Dr. Senerath Jayasekara

Senior Consultant Transfusion Physician - National Blood Transfusion Service

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What is Blood

Functions of blood

Blood is the living fluid that sustains the viability of all living cells in the body.  Blood circulates within the blood vessels due to the pumping mechanism of the heart.  Blood performs three main functions               

• Transportation
• Regulation
• Protection

Blood as a transport system

Transportation of respiratory gases

Oxygen is essential for living cells.  Blood taka up this gas from the lungs and transport to other cells incorporated into red cells in a substance called oxygenated-hemoglobin.  Carbon dioxide is produced due to the metabolic activities of the living cells and blood helps to transport this gas to the lungs to eliminate.  Carbon dioxide is transported mainly as bicarbonate dissolved in blood.

Transportation of Nutrients

Complex substances in our diet are digested into absorbable nutrients like glucose, amino acids, fatty acids, vitamins and minerals.These are absorbed into the blood through the gastrointestinal system and circulated throughout the body.

Transportation of waste products

Waste products are transported to the appropriate organs for excretion. For example urea and ammonia are diverted into kidneys to be excreted as urine and Carbon dioxide is transported into the lungs to eliminate form the body.

Transportation of other substances

Whatever the substance is to reach a destination within the body it needs the help of blood because blood is the transport system of the body. For example if you take a medicine either as a tablet or as an injection it enters into the blood circulation to reach the site of action. Hormones produced by endocrine glands are also released into the blood to reach their destination.

Regulatory Role of Blood

Human bodies have optimal functionality only if the conditions inside the body are regulated in relatively constant and stable levels. This regulation is called homeostasis. Blood and the circulatory system are directly related to the homeostasis. Body temperature, acid base balance or pH and blood pressure are important parameters maintained within stable levels.

Protective Role of Blood

Protection from micro organisms like virus, bacteria and fungi is the responsibility of the immune system. White blood cells directly engulf some of these pathogens and this process is known as phagocytosis. Immuno-globulins and antitoxins dissolved in the blood are also helpful to protect against microbes.

Protection against bleeding is another important role of blood. Platelets and a cascade of plasma proteins known as clotting factors are important to form a blood clot and to stop bleeding when there is damage to a blood vessel by injury.

Who can donate blood?

The person must fulfill several criteria to be accepted as a blood donor. These criteria are set forth to ensure the safety of the donor as well as the quality of donated blood.
 
Donor Selection Criteria

• Age above 18 years and below 60 years.
• If previously donated, at least 4 months should be elapsed since the date of previous donation.
• Hemoglobin level should be more than 12g/dL. (this blood test is done prior to each blood donation)
• Free from any serious disease condition or pregnancy.
• Should have a valid identity card or any other document to prove the identity.
• Free from "Risk Behaviours".

Risk Behaviours

• Homosexuals.
• Sex workers and their clients.
• Drug addicts.
• Engaging in sex with any of the above.
• Having more than one sexual partner

 
Type of Donors

• Voluntory non remunerated donors. (donate for the sake of others and do not expect any benefit. their blood is considered safe and healthy)
• Replacement donors. (donate to replace the units used for their friends or family members)
• Paid donors. (receive payment for donation
• Directed donors. (donate only for a specific patient's requirement)

Replacement and Paid donors are no longer accepted by NBTS. Replacement donation was carried out until recently in some blood banks due to the shortage of blood stocks. Asking patients for replacement donors will lead to more pressure on the relatives, who are already in stress and this in turn results in illegal paid donations.

Directed donations are used in certain conditions such as in rare blood groups.

NBTS achieved the mighty figure of 100% voluntory non-remunerated blood donor base.

Blood is unique among other body tissues due to its liquefied nature.  As a result of this curious researchers could obtain a blood sample and experiment with it more conveniently than any other body tissues. Some of the very early experiments involved blood transfusion from animal to animal and transfusion from animal to human.  However, most of these experiments ended up with detrimental outcome to the recipient due to severe allergic reactions.

In 1901 the Austrian Scientist Karl Landsteiner, for the first time in the history, postulated the concept of blood groups.  He obtained blood samples from several individuals and separated red cells and serum.  Once separated, he mixed the red cells of one individual with serum of another simulating a transfusion in vitro.  He demonstrated that the said mixing resulted in a visible agglutination reaction in some pairs while others did not do so.  Finally he concluded that this observation is due to the existence of blood and the ABO blood group system was discovered.  For this historical discovery Karl Landsteiner was awarded with the Nobel Prize for Medicine in 1030. 

His birthday falls on 14^th of June and World Health Organization selected this to celebrate “World Blood Donor Day” to pay tribute and motivate voluntary non remunerated blood donors. 

Landsteiner’s postulation about ABO blood group system has been scientifically proven and studied in detail.  According to the current understanding differences in the molecular structure of glyco-proteins known as antigens found on the surface of the red cells determine the blood group.  For example in ABO blood Group system there are two antigens known as A antigen and B antigen and they give rise to following blood groups. 

In 1940 Karl Landsteiner and Alexander Wiener discovered another red cell antigen. It was identified as Rhesus factor or Rhesus antigen as it was discovered while they experimented using the red cells obtained from Rhesus Monkey (Rhesus macaque). Rh antigen can be found in more than 90% of the individuals they are typed as Rh-positive. Individuals who do not have the Rh antigen on their red cells are typed as Rh-negative. Accordingly the 4 blood groups in the ABO system can be subdivided further as follows.

The type of antigen present on your red cells and the resulting blood group is genetically determined. If you have the relevant gene responsible for the formation of a particular antigen inherited form your parents your blood group is determined accordingly.

Where there is an antigen there should be an antibody Our immune system is in constant surveillance about the microbes invading our bodies. Various molecules formed by basic elements like carbohydrate and protein found on the outer surface of invading bacteria and viruses are recognized by the immune system as foreign. Since they stimulate the immune system such foreign substances are known as antigens. Once the immune system recognize a foreign antigen a protein that can react with those antigens are produced by the immune system. These proteins are known as antibodies and antibodies helps to neutralize the invading pathogens if the body expose to same pathogen in a subsequent occasion.

While this antibody production naturally occurs in healthy people immunization enhances the antibody production even before the natural pathogens invade the body. For example when you are vaccinated against polio the immune system produces antibodies against polio virus and even if you exposed to the virus subsequently the antibodies in your blood inactivate the virus and you are protected against polio.

Interestingly the immune system produces antibodies only against foreign antigens. Structures similar to A and B antigen found on the red cell are abundant in the microbes and food particles we expose in day today life. As a result of this our immune system produces antibodies against them and they are called A antibody and B antibody. However, the immune system makes sure not to produce an antibody against an antigen inherited on your red cells since it is not foreign. The production of A and B antibodies naturally occurs around 3 to 4 months after birth and the possible combination of antigen and antibodies are as follows.

Compatibility of blood donor and recipient
During a blood transfusion red blood cells obtained from a donor is given to a vein (a blood vessel that carry blood towards the heart) of the patient or recipient through a blood transfusion set. After the transfusion the donor’s red cells dissolves in the recipient’s plasma and circulates all over the recipient’s body. Since our immune system is on constant surveillance on the foreign substances entering into the blood circulation transfused red cells are also subjected to this surveillance.

The immuno-globulins or antibodies in the recipient’s plasma would react against the antigens of donor red cells if they contain a foreign antigen. For example group A blood cannot be given to a group B patient since antigen A on donor red cells reacts with A antibody in the recipients plasma giving rise to a transfusion reaction. Furthermore, prior to a safe transfusion we must consider about the antigen on donor’s red cells and antibodies in the recipient’s plasma. In the transfusion laboratory this is determined by a compatibility testing. Compatible donors and recipients are marked in the following table

Group O red cells are universal donors since they do not have A or B antigens on them they can escape the immune surveillance of any of the recipients. Group AB individuals are universal recipients since they do not have A or B antibodies in their plasma they do not mount a reaction against any of the ABO groups.