Two weeks of breeding and feeding the virus! : Part 1

In the first two articles we noted how HIV penetrates the vaginal or penile mucosa (skin), we also discussed the risks associated with Oral Sex. These are not the only routes of infection: we have direct injection of infected blood like in cases of needle-stick injuries, we have mother-to-child transmission during pregnancy and we also have anal transmission in homosexuals and those who engage in anal sex. It has been proved that anal (rectal) exposure has a greater risk than vaginal exposure but both have a much greater risk than oral exposure.

We also noted that both the foreskin and the vaginal mucosa are rich in cells called dendritic cells whose job is to capture any foreign object like bacteria and viruses. In our quest to follow the path that the Human Immunodeficiency Virus takes until somebody reaches full-blown AIDS let us examine what happens from here.

Before we jump into this, as aspiring virologists we also have to know how the blood is constituted. So for a short period we are going to be hematologists (somebody who studies blood!). This red fluid we call blood contains cells. Red blood cells (erythrocytes) give it the red color because of a chemical called hemoglobin whose function is to trap oxygen from the lungs and deliver it to other cells so that we can produce energy. We also have white blood cells whose function is to protect the body from infection. We have platelets which help in clotting when we bleed. Finally we have the fluid called plasma where everything swims in. One milliliter (1ml) of blood has about 6 billion red cells, 3 million white cells and 1,5 million platelets.

We are more concerned about the White Blood Cells (WBC) because we are worried about how a person can suddenly become frail and fail to defend himself from diseases. These WBC can also be divided into Lymphocytes (because they are associated with lymph nodes) , Granulocytes (because they have granules) and Monocytes ( because they have a single big nucleus). Don’t worry about the big names, they are only there to confuse you, I will explain to you what they all do.

Let us continue with the series and see how lymphocytes are subdivided. The family of lymphocytes is further subdivided into B-cells and T-cells. The “B” refers to a part found in birds called bursa of Fabricius where they mature, in human beings they are produced in the bone marrow and then move to the spleen where they mature. The T cells mature in the Thymus hence the “T”.

In HIV Sciences we are more concerned about this T- cell family and we will basically concentrate on them. However, I promised to explain the basic functions of Monocytes and Granulocytes. Monocytes are a family of white blood cells which swim in the blood looking for any damaged tissues. When they find this tissue they undergo a series of changes that result in them being called macrophages. These macrophages can engulf and digest any bacteria that may want to enter through the damaged tissue eg a wound. Their behaviour is similar to dendritic cells( Langerhans cells on the foreskin). When there is a wound and the bacteria is seeking to enter the body; macrophages and the injured tissue produce chemicals which call on Granulocytes to come and reinforce. These Granulocytes like Monocytes and Macrophages are able to swallow infection agents like bacteria.

Back to T-cells. How many of you have heard somebody saying “He is critically ill, actually his CD4 count was very very low?” Next time you hear them say CD4 blah blah just ask them , “You mean T-Helper cells?”

The T-cells are further categorized into T-Helper cells and T-Cytotoxic cells. The function of the T-Helper cell is to help the body to defend itself by stimulating other cells into action like the B-lymphocyte mentioned above. The T-cytotoxic (“cyto” means cell thus toxic to cell) assist in destroying other infected cells.

You see, on these cells there are certain clusters of glycoprotein (a protein coated with some sugar) attached to the outer surface. Now some Scientists decided to differentiate these clusters of glycoproteins by using numbers. So T-Helper cells carry one of this glycoproteins whose Identity Number is 4 (four). The complete name becomes Cluster Differentiation 4 or CD4 in short. Now just like we say a patient who has HIV infection is HIV-positive; we can also say a cell that carries a CD4 glycoprotein is a CD4 positive cell. That is why T-Helper cells are called CD4 cells and T-Cytotoxic cells are called CD8 cells. Just that!!!!!!!!.

We are now done with the hematology of HIV Sciences back to our beloved virus! The virus is now engulfed by the dentritic cell (Langerhan)or Macrophage . The behaviour of these cells is typically that of a Constabulary Policeman in Zimbabwe: when he catches you he beats you until you soften up and then he parades you to the Officer-in-charge; No Negotiations! These cells engulf the viral particle and begin to bash its outer coating with acids called lysozomes which are found within these cells. The ultimate goal is to present this intruder to the officer-in-charge who in this case is the CD4 (T-Helper) cell.

……….to be continued…………………..