Roll of CD4 cells on HIV/ AIDS

CD4 T-cells & HIV

CD4 is a glycoprotein found on the surface of immune cells such as T helper cells, monocytes and macrophages cells. CD4 cells (sometimes called T-cells, T-lymphocytes, or helper cells) are white blood cells that play an important role in the immune system. Your CD4 cell count gives you an indication of the health of your immune system – your body’s natural defence system against pathogens, infections and illnesses.

CD4 T-cells are considered "helper" cells because they do not neutralize infections but rather triggers the body's response to infections. In response, CD8 T-cells—classified as such because of the type of glycoprotein on their surface—play a part as so-called "killer" cells by producing antiviral substances (antibodies) that help fight off foreign invaders.

Structure of CD 4 cells:

Like many cell surface receptors/markers, CD4 is a member of the immunoglobulin super family

It has four immunoglobulin domains (D1 to D4) that are exposed on the extra cellular surface of the cell:

• D₁ and D₃ resemble  immunoglobulin variable (IgV) domains.
• D₂ and D₄ resemble immunoglobulin constant (IgC) domains.

CD4 uses its D₁ domain to interact with the β₂-domain of MHC class II  molecules. T cells expressing CD4 molecules (and not CD8) on their surface, therefore, are specific for antigens presented by MHC II and not by MHC class II (they are MHC class II-restricted). MHC class I contains Beta-2 macroglobulin.

How HIV Affects CD4 Cells:

As a retrovirus, HIV do not have own enzymes necessary for protein & nuclic acid synthesis and so depend upon synthetic machinery oh host cells. Therefore, HIV needs to infect certain "host" cells in order to make copies of itself. CD4 cells are the prime targets for this in the course of an infection.

During infection, HIV attaches to surface of CD4 cells, emptying its genetic material within so that the host's genetic coding can be altered to produce other HIV virions.

How Do We Measure CD4 T-Cells

By determining how many functioning CD4 cells are circulating in the blood, a doctor can determine the status of a person's immune system. A simple blood tested called the CD4 COUNT estimates the number of functioning CD4 cells in a cubic millimetre of blood. The higher the CD4 count, the stronger the immune function.

Your CD4 cell count is the measurement of the number of blood cells in a cubic millimeter of blood (a very small blood sample). It is not a count of all the CD4 cells in your body.

• The CD4 cell count of a person who does not have HIV can be anything between 500 and 1500.
• People living with HIV who have a CD4 count over 500 are usually in pretty good health.
• People living with HIV who have a CD4 cell count below 200 are at significant risk of developing serious illnesses. While HIV treatment is recommended for all people living with HIV, it is especially important for people with low CD4 counts.

If you have HIV and do not take HIV treatment, your CD4 count will fall over time. The lower the CD4 cell count, the greater the damage to the immune system and the greater the risk of illness.

When you take HIV treatment, your CD4 count should gradually increase