulus external and/or internal. In practice, they behave similarly to sentinels, ready to alert the activated lymphocytes (plasma cells) to produce a very high number of antibodies (up to 2,000 per second), once they come into contact with the attacker.

Within the immune system, immunoglobulins constitute the main component of thehumoral immunitytherefore they have the function of neutralizing foreign substances, recognizing each antigenic determinant as a target, and making them easily identifiable by phagocytes and cytotoxic cells.

Synthesis and Mechanism of ActionImmunoglobulins are secreted by mature B lymphocytes, which house them in their cell membrane (more than 100,000 for each B lymphocyte). At this level they act like antennas, or rather as specific membrane receptors which activate the lymphocyte upon contact with the antigen; a part of the activated lymphocytes is stimulated to reproduce, differentiating into plasma cells capable of synthesizing and secreting an impressive number of new antibodies (up to 2,000 immunoglobulins per second).

The antibodies released by the plasma cells, soluble in the plasma, do not directly destroy the foreign host, but bind to it to make it more visible and susceptible to the action of the other actors of the immune system (phagocytes and cytotoxic cells).

To better understand

Let’s imagine a square crowded with people, among which a certain number of criminals (antigens) are hiding; some gendarmes present in the crowd (immunoglobulins) are able to distinguish ordinary citizens from criminals; as soon as they recognize one, some agents are activated (plasma cells) and begin to fire thousands of special colored cartridges (antibodies), which only hit the bad guys; at the same time the gendarmes alert another group of police forces (phagocytes and cytotoxic cells), who – having arrived en masse on the spot – manage to recognize and arrest the criminals thanks to the colored stains on their clothes.

Antibody memory

When an antigen attacks the body for the first time, immunoglobulins take some time to realize its danger.

However, after the foreign has been eradicated, cells remain in the bloodstream – so-called “memory” – which retain the ability to promptly recognize the antigen should it recur, producing a faster and stronger response; it is precisely on this principle that vaccinations are based.

Types of AntibodiesImmunoglobulins are divided into four general classes: IgG, IgA, IgE, IgM and IgD.

Immunoglobulins A

The immunoglobulin A (IgA) are a group of antibodies present mainly in external secretions, such as saliva, tears, genitourinary secretions, intestinal and bronchial mucus, colostrum and breast milk. They represent an important means of defense against local infections, preventing colonization by pathogens.

Immunoglobulins D

The role of immunoglobulins D (IgD) has not yet been fully elucidated, but is known to be important as an antigen receptor on B cells.

Immunoglobulins E

The immunoglobulin E (IgE) are associated with allergic reactions; in fact, their binding with mast cell receptors causes the massive release of inflammatory mediators, first of all histamine. Immunoglobulin E is also extremely important in protecting against parasitic infestations.

Immunoglobulins G

The immunoglobulin G (IgG) represent approximately 75% of adult plasma antibodies and constitute the fulcrum of secondary immune responses (those that intervene in cases in which there has already been a previous encounter with the antigen). They have a particularly effective defensive action: they can neutralize various toxins, prevent viruses from colonizing cells and facilitate bacterial phagocytosis. During pregnancy, the mother transmits her own IgG to the fetus through the placental membrane, giving the newborn a certain immunity during the first 3-4 months of life.

Immunoglobulins M

The immunoglobulins M (IgM) are antibodies active against blood group antigens and are associated with the primary immune response (initial exposure to the foreign organism); they therefore have a low affinity and are the first to intervene upon contact with a new foreign organism. Mature B lymphocytes, which have never been exposed to an antigen, are known as “naïve lymphocytes” and express only the IgM isoform on their cell surface.

Blood testsIn certain cases, the doctor may prescribe a specific immunoglobulin test, to evaluate – for example – the levels of certain antibodies implicated in a certain disease. Immunoglobulin tests can be useful in determining the cause of liver damage and confirming the suspected diagnosis, or in highlighting the presence of particular auto-antibodies implicated in an autoimmune disease, in the diagnosis of certain types of cancer or allergies.

Preparations based on immunoglobulins can also be injected into the patient, to increase the amount of circulating antibodies, for prophylactic purposes (to prevent the onset of certain pathologies, such as hepatitis A), during the treatment of acute infections or in case of insufficiency antibody.


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