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Immune Deficiency Panel by Flow Cytometry

The introduction of flow cytometry has allowed subclassification of lymphocytes on the basis of immune function. Mature T cells in the peripheral blood express either the CD4 or CD8 antigen. CD4 positive T cells are functionally defined as T helper cells, while CD8 positive cells exhibit either suppressor or cytotoxic activity. Imbalances or deficiencies in the immune system can result from abnormalities in either the CD4 or CD8 population.

HIV-1 infection causes significant changes in the number of CD4 and CD8 positive lymphocytes; within six months of seroconversion, the CD4 count falls about 30%, while the CD8 count increases by 40%, causing a decrease in the CD4/CD8 ratio. Healthy persons generally have absolute CD4 cell counts of about 1000 cells/uL and a CD4/CD8 ratio greater than 1.0. CD4 counts less than 400 cells/uL are generally associated with a progression to AIDS. There is a direct correlation between the CD4 count and the incidence of developing AIDS within four years.

>500 CD4/uL

>250-500 CD4/uL

>100-250 CD4/uL

<100 CD4/uL

Lymphadenopathy

Pneumococcal pneumonia

Pneumocystis carinii pneumonia

Cytomegalovirus retininits

Vaginal candidiasis

Pulmonary tuberculosis

Cervical cancer

Disseminated Mycobacterium avium complex

Herpes zoster

Esophageal candidiasis

Oral candidiasis

Cryptococcosis

Kaposi’s sarcoma

Persistent cryptosporidiosis

Non-Hodgkins lymphoma

Toxoplasmosis encephalitis

Anemia

Disseminated histoplasmosis

Cervical intraepithelial neoplasia

Immune restoration after treatment with highly active antiretroviral therapy (HAART) can be divided into two distinct phases with distinguishable kinetics and mechanisms. The first phase consists of a rapid increase in circulating CD4 cells within the first 8 weeks of therapy due to redistribution of lymphocytes that had been trapped in inflamed lymph nodes. The second phase is characterized by a slower increase in both CD4 and CD 8 cells, composed largely of naïve cells produced in the thymus. This slow phase persists in most persons for at least the first 2 years of therapy.

The increase in CD8 positive cells is not unique to HIV-1 infection, since many viruses and vaccinations cause a transient increase in the CD8 population. Generally, the CD4/CD8 ratio is increased in autoimmune diseases and decreased in viral infections. Cyclosporine and prednisone therapy decreases the CD4/CD8 ratio. It is important to determine absolute numbers of CD4 and CD8 positive cells (cells per uL), not just the CD4/8 ratio, in order to distinguish HIV-1 from other viral infections.

Lymphocyte

Absolute Count

Percent

Mature T Cells (CD3)

650 - 3036

65 - 92

Helper T Cells (CD4)

310 - 2112

31 - 64

CD4:8 ratio

1.0 - 1.5

Suppressor T Cells (CD8)

80 - 1353

8 - 41

The immune deficiency panel was originally designed to enumerate mature T cells (CD3), helper T cells (CD4), and suppressor T cells (CD8). The CD4:CD8 ratio was also calculated. Current clinical guidelines for monitoring patients with HIV infection recommend following only the absolute CD4 count.

Specimen requirement is one 5-mL green top (sodium heparin) tube and one 5 mL lavender top (EDTA) tube of blood. Samples should remain at room temperature and transported to the laboratory within 24 hours of phlebotomy.

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