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Immune Basics

Getting sick is often a sign that your body is responding to invaders from the outside. For example, the pus around a splinter that gets infected is a sign of immunological effort — white blood cells building up to fight an infection. The same is true when your body’s immune system responds to HIV. Your immune system is a complex network of cells and organs that work together to defend your body from viruses, bacteria, fungi, and protozoa. There are two types of immune response: innate and acquired. The innate response starts very quickly after infection and does not depend on recognizing the specific type of foreign substance that is present in your body. Natural killer cells are a component of innate immunity. Acquired immune response starts more slowly, but it also lasts longer. In addition, the acquired response is specific to certain foreign substances and it can "remember" past infections. This memory allows the immune system to react quickly if you are ever re-infected with the same pathogen.

Two important components of acquired immunity are antigens and antigen-presenting cells. An antigen is a foreign substance, usually a protein, which can harm your body and cause your immune system to respond. Antigen-presenting cells are already in your body and digest these intruders and display the antigen on their surface. Antigen-presenting cells then look for other immune cells that respond to that specific antigen.

One type of cell that antigen-presenting cells look for is called a B cell. Each B cell recognizes one and only one type of antigen. When the antigen-presenting cells find B cells that recognize the antigen, those B cells become active and begin to reproduce. Most of the new B cells release antibodies. Antibodies play several roles, including marking foreign matter and infected cells for destruction by other parts of the immune system. (The standard test for HIV does not actually look for the presence of HIV but antibodies that your body has produced to fight HIV. It usually takes about a month or six weeks for your body to produce enough antibodies to register on an HIV test, a point known as seroconversion.) Once the infection is under control, these antibody-creating B cells are no longer needed and they die. However when B cells reproduce, they also produce "memory B cells" that can live for years. If the same antigen is encountered again, the memory B cells activate and reproduce more quickly than the first time.

Antigen-presenting cells can also look for T cells. Like B cells, T cells respond to only one type of antigen. Again, once the right T cells are found, they become active and reproduce. But instead of producing antibodies, T cells release cytokines. Cytokines direct immune cells to attack a foreign substance. Furthermore, there are two types of T cells: CD4 T cells and CD8 T cells. CD4 T cells are also called "helper T cells" because they help other parts of the immune system respond to infection. Specifically, CD4 T cells stimulate B cells to respond and increase the production of CD8 T cells. Although CD8 T cells have many roles, one of the most important is to kill the cells that have become infected by an antigen. These CD8 cells are called "killer cells" or "cytotoxic T lymphocytes" (CTLs). As with B cells, once the infection is under control, many of these T cells are no longer needed and they die. However, some T cells become memory cells, which can respond quickly if the antigen is encountered again.

HIV, however, is more formidable an enemy to the immune system than most. It reproduces by installing itself in the CD4 cells that coordinate the rest of the immune response. Once HIV is inside them, these T-cells become virus factories, producing thousands of viral particles and then dying off. A person infected with HIV produces millions and millions of new viral particles every day. This is the process of viral replication.

The more CD4 cells HIV kills off, the more powerless your immune system becomes to fight off other bacteria, fungi, parasites, and viruses. It is often these germs and bugs - some of which may have been inside your body and controlled by your immune system since childhood — that make you sick in the course of HIV illness. Because they take advantage of the opportunity provided by your weakened defenses, many of the most serious of these illnesses are known as opportunistic infections, or OIs. Many of these, and ways to treat them, are discussed in the fact sheets produced by GMHC's Treatment Education and Adherence Program.

What can you do? Deciding whether to treat HIV or not is a tough decision. Though it can take years and years before HIV will destroy your immune system without treatment, for 95% of HIV-positive people the virus will eventually win unless you take medications to help.

For more information on HIV/AIDS please visit the CDC and DOH websites.