HIV attacks certain white blood cells, called CD4 cells. While acute HIV might lower your white blood cell count, it might not be noticeably lower.

Your white blood cells fight off infections, forming an important part of your immune system.

During the acute stage of HIV infection, your white blood cell count may decrease slightly. But the difference might be so slight that it might not fall out of the “normal” range for a white blood cell count.

A white blood cell count is a type of blood test that measures the number of white blood cells in your body. HIV directly attacks CD4 cells, which make up a small percentage of your white blood cells.

At any stage of HIV infection, including acute HIV infection, your CD4 cell count may be low. HIV treatments can help your CD4 count stay at a healthy level.

There are a number of different kinds of white blood cells, including:

CD4 cells are a type of T-cell, which is a subtype of lymphocytes. Lymphocytes make up 20–40% of your white blood cells.

Acute HIV infection, which is the initial stage of HIV infection, generally occurs 2–4 weeks after someone contracts the virus. During this stage, the virus attacks and destroys CD4 cells, leading to a lower white blood cell count.

But because CD4 cells are only a small part of your overall white blood cell count, the decrease may only be slight. For this reason, a white blood cell count alone can’t determine whether you have acute HIV infection or not.

Usually, HIV tests look at your antibodies and antigens. They only test positive 18–90 days after exposure, depending on the type of test that’s done. As such, standard HIV screening tests won’t necessarily detect acute HIV infection.

If you suspect you’ve been exposed to HIV, it’s important to get medical help as soon as possible. Taking postexposure prophylaxis (PEP) medications within 72 hours after exposure can help prevent the development of HIV.

If more than 72 hours have passed, you should still consult with a doctor or other healthcare professional. Early diagnosis and treatment with antiretroviral drugs can help reduce the risk of long-term complications.

A low white blood cell count is sometimes called leukopenia. HIV — at any stage — can cause leukopenia. But a low white blood cell count can be caused by other conditions.

Common reasons for a low white blood cell count can include:

Because white blood cells fight off infection, your white blood cell count might increase when your body detects a threat. This “threat” could be an illness or infection.

A high white blood cell count, called leukocytosis, can be triggered by:

Chronic (long-term) high white blood cell counts can be caused by:

If you have an abnormal white blood cell count, whether it’s low or high, a clinician might order additional blood tests to determine the cause.

Standard HIV screening tests seldom detect acute HIV infection.

HIV tests can include:

  • antibody tests
  • combined antigen and antibody tests (also called 4th generation tests)
  • viral load tests
  • HIV RNA viral load test
  • p24 antigen blood test

If your first HIV test result is positive, a healthcare professional will order follow-up testing to confirm your result.

All HIV tests have a window period, which differs from one test type to another. This means they won’t give you a positive result immediately after you’re exposed to the virus.

Anemia occurs when your red blood cell count is low. HIV doesn’t usually affect your red blood cells, although some people do experience HIV-related anemia, typically at a later stage of infection. But red blood cells are seldom affected during the acute stage.

HIV attacks your CD4 cells. Although your white blood cell count may be slightly lower during the acute stage of HIV infection, the change is usually so slight that you may still fall in the “normal” range for white blood cells.

If you think you’ve been exposed to HIV, seek emergency medical help. The earlier you receive HIV treatment, the better.

Sian Ferguson is a freelance health and cannabis writer based in Cape Town, South Africa. She’s passionate about empowering readers to take care of their mental and physical health through science-based, empathetically delivered information.