Disorders of the immune system

Our immune system does the best to protect us against pathogens ensuring that we fight off disease or prevent getting it in the first place. However, sometimes things in the immune system do not function in our best interests and often have dire consequences.

Autoimmune diseases

An autoimmune disease is a disease in which the body identifies some self cells as non-self cells and attacks them. (VCAA examiners report 2008). As a result the B and T lymphocytes attack and destroy self- cells as if they were invading pathogens. This kind of attack leads to a number of diseases such as Diabetes mellitus, Rheumatoid arthritis and Multiple sclerosis.

Diabetes mellitus (type 1)

A disease characterised by the attacking of self cells in the pancreas. This causes it to not produce insulin, that is  normally used to reduce blood sugar levels and as a result blood sugar levels are not regulated.

Rheumatoid arthritis

A disease characterised by inflammation the affects the joints caused by the lymphocytes attacking the collagen in the joints.

Multiple sclerosis

A disease characterised by the breakdown of the myelin sheath surrounding the axon on the nerve cells through the attack of immune cells. 

Allergies

An allergic response occurs when the body makes antibodies (specifically IgE antibodies) against harmless antigens known as allergens such as dust, pollen and plant spores causing an immune response. When a person contains antibodies to a specific allergen they are said to be sensitised to that allergen. Further exposure to the allergen causes cross links to be formed between the IgE antibodies on the mast cells and the allergen, this triggers the mast cells to release histamine. The release of histamine causes contraction of the smooth muscle, such as in the muscle tubules leading to the lungs decreasing the diameter of the tubules leading to difficulty breathing. Blood vessels also dilate in response to histamine allowing cells and serum to move into the surrounding tissues causing swelling and inflammation.

Allergic responses can be fatal and in order to be avoided an individual needs to be desensitised from a particular allergen. This involves exposing the person will small dose of the allergen that produces the allergy. This allergen links with the IgE making less IgE antibodies available for any further reaction, reducing the severity of further reaction.

Rejection of transplanted tissue

Successful transplantation relies on the organ to be inserted to be as close as the MHC markers on the individual receiving it. With the exception of identical twins the antigens on an individual’s tissues are not the same as anybody else’s tissues, so it important the individuals are “typed” on the basis of their antigens. Selection of the appropriate transplanted tissue is chosen on the basis of minimising differences by selecting to transplant organs between individuals of the same type. Failure to successfully transplant organs of the same type causes the individual to generate an immune response against the tissue containing the foreign antigens, causing the body to reject the tissue.  Drugs are available to inhibit the immune response and often organ donor recipients need to take immunosuppressants such as Cyclosporine for the rest of their life that acts specifically against the T cells to prevent tissue rejection.

Rhesus incompatibility

Rhesus is a protein that resides on the red blood cells. People who have this protein are considered to be Rhesus positive and if an individual does not have this protein they are termed Rhesus negative.The presence of the protein on the red blood cells is genetically  determined. The Rhesus protein is of a concern during pregnancy as if a Rhesus negative mother has a rhesus positive child and the Rhesus positive proteins in the baby’s blood comes into contact with the Rhesus negative mother’s blood an immune response in the mother is generated, producing antibodies in response to the Rhesus protein (what the mother’s immune system deems as foreign antigens) resulting in implications for the child. This is generally not an issue for the first pregnancy because few if any blood cells from the fetus move across the placenta into the mother. It is during the birthing process whereby the placenta separates away from the uterus wall that a significant number of the baby’s red blood cells cross into the mother’s blood stream whereby the immune response generated results in antibodies being made that reside in her blood stream. During a second pregnancy these antibodies  and memory B cells that have been circulating in the mother’s blood stream since the first pregnancy that are able to pass the placenta into the fetus reacting with the Rhesus proteins on the child causing damage to the tissues and organs.   The baby’s blood cells are destroyed and their contents circulate in the blood stream giving the baby a jaundiced appearance (yellowed skin). This is known as hemolytic disease.

 Treatment is available to Rhesus negative women who have Rhesus positive babies as the mothers can get injected with immunoglobulin high in Rhesus antibody, that react with and remove the fetal red blood cells in the mothers circulation so the mother does not have to generate her own antibodies in response to the Rhesus protein. Consequently, during the  second pregnancy no pre-made antibodies or memory B cells against the Rhesus protein are present in the mother’s blood to generate an immune response. This immunoglobulin is usually administrated within 72 hours of the birth and needs to be repeated for each subsequent pregnancy.

It is important to note the immune response to the Rhesus factor is only a concern for Rhesus negative mothers having Rhesus positive children not any other possible rhesus type combination in mother and child is of a concern.

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