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CD4 cells are a type of lymphocyte (white blood cell). They are an important part of the immune system. CD4 cells are sometimes called T-cells. There are two main types of T-cells. T-4 cells, also called CD4+, are "helper" cells. They lead the attack against infections. T-8 cells, (CD8+), are "suppressor" cells that end the immune response. CD8+ cells can also be ?killer? cells that kill cancer cells and cells infected with a virus. Researchers can tell these cells apart by specific proteins on the cell surface. A T-4 cell is a T-cell with CD4 molecules on its surface. This type of T-cell is also called ?CD4 positive,? or CD4+.


When HIV infects humans, the cells it infects most often are CD4 cells. The virus becomes part of the cells, and when they multiply to fight an infection, they also make more copies of HIV.

When someone is infected with HIV for a long time, the number of CD4 cells they have (their CD4 cell count) goes down. This is a sign that the immune system is being weakened. The lower the CD4 cell count, the more likely the person will get sick.

There are millions of different families of CD4 cells. Each family is designed to fight a specific type of germ. When HIV reduces the number of CD4 cells, some of these families can be totally wiped out. You can lose the ability to fight off the particular germs those families were designed for. If this happens, you might develop an opportunistic infection


The CD4 cell value bounces around a lot. Time of day, fatigue, and stress can affect the test results. It's best to have blood drawn at the same time of day for each CD4 cell test, and to use the same laboratory.

Infections can have a large impact on CD4 cell counts. When your body fights an infection, the number of white blood cells (lymphocytes) goes up. CD4 and CD8 counts go up, too. Vaccinations can cause the same effects. Don't check your CD4 cells until a couple of weeks after you recover from an infection or get a vaccination.


CD4 cell tests are normally reported as the number of cells in a cubic millimeter of blood, or mm3. There is some disagreement about the normal range for CD4 cell counts, but normal CD4 counts are between 500 and 1600, and CD8 counts are between 375 and 1100. CD4 counts drop dramatically in people with HIV, in some cases down to zero.

The ratio of CD4 cells to CD8 cells is often reported. This is calculated by dividing the CD4 value by the CD8 value. In healthy people, this ratio is between 0.9 and 1.9, meaning that there are about 1 to 2 CD4 cells for every CD8 cell. In people with HIV infection, this ratio drops dramatically, meaning that there are many times more CD8 cells than CD4 cells.

Because the CD4 counts are so variable, some health care providers prefer to look at the CD4 percentages. These percentages refer to total lymphocytes. If your test reports CD4% = 34%, that means that 34% of your lymphocytes were CD4 cells. This percentage is more stable than the number of CD4 cells. The normal range is between 20% and 40%. A CD4 percentage below 14% indicates serious immune damage. It is a sign of AIDS in people with HIV infection. A recent study showed that the CD4% is a predictor of HIV disease progression.


The meaning of CD8 cell counts is not clear, but it is being studied.

The CD4 cell count is a key measure of the health of the immune system. The lower the count, the greater damage HIV has done. Anyone who has less than 200 CD4 cells, or a CD4 percentage less than 14%, is considered to have AIDS according to the US Centers for Disease Control.

CD4 counts are used together with the viral load to estimate how long someone will stay healthy. See Fact Sheet 125 for more information on the viral load test.

CD4 counts are also used to indicate when to start certain types of drug therapy:

When to start antiretroviral therapy (ART):
When the CD4 count goes below 350, most health care providers begin ART

Also, some health care providers use the CD4% going below 15% as a sign to start aggressive ART, even if the CD4 count is high. More conservative health care providers might wait until the CD4 count drops to near 200 before starting treatment. A recent study found that starting treatment with a CD4% below 5% was strongly linked to a poor outcome.

When to start drugs to prevent opportunistic infections:

Most health care providers prescribe drugs to prevent opportunistic infections at the following CD4 levels:

Because they are such an important indicator of the strength of the immune system, official treatment guideline in the US suggest that CD4 counts be monitored every 3 to 4 months.

Testing for acute HIV infection

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The normal HIV blood test will come back negative for someone who was infected very recently. The test looks for antibodies produced by the immune system to fight HIV. It can take two months for these antibodies to be produced.

However, the viral load test measures the virus itself. Before the immune system produces antibodies to fight it, HIV multiplies rapidly. Therefore, this test will show a high viral load during acute infection.

A negative HIV antibody test and a very high viral load indicate recent HIV infection, most likely within the past two months. If both tests are positive, then HIV infection probably occurred a few months or longer before the tests. A special "detuned" version of the HIV antibody test is less sensitive. It detects only those infections that occurred at least four to six months before testing. It can be used to help identify cases of acute HIV infection.

Some people think that there?s not much harm done in the early stages of HIV infection. They believe that any damage to their immune system will be cured by taking antiretroviral therapy (ART). This is not true!

Up to 60% of infection-fighting ?memory? CD4 cells are infected during acute infection, and after 14 days of infection, up to half of all memory CD4 cells can be killed. Also, HIV quickly reduces the ability of the thymus gland to replace lost CD4 cells. The lining of the intestine is also damaged very quickly. This can all occur before a person tests positive for HIV.

The number of HIV particles in the blood is much higher during acute HIV infection than later on. Exposure to the blood of someone in the acute phase of infection is more likely to result in infection than exposure to someone with long-term infection. One research study estimated that the risk of infection is approximately 20 times higher during acute HIV infection.

At first, the immune system produces white blood cells that recognize and kill HIV-infected cells. This is called an "HIV-specific response." Over time, most people lose this response. Unless they use antiretroviral drugs (ARVs), their HIV disease will progress.

Guidelines for using HIV medications recommend waiting until the immune system shows signs of damage. However, starting ARVs during acute HIV infection might protect the HIV-specific immune response.

Researchers have studied people who start treatment during acute infection and then stop taking ARVs. One study showed that this treatment may delay the time until ART is needed. Researchers are doing more studies.

Starting ART is a major decision. Anyone thinking about taking ARVs should carefully consider the benefits and disadvantages.

Taking ART changes your daily life. Missing doses of drugs makes it easier for the virus to develop resistance to medications, which limits future treatment options.

The medications are very strong. They have side effects that can be difficult to live with for a long time, and they can be very expensive.

Early treatment can protect the immune system from damage by HIV. Immune damage shows up as lower CD4 cell counts and higher viral loads. These are associated with higher rates of disease. Older people (over 40 years old) have weaker immune systems. They do not respond to ARVs as well as younger people.

However, not everyone with HIV gets sick right away. Someone with a CD4 cell count over 350 and a viral load under 20,000, even if they don?t take antiviral drugs, has about a 50/50 chance of staying healthy for 6 to 9 years.

At first, researchers believed that early treatment might allow a patient to stop taking ART after a period of controlling HIV. However, newer reports indicate that this is very unusual.

Acute HIV infection

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The amount of HIV in the blood gets very high within a few days or weeks after HIV infection. Some people get a flu-like illness. This first stage of HIV disease is called "acute HIV infection" or "primary HIV infection."

About half of the people who get infected don?t notice anything. Symptoms generally occur within 2 to 4 weeks. The most common symptoms are fever, fatigue, and rash. Others include headache, swollen lymph glands, sore throat, feeling achy, nausea, vomiting, diarrhea, and night sweats.
It is easy to overlook the signs of acute HIV infection. They can be caused by several different illnesses. If you have any of these symptoms and if there is any chance that you were recently exposed to HIV, talk to your health care provider about getting tested for HIV.

Most risky activities

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The highest risk of becoming infected with HIV is from sharing needles to inject drugs with someone who is infected with HIV. When you share needles, there is a very high probability that someone else's blood will be injected into your bloodstream. Hepatitis virus can also be transmitted by sharing needles.

The next greatest risk for HIV infection is from unprotected sexual intercourse (without a condom). Receptive anal intercourse carries the highest risk. The lining of the rectum is very thin. It is damaged very easily during sexual activity. This makes it easier for HIV to enter the body. The "top" or active partner in anal intercourse seems to run a much lower risk. However, the risk still seems higher than for the active partner in insertive vaginal intercourse.

Receptive vaginal intercourse has the next highest risk. The lining of the vagina is stronger than in the rectum, but is vulnerable to infection. Also, it can be damaged by sexual activity. All it takes is a tiny scrape that can be too small to see. The risk of infection is increased if there is any inflammation or infection in the vagina.

The risk is higher for the receptive partner. However, there is some risk for the active partner in anal or vaginal sex. It's possible for HIV to enter the penis through any open sores, through the moist lining of the opening of the penis, or through the cells in the mucous membrane in the foreskin or the head of the penis.

Safe and unsafe activities

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Unsafe sex has a high risk of spreading HIV. The greatest risk is when blood or sexual fluid touches the soft, moist areas (mucous membrane) inside the rectum, vagina, mouth, nose, or at the tip of the penis. These can be damaged easily, which gives HIV a way to get into the body.

Vaginal or rectal intercourse without protection is very unsafe. Sexual fluids enter the body, and wherever a man's penis is inserted, it can cause small tears that make HIV infection more likely. The receptive partner is more likely to be infected, although HIV might be able to enter the penis, especially if it has contact with HIV-infected blood or vaginal fluids for a long time or if it has any open sores.

Most sexual activity carries some risk of spreading HIV. To reduce the risk, make it more difficult for blood or sexual fluid to get into your body.

Be aware of your body and your partner's. Cuts, sores, or bleeding gums increase the risk of spreading HIV. Rough physical activity also increases the risk. Even small injuries give HIV a way to get into the body.

Use a barrier to prevent contact with blood or sexual fluid. Remember that the body's natural barrier is the skin. If you don't have any cuts or sores, your skin will protect you against infection. However, in rare cases HIV can get into the body through healthy mucous membranes. The risk of infection is much higher if the membranes are damaged.

The most common artificial barrier is a condom for men. You can also use a female condom to protect the vagina or rectum during intercourse. Fact Sheet 153 has more information on condoms.

Lubricants can increase sexual stimulation. They also reduce the chance that condoms or other barriers will break. Oil-based lubricants like Vaseline, oils, or creams can damage condoms and other latex barriers. Be sure to use water-based lubricants.

Oral sex has some risk of transmitting HIV, especially if sexual fluids get in the mouth and if there are bleeding gums or sores in the mouth. Pieces of latex or plastic wrap over the vagina, or condoms over the penis, can be used as barriers during oral sex. Condoms without lubricants are best for oral sex. Most lubricants taste awful.
Safe activities have no risk for spreading HIV. Abstinence (never having sex) is totally safe. Sex with just one partner is safe as long as neither one of you is infected and if neither one of you ever has sex or shares needles (see Fact Sheet 154) with anyone else.

Fantasy, masturbation, or hand jobs (where you keep your fluids to yourself), sexy talk, and non-sexual massage are also safe. These activities avoid contact with blood or sexual fluids, so there is no risk of transmitting HIV.

To be safe, assume that your sex partners are infected with HIV. You can?t tell if people are infected by how they look. They could be lying if they tell you they are not infected, especially if they want to have sex with you. Some people got HIV from their steady partners who were unfaithful "just once".

Even people who got a negative test result might be infected. They might have been infected after they got tested, or they might have gotten the test too soon after they were exposed to HIV.
Some people who are HIV-infected don't see the need to follow safer sex guidelines when they are sexual with other infected people. However, it still makes sense to "play safe". If you don't, you could be exposed to other sexually transmitted infections such as herpes or syphilis. If you already have HIV, these diseases can be more serious.

Also, you might get "re-infected" with a different strain of HIV. This new version of HIV might not be controlled by the medications you are taking. It might also be resistant to other antiretroviral drugs. There is no way of knowing how risky it is for two HIV-positive people to have unsafe sex. Following the guidelines for safer sex will reduce the risk.


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To spread HIV during sex, HIV infection in blood or sexual fluids must be transmitted to someone. Sexual fluids come from a man's penis or from a woman's vagina, before, during, or after orgasm. HIV can be transmitted when infected fluid gets into someone's body.

You can't spread HIV if there is no HIV infection. If you and your partners are not infected with HIV, there is no risk. An "undetectable viral load does NOT mean "no HIV infection." If there is no contact with blood or sexual fluids, there is no risk. HIV needs to get into the body for infection to occur.

Safer sex guidelines are ways to reduce the risk of spreading HIV during sexual activity.

How can I get infected by HIV?

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Unless you are 100% sure that you and the people you are with do not have HIV infection, you should take steps to prevent getting infected. People recently infected (within the past 2 or 3 months) are most likely to transmit HIV to others. This is when their viral load is the highest. In general, the risk of transmission is higher with higher viral loads. This fact sheet provides an overview of HIV prevention, and refers you to other fact sheets for more details on specific topics.

Sexual Activity
You can avoid any risk of HIV if you practice abstinence (not having sex). You also won't get infected if your penis, mouth, vagina or rectum doesn't touch anyone else's penis, mouth, vagina, or rectum. Safe activities include kissing, erotic massage, masturbation or hand jobs (mutual masturbation). There are no documented cases of HIV transmission through wet clothing.

Having sex in a monogamous (faithful) relationship is safe if:

* Both of you are uninfected (HIV-negative)
* You both have sex only with your partner
* Neither one of you gets exposed to HIV through drug use or other activities

Oral sex has a lower risk of infection than anal or vaginal sex, especially if there are no open sores or blood in the mouth. See Fact Sheet 152 for more information on the risks of various behaviors.

You can reduce the risk of infection with HIV and other sexually transmitted diseases by using barriers like condoms. Traditional condoms go on the penis, and a new type of condom goes in the vagina or in the rectum.
Some chemicals called spermicides can prevent pregnancy but they don't prevent HIV. They might even increase your risk of getting infected if they cause irritation or swelling.

Drug Use
If you're high on drugs, you might forget to use protection during sex. If you use someone else's equipment (needles, syringes, cookers, cotton or rinse water) you can get infected by tiny amounts of blood. The best way to avoid infection is to not use drugs.

If you use drugs, you can prevent infection by not injecting them. If you do inject, don't share equipment. If you must share, clean equipment with bleach and water before every use.

Some communities have started exchange programs that give free, clean syringes to people so they won't need to share.

Vertical Transmission
With no treatment, about 25% of the babies of HIV-infected women would be born infected. The risk drops to about 4% if a woman takes AZT during pregnancy and delivery, and her newborn is given AZT. The risk is 2% or less if the mother is taking combination antiretroviral therapy (ART). Caesarean section deliveries probably don't reduce transmission risk if the mother's viral load is below 1000.

Babies can get infected if they drink breast milk from an HIV-infected woman. Women with HIV should use baby formulas or breast milk from a woman who is not infected to feed their babies.

Contact with Blood
HIV is one of many diseases that can be transmitted by blood. Be careful if you are helping someone who is bleeding. If your work exposes you to blood, be sure to protect any cuts or open sores on your skin, as well as your eyes and mouth. Your employer should provide gloves, facemasks and other protective equipment, plus training about how to avoid diseases that are spread by blood.