A little on the Immune System…
Dear Friends,
In order to better understand this issue, we must take a look at the surrounding elements involved in the Immune System – without looking at the whole system:
Lymphocytes
1. Lymphocytes occur in two generaal types. T lymphocytes and B lymphocytes.
2. [B]T lymphocytes[/B] are developed in the bone marrow and migrate to the thymus where they become imprinted. (a). T cells provide [B]cell-mediated immunity[/B] against viruses, fungi, transplanted human cells and cancer cells.
3. [B]B lymphocytes[/B] are developed in the bone marrow and are responsible for [B]humoral immunity[/B] against bacterial and fungal infections.
Functions of T lymphocytes
A. Types of T lymphocytes
1. [B]Killer T Cells[/B] destroy cells that are identified by specific antigens on their cell surface. This is termed [B]cell-mediated immunity[/B] as the T cells come in physical contact with the destroyed cell.
2. Helper and suppressor T cells regulate the responses of B cells and killer T cells. (a) [B]helper T cells[/B] increase the activity of B cells and killer T cells and [B]suppressor T cells[/B] decrease their activities.
3. T cells and macrophages (function in the destruction of foreign antigens (bacteria/viruses) secrete polypeptides (molecular chain of amino acids) that regulate many aspects of the immune system. These products are called cytokines (interleukin or interferon), or lymphokines if secreted by lymphocytes.
4. T cells do not make antibodies and have no antibodies on their surface. They have specific antigen receptors on their membrane surface. (a) T cells can’t combine to free antigens. Antigens must be presented to them by [B]antigen-presenting cells[/B] such as macrophages. (b) the macrophages have antigens on their surface called [B]histocompatibility antigens[/B] which identify them as belonging to the host. These antigens are called [B]MHC[/B] antigens after the gene which codes for their manufacture.
B. Interactions between macrophages and T lymphocytes
1. The MHC genes produce two classes of molecules: (a) class 1 are made by all cells in the body except red blood cells. (b) class 2 are made only by macrophages and B lymphocytes.
2. Killer T cells can interact only with antigens presented with class 1 MHC molecules; helper T cells can interact only with antigens presented with class 2 MHC molecules.
3. When a foreign particle such as a virus enters the body the following sequence of events occurs: (a) a macrophage digests the particle and foreign antigens are moved to the surface of the cell membrane to form a complex with the class 2 MHC molecules. (b) the macrophage presents the antigen to a helper T cell and sttimulates it to secrete a cytokine (interleukin 1). (c) interleukin 1 stimulates cell division of helper T cells which secrete [B]macrophage colony stimulating factor[/B] to stimulate macrophage division and [B]interleukin 2[/B] which stimulates division of killer T cells. (d) killer T cells destroy cells infected with virus particles. (e) helper T cells also stimulate the activity of B cells which secrete antibodies.
4. Antibodies are usually only produced against non-self antigens and not self antigens. This is called immunological [B]tolerance.[/B]
C. Tumor Immunology
1. Tumor antigens activate the immune system and promote an attack by killer T cells and natural killer cells. (a) natural killer cells destroy tumors in a non-specific fashion and do not require prior exposure for sensitization to the tumor antigens.
2. According to the concept of [B]immunological surveillance[/B] against cancer, the immune system constantly recognizes and destroys tumor cells.
V. Diseases caused by the immune system
A. Autoimmunity
1. When the immune system doesn’t recognize and tolerate self-antigens it causes an autoimmune disease by the production of antibodies against self-antigens.
2. Autoimmune diseases can result from several different mechanisms: (a) an antigen that does not normally circulate in the blood may become exposed to the immune system. (b) antibodies may be produced aainst other antibodies. (c) antibodies may cross react with self-antigens.
B. Allergies
[B]1. Allergies[/B] are abnormal immune responses to antigens or [B]allergens.[/B]
2. There are two types of allergy: (a) [B]immediate hypersensitivity -[/B]a B cell response that can produce symptoms within minutes. (b) [B]delayed hypersensitivity -[/B]a T cell response that can produce symptoms within 48 hours.
Regards to all.
Jethro