Glossary

Annealing: Capable of causing the association of single-stranded nucleic acids so that double-stranded molecules are formed, often by heating and cooling.⁴¹⁽²⁾

Budding: This final step in the viral replication cycle occurs when an individual virus pushes out ("buds") from the host cell. The virion then steals part of the cell's outer envelope and frees itself to attach to - and infect - another host cell.²⁹⁽³⁾

CCR5: Chemokine receptor 5 (CCR5) is a protein on the surface of some immune system cells. It is one of two co-receptors that HIV can use along with the CD4 receptor to bind to and enter host cells (the other co-receptor is CXCR4).³⁰⁽¹⁾

CD4+ T-cell: Also known as helper T-cell or CD4 lymphocyte, this type of white blood cell carries the CD4 cell surface receptor and helps the body fight infection. CD4 cells coordinate the immune response, signaling other cells in the immune system to perform their special functions. The number of CD4 cells in a sample of blood is an indicator of the health of the immune system. HIV infects and kills CD4 cells, leading to a weakened immune system.³⁰⁽¹⁾

CD4+ T-cell count: A measurement of the number of CD4 cells in a sample of blood. The CD4 count is one of the most useful indicators of immune system health. It can also serve as a predictor for the progression of HIV/AIDS. A CD4 cell count is instrumental in determining when to begin, interrupt, or halt anti-HIV therapy; when to give preventive treatment for opportunistic infections; and to measure response to treatment. A normal CD4 cell count is between 500 and 1,400 cells/mm3 of blood, but an individual's CD4 count can vary. In HIV-infected patients, a CD4 count at or below 200 cells/mm3 is considered an AIDS-defining condition.^30(1-2)

CD4 receptor: A specific molecule present on the surface of a CD4 cell. To gain entry into a host cell, HIV recognizes and binds to a CD4 receptor as well as a co-receptor.³⁰⁽²⁾

Chemokines: Chemical messengers that control the activities of the immune system, these proteins are involved in a wide variety of processes, including: the control of infectious diseases, cancers, and inflammation. Chemokines include interferons, interleukins, and many other small proteins.³⁰⁽⁵⁾

Co-receptor: HIV entry into host cells involves several distinct steps beginning with CD4 attachment followed by interaction with a co-receptor, generally CCR5 and/or CXCR4 (cell-surface chemokine receptors that promote cell migration). Chemokine-receptor binding brings viral and host-cell membranes together, activating the fusion process.

CXCR4: Chemokine (receptor 4 (CXCR4, also known as fusin) is a protein on the surface of some immune system cells. It is one of two co-receptors that HIV can use along with the CD4 receptor to bind to and enter host cells (the other co-receptor is CCR5).³⁰⁽³⁾

Dendritic cells: A type of antigen-presenting cell that picks up foreign substances from the bloodstream and "presents" them to other parts of the immune system - activating an immune response against the foreign invader.³¹⁽²⁾

Envelope: Composed of 2 layers of fat-like molecules (lipids), this is the outer protective membrane of HIV. The virion uses specific proteins embedded in the envelope to attach to and enter host cells.³²⁽²⁾

Express: To manifest or produce (a molecule, character, or effect) by a genetic process.²⁸⁽³⁾

gp120: One of the proteins embedded in the outer envelope of HIV, gp120 projects from the surface of HIV and binds to the CD4 receptor on CD4 cells. This initiates the process by which HIV enters and infects a host cell.³³⁽³⁾

gp41: One of the proteins embedded in the outer envelope of HIV, gp41 plays a key role in HIV infecting CD4 cells. By fusing the virion's envelope with the host cell membrane, gp41 allows HIV to enter the cell.^33(3-4)

Integrase: An HIV protein that inserts the virion's genetic information into the DNA of the host cell.³⁵⁽⁵⁾

Integration: The process by which HIV integrase inserts HIV's genetic material into an infected cell's own DNA.³⁵⁽⁵⁾

Macrophage: By stimulating other immune system cells to fight infection, this type of white blood cell destroys foreign invaders.³⁷⁽³⁾

Messenger RNA (mRNA): Carrying the code for a particular protein from the nuclear DNA to a ribosome in the cytoplasm, this molecule serves as a blueprint for the formation of that protein.³⁷⁽³⁾

Mutation: Having the ability to be passed down to future generations, this change or adaptation can only occur when a virus is actively replicating - not when anti-HIV drugs have suppressed the viral load to undetectable. If HIV replication is not well-controlled, an individual's original HIV strain can adapt to infect different T-cell types or resist different anti-HIV drugs.³⁷⁽⁴⁾

Nucleoside: This precursor to a building block of DNA or RNA that must be chemically changed into a nucleotide before it can be used to make DNA or
RNA.³⁸⁽⁴⁾

Protease: An enzyme that breaks down long polypeptides into smaller protein units. The HIV protease enzyme cuts long chains of HIV polypeptide into the smaller, active proteins used in HIV replication.⁴⁰⁽⁹⁾

Provirus: This virus form has been integrated into host cell's own DNA - transmitting a DNA version of HIV genetic material.⁴⁰⁽¹⁰⁾

p24: This HIV protein makes up the virus core surrounding HIV's genetic material.⁴⁰⁽¹¹⁾

Receptor: Located on a cell surface, this protein serves as a binding site for a specific chemical group, molecule, or virus outside the cell - such as HIV floating free in the blood.⁴¹⁽²⁾

Retrovirus HIV is an example of this type of virus. Storing its genetic information in a single-stranded RNA molecule, a retrovirus then proceeds to construct a double-stranded DNA version of its genes using the enzyme, reverse transcriptase. The DNA copy is then integrated into the host cell's own genetic material.⁴¹⁽³⁾

Reverse Transcriptase (RT): An enzyme found in HIV and other retroviruses, RT converts single-stranded HIV RNA into double-stranded HIV DNA.⁴¹⁽³⁾

Reverse transcription: In order to achieve productive infection, retroviral RNA must be transcribed into DNA and inserted into the host chromosome. This is achieved by the retroviral enzyme, reverse transcriptase (RT), which synthesizes a linear, double-stranded DNA copy from two molecules of single-stranded viral RNA and assures that the integrated viral DNA, called a provirus, is able to highjack the host's DNA machinery to replicate.

Transcription: One of the steps in the viral replication cycle, this is the process of constructing a messenger RNA molecule using a DNA molecule as a template. The result is a transfer of genetic information to the HIV messenger RNA (mRNA), which is then used in a translation to create HIV proteins and continue the viral replication cycle.^{42(4) 29(4)}

Translation: The step in the viral replication cycle that follows transcription, translation is the process of forming a protein molecule at a ribosomal site of protein synthesis from information contained in messenger RNA (HIV mRNA). Once these HIV proteins are produced, they can combine with copies of HIV's RNA genetic material to form new, complete copies of HIV.^{42(4) 28(5)}

Tropism: The affinity of a virus to invade and replicate in a particular cell type, making it an important aspect of co-receptor binding.⁴²⁽⁴⁾

Virion: A mature virus particle existing freely outside a host cell, consisting of an RNA or DNA core. The virion has a protein coat and at times retains external envelopes.^{28(7), 43(3)}

To learn more about terminology related to HIV/AIDS, visit the Centers for Disease Control and Prevention at www.cdc.gov as well as AIDS info at www.aidsinfo.nih.gov and MedlinePlus at www.nlm.nih.gov/medlineplus/.

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