Immunoglobulins: what are they, types, characteristics and functions

According to the Global Study of the Burden of Disease, 95% of the world population has some health problem, at least in the sample group analyzed between 1990 and 2013. This is not surprising, since 15% of the world's biomass in the form of carbon is made up of bacteria (70 gigatons), some of them beneficial to humans, others for diners and others directly pathogens.

Beyond bacteria, there are thousands of non-living infectious agents in the form of viruses, which mutate at a frantic rate and evolve to circumvent the long-term immunity of organisms. Human competition with pathogens is a veritable arms race: when a response develops specific to a pathogen, it is expected that it ends up mutating to stop being recognized by lymphocytes and other bodies specific.

For this reason, influenza vaccination campaigns are annual, while other vaccines provide lifelong immunity to a given pathogen. Depending on the mutation rate and adaptability of the organism, the chances of infection may increase or decrease over time. Based on these interesting premises, we tell you everything you need to know about

immunoglobulins.

• Related article: "Immune system: what is it, parts, functions and characteristics"

What are immunoglobulins?

According to the National Cancer Institute (NIH), an immunoglobulin or antibody is a protein made by plasma cells (types of white blood cells) in response to the presence of an antigen, a substance that causes the human immune system to activate, when it is recognized as a threat. The key to understanding immunity is based on the antibody (Ig) - antigen (Ag) dyad or what is the same, Ig-Ag.

Each immunoglobulin binds to a single antigen, allowing specialized killing immune cells (such as macrophages) to more effectively recognize and engulf the pathogen, but some of them can also destroy the antigen directly. Each antibody has a specialized paratope or antigen binding site, which is specific to the epitope of the antigen itself. In other words, each Ig-Ag complex involves a non-transferable lock and key.

The clearest utility of immunoglobulins in general society is, without a doubt, the development of vaccines. When a weakened virus or bacteria is introduced into the body (or a section of it that promotes a response immune), the proliferation of lymphocytes and the release of specific immunoglobulins for said antigen. A) Yes, the body "learns" which is the dangerous microorganism, always from the safety of a previous pathogenic inactivation.

Thanks to this safe immunization mechanism, it is estimated that more than 37 million lives have been saved around the globe in the last 20 years, especially in children. A clear example of this is smallpox: in the 18th century, 400,000 people died annually from this disease, resulting in a fatality rate of the agent of almost 30%. Thanks to vaccination, the last case of smallpox was diagnosed in 1977, and the WHO declared the world free of the pathogen in the 1980s. Undoubtedly, the knowledge of immunoglobulins has allowed us to free ourselves as a species of epidemiological ravages.

Structure of these proteins

Immunoglobulins have a typical “Y” shape, made up of two different halves.. You must picture this conformation clearly in your mind before continuing, as we are going to rely on this pattern to describe the general conformation of antibodies.

Like all protein, an immunoglobulin has the amino acid as its basal unit, each of the subunits that, joined by peptide bonds, gives rise to peptides (less than 10 amino acids), polypeptides (more than 10) and proteins (many concatenated amino acids). In this case, the standard immunoglobulin is composed of 4 polypeptide units: two equal heavy chains (Heavy, in the base and cleavage of the "Y") and 2 light chains identical to each other (Light, each of the lateral tips of the branches of the "Y").

Each "H" region is composed of a variable region (VH) and 3-4 constant regions (CH1, CH2, CH3, etc.). On the other hand, "L" light chains are made up of a variable region (VL) and a constant region (CL). All this may sound very confusing, but you only need to stick with the following concept: the ends of the heavy chains (H) and light (L) are variable, while the general conformation of “Y” is constant between immunoglobulins of the same type.

The “Y” shape is the typical one that is exhibited in biology and immunology classes, but not the only one. This monomeric form encompasses immunoglobulins D, E, and G, while Ig A is a dimer and Ig M is a pentamer. As you can imagine these anatomical changes also imply a clear variability in functionality.

• You may be interested in: "The 4 types of pathogens (and their characteristics)"

The types of immunoglobulins

We leave the molecular forest to return to a little more general themes, this time, the different functions of immunoglobulins according to their designation. We describe them briefly.

1. Immunoglobulin A

It is found in the mucous linings of the respiratory tract, urogenital tract, and lumen of the digestive system, in addition to saliva, tears, and breast milk. Curiously, in blood it is found in a monomeric form (like the "Y" described above), but in mucosa its arrangement is dimeric.

Due to their proximity to the only open systems within the human body (excretory, respiratory, and digestive), these immunoglobulins are the first to come into contact with viruses that invade the oropharyngeal cavity and other intestinal microorganisms.

2. Immunoglobulin G

This immunoglobulin is the one that is most represented in the blood, the cerebrospinal fluid and peritoneal fluid (from the abdominal cavity). Constitutes 80% of total immunoglobulins, so without a doubt it is the predominant one.

Furthermore, it should be noted that there are 4 subvariants of this type of immunoglobulin, from IgG 1 to IgG4. Each of them are especially skilled on a specific front, detecting antigens and toxins from different microorganisms.

3. Immunoglobulin M

It is expressed on the surface of B lymphocytes, the main effectors of the humoral response of the adaptive immune system.

They are the contingency response to an infection, since they eliminate pathogens in early stages until the immune system synthesizes enough IgG-type s. They make up 6% of circulating immunoglobulins in the human bloodstream and are present in the vast majority of animals, hence they are considered the oldest antibodies in evolutionary history in vertebrates.

4. Immunoglobulin E

The antibody that is clinically related to allergic conditions. Normally, this immunoglobulin is found in small amounts circulating in the blood, but increases dramatically when the body is exposed to an allergen, or what is the same, a harmless substance that causes an unjustified reaction in the immune system of the individual. It is also expressed in atypical amounts in parasitic infections.

5. Immunoglobulin D

This is one of the immunoglobulins that is least expressed, but no less important for that. It only represents 1% of the total immunoglobulins in the body and It is the largest component of the surface of many type B lymphocytes in their maturation stage. Due to its scarcity, its function is less defined than that of the rest of the variants already described.

Summary

As you may have seen, immunoglobulins come in various forms (isotypes) and arrangements morphological, but all of them have a very clear function: to protect the body from possible infections and pathogens. From viruses to morphologically more complex parasites (such as helminths), immunoglobulins are able to recognize them, activate the rest of the immune cells, mark them based on their surface antigens and, after the relevant cascade reaction, remove them.

In summary, immunoglobulins are proteins secreted by B lymphocytes and plasma cells in response to an antigen that has infiltrated the host's body. From immune responses to allergic reactions, antibodies have a variety of protective functions.