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

How does HIV spread? Why can't the body kill the HIV virus? How does AIDS occur? What is the difference between HIV 1 and HIV 2? Can HIV infect brain cells? read to find out...

Routes of transmission of HIV:

HIV is transmitted from person to person by:

Sexual transmission (>75%):
More common among the male homosexuals than heterosexuals

1. Vaginal sex, Anal sex
2. Oral sex

Source

Parenteral transmission:

1. By infected blood and blood products
2. Contaminated needles, syringes, and surgical instrument
3. I/V drug abusers

Vertical transmission:

1. Trans-placental
2. During birth / through birth canal
3. Breast feeding

Probable other methods:

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

Source: Aids.gov

Pathogenesis of HIV:

HIV virus preferentially attacks and kills the white blood cells that contain a specific surface antigen, CD4 (eg 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:

HIV enters the body through different routes (see above) and goes to the blood

In the blood HIV infects CD4+ T cells and kills them.

T cells are directly responsible for cell mediated immunity and also indirectly responsible for humoral immunity (antibody mediated immunity) of the body.

Destruction of T cell means destruction of cell mediated immunity and humoral immunity.

As the immunity decreases, the body is now more prone and susceptible to infection by oppurtunistic pathogens.

Leading to the development of oppurtunistic infections.

AIDS

Source: Aids.gov

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.

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 (See genes 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	Cryptoccosis, persiste