Acquired Immunodeficiency Syndrome (AIDS) is characterized by weakening of the immune system on contracting the notorious Human Immunodeficiency Virus (HIV). This virus specifically attacks the immune cells called T-helper cells, which leads to increased susceptibility to infections.

Did You Know?
A specific genetic defect in a set of people with European and Asian heritage has rendered them resistant to AIDS.
The human immunodeficiency virus (HIV) is one of the most notorious viruses, and affects about 50,000 people ever year in the USA alone. This virus infects several types of cells in the body including a set of white blood cells called T-helper (Th) cells. Reduction in the number of T-helper cells weakens the immune system, ultimately leading to Acquired Immunodeficiency Syndrome (AIDS), which is characterized by increased bacterial, fungal, and parasitic infections.

In an infected individual, HIV is present in the genital fluids and blood. Hence it gets transmitted sexually as well as through the use of infected syringes, transfusion of infected blood, and on contact of a wound with the infected fluids. The virus also spreads from an infected mother to the child during pregnancy, delivery, or through breast milk. After transmission, the virus may persist in a latent state for several years before leading to AIDS.

The interaction of HIV with T-helper cells, as well as the different stages of HIV infection have been explained below.

HIV and T-helper Cells
HIV belongs to the family of retroviruses, which are RNA (ribonucleic acid) viruses characterized by their ability to synthesize DNA (deoxyribonucleic acid) from RNA by a process called reverse transcription. The HIV genome comprises two copies of single stranded RNA (ssRNA). These RNA molecules and the enzymes required for multiplication of the virus are enclosed inside a conical protein structure called capsid. The capsid is surrounded by an envelope that is similar to the cell membrane of human cells.

T-helper cells are the main targets of HIV, and are also called CD4 positive cells due to the presence of a specialized protein called CD4 (cluster of differentiation 4) on their cell surface. These cells are a type of soldier cells present in the body, and play an important role in detecting foreign bodies and pathogens, as well as activating other cells of the immune system. HIV attacks these cells, multiplies inside them, and ultimately destroys them. In simple terms, HIV reprograms the T-helper cells into HIV-producing cells. This replication cycle of HIV occurs through the following sequence of events.

Attachment and Entry

The HIV envelope contains specialized protein stems onto which a set of glycoproteins called gp120 are attached. These glycoproteins can identify and bind to CD4 molecules present on T-helper cells. This interaction is analogous to lock and key interaction, and serves as a gate opener for HIV. Once HIV docks onto the cell membrane of T-helper cells, fusion of the viral and cellular membranes occurs, and the viral capsid is released into the cytoplasm.