HIV Infection and T Cell Counts Lab

BACKGROUNDHIV, Human Immunodeficiency Virus, is a virus that targets cells of the human immune system with a CD4 receptor in their cell membrane. Infected cells are eventually killed by HIV. Helper T lymphocytes (TH) have CD4 receptors. As the TH cell population declines, so does the person’s ability to mobilize an immune response to any microbe. HIV can be contracted through contact with bodily fluids such as semen, vaginal fluid, and blood, from mother to unborn child or through breastfeeding, through contaminated needles, but NOT by contact with saliva, sweat, or tears.

When the TH cell count drops below 500 cells per ml, the body has difficulty defending itself against certain microbes. The HIV-infected individual may begin to suffer from a number of diseases called opportunistic infections, such as skin rashes and fungal infections. Beneath a critical level of 200 TH cells per mL, life-threatening illnesses, including pneumonia, tuberculosis, and cancer, may appear. At this point, the TH-depleted individual is diagnosed with the disease AIDS, Acquired Immune Deficiency Syndrome.

In this activity, you’ll monitor TH counts using drawings that represent hemacytometer fields. Hematocytometers are special microscope slides that are designed to be used in performing blood counts. Etched into the surface of the slide are tiny grids called counting chambers.

In order to make a blood cell count, a volume of diluted blood is placed within the counting chamber. The cells within the grid are tallied. This cell number is adjusted to reflect both the dilution and volume within the counting chamber. The final number represents the number of blood cells per 1 mL of blood. (Note: a teaspoon holds about 5mL of fluid.)

PURPOSEYou will determine the TH cell counts of a number of blood samples taken over time from an individual infected with HIV.

MATERIALSCounting chamber diagrams.

PROCEDUREDetermining TH Cell Counts1. Develop a counting pattern that is easy to remember and repeat. Make sure you don’t count the cells more than

once.2. The following counting chambers represent stained TH cells measured once per year. To obtain the numbers of

TH cells in 1 mL, you’ll need to multiply the cell chamber counts by 20.3. Count and record the number of cells in each sample. Then, calculate and record the number of cells found in 1

mL of that sample’s blood.

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ANALYSIS

Table 1. _____________________________________________________________________________

TH Cell Samples TH Cells in Chamber TH Cells per mL1985

1986

1987

1988

1989

1990

1991

1992

1993

1994

Create a line graph of the data from Table 1. The first column is your independent variable. Use the 3 rd column of data for the dependent variable.

Graph 1. Depletion of TH Cells Over a Nine Year Period

Use the background information provided with this lab, the data and analysis, and your knowledge of viruses to answer the questions below.

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1. What system of the body does the HIV virus affect? _____________________________

2. What type of cell is killed by the HIV virus? _____________________________

3. What does HIV stand for? ___________________ _______________________________________ ____________________ What does AIDS stand for? ________________________ __________________________ ________________________________ _____________________________

4. Distinguish between “HIV” and “AIDS”.

5. How can HIV be spread from person to person?

6. What is an opportunistic infection?

7. Using the data, identify the most likely point on the graph at which opportunistic infections will begin to appear. According to the background, how did you know this was the point?

8. Using the data, identify the point on the graph when the HIV-infected person will be diagnosed with AIDS. According to the background, how did you know this was the point?

9. What is a hemacytometer (define it with word parts first, then use the information available to you to give a better description of one)?

10. Why do you have to multiply the grid counts by 20? (HINT: Look at the 4th paragraph of the background)?

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