Immune disorders: HIV infection, the mechanism of action



HIV is a viral infection that affects the immune system, if not treated the infection will progressively damage the white blood cells that play an important role in shielding you from common infections and illnesses. As this protective shield is broken down your body is left susceptible to infections which leads to the development of cancers, serious infections and then AIDS (Acquired Immune Deficiency Virus).

The HIV virus is carried in blood, semen, vaginal secretions, breast milk and saliva, although the level in saliva is extremely low and the passing of the virus from saliva is incredibly rare. To become infected, and infected fluid from a HIV+ person must mix with the fluids of a HIV- person.

The HIV virus enters the body and infects white blood cells called lymphocytes that are coated with a protein structure called CD4+ these cells are sometimes called T-cells or T helper cells. These T helper cells play a crucial role in triggering the immune response.

In a HIV- person when a bacteria or virus infect the body it is picked up by an antigen presenting cell (APC). The APC then presents this cell to the T helper cells (CD4+ cells). These T helper cells then respond by rapidly dividing and cause other immune cells (B cells) to mature and release antibodies for that particular invading bacteria or virus. Once the mature immune cells (called plasma or mature B cells) begin releasing antibodies into the blood, the invading bacteria and viruses can easily be identified and destroyed by matured immune cells including Macrophages, cytotoxic T cells and natural killer cells.

In a person infected with HIV the virus enters the T cells / T Helper / CD4+ cells and hijacks its internal copying machinery. The virus inserts its DNA into the cells own DNA and begins to copy over and over again filling the cell with millions of new HIV virus particles. Eventually the cell is overwhelmed and bursts releasing millions of new viral particles into the blood stream each which moves on to infect another cell. These new viral particles go on to infect more T cells / T helper cells / CD4+ cells which if damaged stop the immune system from making antibodies as it disrupts the APC from triggering B cells to mature B cells and plasma cells.

Treatment of HIV uses a group of drugs called ART Anti-Retroviral Therapy. ART halts the copying machinery inside the cell (effectively switching it off) therefore preventing the virus from replicating.

HIV infection can be controlled and is now a long-term medical condition, it is important that ART therapy is started straight away and it is taken routinely for the rest of the HIV+ persons life. It is possible to Iive a healthy, prosperous life with HIV if it is managed well. Many studies are now showing that if ART is started and taken routinely that the HIV can be kept to an undetectable level meaning the likelihood of the HIV being passed on is very low.

What is viral load?

Viral load is a measure used to check the health of a person infected with HIV. It is a measure of the amount of HIV genetic material in 1ml drop of blood. This is measured using complex laboratory equipment and given as number in the thousands. In terms of scale 10,000 is low and 100,000 is high. It can change depending on how well ART is being managed. When the viral load falls that low that the lab equipment cant detect it, the person is said to be undetectable.

HIV infection is tested at a sexual health clinic using a persons blood. The blood is sent off to a science lab where an ELISA test is performed. The principles of ELISA testing are now in the A level biology specification.

We will discuss in more detail HIV infection on the Anatomy Lab LIVE UK tour 2017.

More to follow soon...


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