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By: - at May 29, 2013

Introduction to Viruses and the Immune System Response

illustration of virusThe word virus ignites fear in many people’s minds; they are capable of inflicting extensive cell damage, can tolerate extreme temperatures and much like an unwanted house guest, are very resistant to leaving the body. Viral infections can range from being a minor inconvenience to a potentially life threatening disaster. They are able to infect most living cells – belonging to animals, plants or bacteria. The virus has one aim; to invade. The battle between the virus and the immune system on a molecular level is often overlooked, and although very complex – it is a fascinating world.

An Introduction to Viruses

diagram of a virusOnly able to perform any metabolic activity inside living host cells, a virus cannot multiply unless in these conditions. It is for this reason that it is highly debated whether viruses should be considered ‘living’ or not. Viruses are microbes, consisting of only a nucleic acid core (DNA or RNA) surrounded by a protein coat known as a capsid. Viruses are the smallest known microbe, although sizes can vary drastically. A singular virus particle is known as a virion. They are generally between 10nm to 300nm, compared to the approximate size of bacteria being 1000nm. The protein coat surrounding the virus serves as a protective barrier of the genetic material, ensuring that it is safe from inactivation as a result of environmental factors. The capsid is composed of structural protein units known as capsomeres.

Many virus particles (though not all) are also surrounded by an envelope composed of lipoprotein, containing viral antigens. An antigen is a foreign substance that is often toxic in nature, promoting antibody production in the blood. In the event of a virus finding a suitable cell and deciding to replicate itself, it essentially ‘reprograms’ the cells synthesizing apparatus to instead produce virus components. It does this by transcription of the core nucleic acid into mRNA, which is virus specific messenger. This directs the cell to carry out the process of replicating the virus. The virus essentially takes hostage of the cell.

Anatomy of a Virus:
Anatomy of a Virus

There are four primary ways that a virus can affect a cell; death, transformation, latent infection and haemadsorption. Transformation, as the name suggests, changes the properties of the cell from normal to potentially cancerous or malignant. Latent infection means that though the virus is within a cell, there are no clearly noticeable effects on the cells activities and functions. Haemadsorption occurs when the outer protein coat of the virus adheres to erythrocytes (red blood cells), resulting in them clumping together.

An Introduction to the Immune System

Single Human Lymphocyte:
Lymphocytes - White blood cell Single Human Lymphocyte

To first understand how the immune system combats viral infection, it is essential to understand the basic function and components. The function of the complex immune system is to defend the body from potentially harmful invaders. An invader may be in the form of a bacterial, fungal, viral or parasitic infection. There are several well equipped components involved that are continually working together to eliminate the unwanted guest(s). Lymphocytes are a type of white blood cell that are heavily involved in immune system function, and a main component of adaptive immunity. This is when the Immune system adapts and evolves to become better equipped to defend itself against specific attackers. The main function of lymphocytes is to provide immune defense against invaders which they are specifically programmed to respond to.

How it Works
Despite what the name suggests, the immune system is rather more like a scattering of organs and tissue throughout the body – it is not contained to one area. Lymphocytes are present in the adenoids, tonsils, thymus, lymph nodes, spleen, appendix, lymphatic vessels and bone marrow. The origin of all the immune systems cells is the bone marrow, where some stem cells develop further into more specialized cells through a process called haematopoiesis.

Click to Enlarge - Haematopoiesis in Humans
Haematopoiesis in Humans

It is here that the cells ‘branch off’ into separate groups with different tasks within the immune system. Cells may develop into the aforementioned lymphocytes, or phagocytes, which are able to consume microorganisms. Lymphocyte cells are then separated again to become either B lymphocytes (which mature in the original bone marrow) or T lymphocytes, which travel to the Thymus before developing further. There is especially a heavy concentration of B and T cells in the spleen and lymph nodes. B cells are able to produce antibodies whereas T cells cannot; they instead directly attack their target. Both T and B cells possess receptors which respond only to specific antigen molecules, increasing their ability to successfully eliminate infection. B cells specifically operate by binding their antibodies to specific antigens, in order to effectively ‘neutralize’ their harmful effects.

Antibodies are able to attack or order other cells to attack on their behalf. When T cells have traveled to the thymus for maturity, they undergo a process which enables them to distinguish cells from the body they live in (self-cells) apart from potential invaders (nonself cells). During this time the T cells become one of several types, each with their own specialty and functions. There are memory T cells, helper T cells, regulatory T cells, suppressor T cells and cytotoxic T cells (otherwise known as natural killer cells). If a cell is destined to become a phagocyte instead, it will aid the immune system by consuming the target microorganism or virus, resulting in destruction of it entirely. Some examples include neutrophils, eosinophils, and macrophages.

formed elements of blood
By BruceBlaus via Wikimedia Commons

Phagocytes are white blood cells that ingest microorganisms, and so have a very important role in the immune systems constant fight for health. Neutrophils are the main component of pus, and they completely engulf and dispose of invading organisms through a process known as phagocytosis. Eosinophils are primarily involved in the host defense against parasites, whereas macrophages play a similar role to neutrophils but are more specialized in chronic inflammation. The aforementioned examples are all involved in the inflammatory response; this is when your immune system responds to wounds by sending phagocytes to the inflamed area to ingest the offending bacteria. This is an example of a non-specific immune response. The inflammatory response does not target specific invaders, but is part of the skins role as a ‘barrier’ in the immune system. This makes up some of the innate immune system; including skin and mucous membranes, nasal secretions and cilia present on the upper respiratory tract cells to defer bacteria. It is the first nonspecific line of defense to invasion. When this defense fails, other immune responses are activated.

Immune Response to Viral Infection

The other branch of immune response is known as adaptive immunity; this responds to specific invaders such as viruses. It is composed mainly of B and T cells, discussed previously in the introduction to the immune system. This type of response is separated again into two areas; B cells participate in what’s known as the humoral immune response, and T cells participate in cell mediated immune response. Humoral immunity is the principle defense mechanism against invading microbes and involves antibodies, produced by B cells and circulated to effectively disable the virus before it can enter and cause irreparable damage to cells.

cells of the immune system

The antibodies are able to attach themselves to the virus and remove it from the body, as well as using various effector mechanisms to eliminate them. They are specialized by function, and so some antibodies specialize in phagocytic processes such as the ingestion of viruses, while others may trigger the inflammatory response in cells.

How the Cells Defend
immune systemCell mediated response is more specific, and involves T cells directly attacking the virus. Some viruses are able to use phagocytes as host cells, and so circulating antibodies cannot attach themselves to or eliminate them. This is where cell mediated immunity is important; it is able to destroy even viruses residing in other cells. The infected cell is destroyed entirely, by 3 subgroups of T cells consisting of helper T cells, suppressor T cells and cytotoxic T cells. Cytotoxic T cells are cells that attack viruses expressing abnormal antigens and produce lymphokines, which are proteins that promote the inflammatory response. The adaptive immune system is extremely efficient and selects the most useful cells to deal with different types of invasions. Antibodies are also made in this type of immune response, binding themselves to the virus in order to disable them, stop them from further invading any host cells and promote their ingestion through phagocytosis. The virus is essentially destroyed and entirely removed from the body during this process.

The current modern research into viral infection has proved immensely useful to both scientists and the pharmaceutical industry. Understanding how exactly these microbes can invade is the key to prevention; it is an area of study that is hugely important. Though there are still gray areas of the immune system that need to be explored, modern findings are getting closer and closer to discovering how to cure viral infection through studying the microbiology of viruses and immune system response.





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