Human immunodeficiency virus (HIV)
The above image is a coloured SEM of HIV (yellow) budding from host cells (red).
HIV is part of the retrovirus family, which causes AIDS (acquired immunodeficiency syndrome), resulting in progressive failure of the immune system. It can be transferred in blood, semen, vaginal fluid, pre-ejaculate and breast milk. HIV has a long latency/incubation period.
HIV infects vital cells in the immune system, particularly CD4+T helper cells (a type of white blood cell called lymphocytes). The infection of the HIV virus causes the number of T-helper cells to decline, making the body more susceptible to opportunistic infections (such as tuberculosis). It is these secondary infections (or malignancies) that will eventually cause death.
A2 level notes on how HIV works:
The HIV envelope has surface proteins, which recognise and bind to surface proteins on T-helper cells. HIV then infects the T helper cell and viral mRNA enters the host.
Note: Reverse transcriptase found in the virion is an enzyme that makes DNA from mRNA. This DNA codes for viral coat proteins and recognition proteins. DNA made by reverse transcriptase can be called complementary DNA (cDNA) as it is complementary to mRNA.
cDNA passes from the cytoplasm of the T-helper cell into the nucleus (integrase protein helps cDNA pass through the nuclear pore and inserts DNA into the host cell chromosome).
Viral cDNA is transcribed to viral mRNA. Viral mRNA is then translated to protein capsomeres. Virions self-assemble and cell lyses as virions exit the host cell (exocytosis).
A2 level notes on the course of a HIV/AIDS infection:
Stage 1: Acute HIV syndrome. Between 3-12 weeks post-infection, HIV antibodies appear in the blood (now HIV positive). Symptoms experienced during this time may include fever, headache, dizziness and swollen glands.
Stage 2: Asymptomatic/chronic stage. All symptoms disappear once the infection has been established. Virus is now latent. The duration of this stage depends on health, immunity, genetics and access to medication. During this stage the virus replicates, infecting T helper cells but being restrained by T killer cells. Towards the end of this stage the immune system becomes overwhelmed and secondary infections begin to develop.
Stage 3: Symptomatic stage. Eventually the viral load becomes so great that the whole immune system starts to fail. The normal T helper cell count falls from 500 to 200 per mm3 of blood. Symptoms now suffered include weight loss, fatigue, diarrhoea, night sweats and low-grade infections such as candidiasis (thrush). This stage rapidly progresses to the final stage.
Stage 4: Advanced AIDS. T helper cell numbers continue to fall. Symptoms include severe weight loss, dementia (as brain cells become infected), cancers (e.g. Kaposi’s sarcoma) and serious infections (e.g. tuberculosis (note: HIV is the main cause for an increase in TB infection) and cryptococcal meningitis). The final stage of advanced AIDS is always death.
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