Understanding the Immune Response

How the Body Defends Itself During an HIV Infection

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3D ribbon conformation of the antibody IgG2. Image by Tim Vickers was released into the public domain

When a foreign substance such as a bacteria or virus enters the body, the immune system activates itself to protect against the invader. The system itself is composed of a complex networks of cells and cellular responses that work in tandem to identify, tag, and neutralize the infecting agent.

In many cases, the body is able to defend itself. In some, however, the immune system can be overwhelmed and unable to cope, requiring medical intervention to bring the invader under the control.

The Immune Response During HIV Infection

When HIV first enters the body, the immune system sends in its first-line defense. These early defenders include white blood cells called macrophage (literally "big eater") and dendritic ("finger") cells, which aim to corral and kill the viruses at the site of exposure.

Both macrophage and dendritic cells are considered part of the innate immune system, meaning that they are always around to mount a general attack. However, when viral infiltration is more aggressive (for example, in cases of blood-to-blood exposure or unprotected sex), these cells are often unable to contain the infection.To do so requires a more targeted (adaptive) immune response.

Once the body is alerted to the presence of the virus, biochemical signals are sent to cells which attach themselves to the invaders and "present" them to another set of specialized cells called T-cells.

By doing so, a subset of "helper" CD4 T-cells signal "killer" CD8 T-cells to multiply and neutralize the invading viruses. The body also produces what are known as antibodies, which target and kill specific invaders though cellular markers known as antigens.

What are Antigens and Antibodies?

Antigens are a type of protein that reside on the surfaces of all cells. They function as identifiers and tell the body whether a cell belongs in the body or must be destroyed. Each and every cell in our body has an antigen which differentiates good substances from bad substances. It is through antigens that the immune system is able to mount a targeted defense.

Antibodies are also proteins that work in tandem with antigens to neutralize foreign agents. When the body detects a foreign antigen, it produces a specific antibody that will join with the antigen like a lock and key. When the key is in the lock, the antigen's cell is unable to reproduce. By stopping the invader's ability to reproduce, it is effectively killed and the infection averted.

Unfortunately, during an HIV infection, these antibodies are usually not strong enough to fight off the infection, leaving the HIV free to multiply and damage the immune system.

How HIV Does the Damage to the Immune System

After the acute (early) stage of HIV has occurred, the immune system is most often able to contain the infection to a point where the virus is not eradicated but levels off to a so-called "set point." The person with HIV can usually maintain at this level for years, often with few if any symptoms.

But the problem is that, while the initial immune response is robust, it is undermined by two things:

  • Firstly, very early in the infection, a form of HIV (called a provirus) can escape and "hide" in cellular havens called latent reservoirs, where the body is unable to detect them.
  • Meanwhile, active and free-circulating HIV undermine the immune defense by infecting the very CD4 T-cells meant to initiate a response. By doing so, the immune system is left blind and becomes less and less capable of defending itself.

Once enough CD4 cells are killed, the immune system becomes "compromised," no longer able to stop the invaders or prevent other opportunistic diseases from taking hold and causing illness

This is the stage classically designated as AIDS, which we define as having a CD4 count of less than 200 cells/mL and/or having an AIDS-defining illness.

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