Immune System Introduction


   So, you ask, what does it mean to have SCID? Well, in order to answer this question, we have to go through a quick tour of the immune system.

   The immune system of the human body is comprised of a vast array of cells that fight off diseases (antigens) that are harmful to the well-being of the body. In an individual with a properly functioning immune system, the body has multiple genes that encode specific instructions for the proper design and function of the cells of the immune system.

   The immune system has two different arms: the cellular response system and the humoral response system. The main cell in the immune system is the T-cell. There are several types of T-cells, two of which are cytotoxic T-cells and helper T-cells. Cytotoxic T-cells kill cells infected with a virus (cyto-cell, toxic-deadly). Helper T-cells are signal cells in the immune system and will be discussed in detail later. Natural killer cells are also very important; they "naturally" attack antigens without having to recognize them.

   The other major cells are B-cells. B-cells produce and secrete antibodies, the cells that attach to invading bacteria and cause them to be destroyed.

   The humoral response system is composed of B-cells while the cellular response system is composed of cytotoxic T-cells. The helper T-cell, however, is very important to both parts of the immune system.


The Almighty Helper T-cell


   It can be said that there is a war going on within your body. Your immune system is fighting antigens at all times to keep your body free of viruses and bacteria. Using this analogy, we can say that our humoral and cellular response systems are like the Army and the Navy of our immune system. With two different areas of battle, however, a "Commander-in-Chief" is needed. This is where the helper T-cell fits in.

   How does the T-cell perform this function? Well, the key is in molecules called cytokines. Cytokines are small molecules that carry messages to and from different cells. Almost every cell has molecules on its surface called cytokine receptors. These receptors are intended to accept a specific cytokine and relay that message to the DNA in order to cause the cell to perform a specific action. Cytokines act in concert on different types of cells and can elicit a variety of responses depending on the situation and the environment. When a cell is infected, it releases cytokines into the bloodstream that carry a message saying that it is infected. The first cell to receive these messages is the helper T-cell. The helper T-cell can then send the proper cytokines to the humoral and cellular response systems in order to coordinate an attack against the antigen.
This is a picture of the information through cytokines. Credit for this picture goes to Peter Parham's The Immune System. As you can see, a cytokine approaches the membrane of a cell. The receptors then combine. The message is then processed and sent to the DNA in which a change is made to reflect the message delivered by the cytokine.


So, what's so bad about SCID?


   Well, the cytokine receptors on the helper T-cells are especially important. A chain of these receptors is the gamma-c cytokine receptor. This is one of several polypeptide chain that forms the IL-2, IL-15, IL-9, IL-7, and IL-4 receptors. This is one of several mutations that can lead to SCIDs. Each of these cytokines is crucial to the immune system. With a defect in the gamma-c chain, the T-cell cannot respond to an invasion. Children born with SCIDs are missing the central component of the immune system: the helper T-cell.

Essentially: To have SCID means to have no functioning immune system!!!


   Unlike HIV, which slowly attacks the pre-existing helper T-cells, SCIDs leaves a person vulnerable from the day they are born. They have to be kept in solitary protection to prevent any diseases from attacking the body. No direct contact from the outside world is safe. Thus, this disease has been labeled the "Baby in a Bubble" disease due to the fact that these children were placed in a de-infected plastic bubble to protect them. This type of existence is inhuman.



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