HIV/AIDS: How HIV spreads and evades the immune system, Pathogenesis and Differences between HIV 1 and HIV 2

HIV stands for human immunodeficiency virus. It is the virus that can lead to acquired immunodeficiency syndrome, or AIDS, if not treated. Unlike some other viruses, the human body can’t get rid of HIV completely, even with treatment. So once you get HIV, you have it for life.

Route of transmission of HIV virus

HIV is transmitted from person to person by the following ways.

Sexual transmission (>75%)

More common among the male homosexuals than heterosexuals. HIV can be transmitted via:

• Vaginal sex
• Anal sex 
• Oral sex

Parenteral transmission

• By infected blood and blood products 
• Contaminated needles, syringes, and surgical instrument 
• Injectable drug abusers 

Vertical transmission 

• Trans-placental 
• During birth through the birth canal 
• Breast feeding

Probable other methods 

• Through donated organs and tissues such as organ transplantation via donated semen 
• Any skin piercing: injection, ear, nose piercing, tattooing, acupuncture 
• By sharing razors, combs, toothbrush (rarely)

Most Common Routes of Transmission of HIV

Pathogenesis of HIV:

HIV virus preferentially attacks and kills the white blood cells that contain a specific surface antigen, CD4 (for example, T cells, monocytes/macrophages).

These cells are vital for the maintanence of the immunity of the body against infection and malignant cells.

The pathogenesis can be summarized as:

1. HIV enters the body through different routes (see above) and goes to the blood.
2. In the blood HIV infects CD4+ T cells and kills them.
3. T cells are directly responsible for cell mediated immunity and also indirectly responsible for humoral immunity (antibody mediated immunity) of the body.
4. Destruction of T cell means destruction of cell mediated immunity and humoral immunity.
5. As the immunity decreases, the body is now more prone and susceptible to infection by opportunistic pathogens.
6. Leading to the development of opportunistic infections.
7. AIDS.

HIV also infects the macrophages present in brain. These macrophages are called microglia. 

Microglial cells, when infected by HIV, do not die as quickly as T cell.

But since they are infected by HIV they release abnormal chemicals which damage the nerve cells (neurons). This produces significant neurological sign and symptoms.

So HIV does not directly attack the brain cells but indirectly through microglial cells it damages the brain cells.

HIV and AIDS in a nutshell

How HIV evades the immune system:

There are three main reason why our body can't kill the HIV virus:

1. Integration of the viral DNA into the host cell DNA, resulting in a persisting infection.
2. A high rate of mutation of the env gene (Read about the genetics of HIV).
3. The production of the tat and nef proteins that down-regulates class 1 MHC proteins required for cytotoxic T-cell to recognize and kill HIV infected cells.

Differences between HIV 1 and HIV 2:

Points	HIV 1	HIV 2
Geographical distribution	World wide	In west Africa
Virulence	More	Less
Incubation period	Short	Long
Tumor	Kaposi sarcoma is more	Not so common
Opportunistic infection	Pneumocystic pneumonia	Cryptococcosis

That's all for today!
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