Why do we have different blood types?

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The importance of blood in our life is unmatched. Blood is a type of CONNECTIVE TISSUE (a type of tissue that connects two organs. Blood is considered the liquid connective tissue since it connects all the organs in a body to each other) that carries oxygen in its structure to supply it to their organs to facilitate in cellular metabolism and processes.

Blood is composed of BLOOD CELLS suspended in BLOOD PLASMA. The components of blood are-

  1. RBCs, i.e., red blood cells or ERYTHROCYTES. RBCs possess a pigment called haemoglobin, which gives blood its red color. Haemoglobin is an iron-containing chemical structure that can bind to oxygen, thereby supplying oxygen to the cells of the body.
  2. WBCs, i.e., white blood cells or LEUCOCYTES. WBCs are actively engaged in the immune system and are of various types, all of which (directly or indirectly) partake in immune response in case of the arrival of a pathogen.
  3. Platelets or THROMBOCYTES. Platelets are involved in the clotting of blood as observed on a site of injury. Platelets contain substances which react with blood vessels at the site of injury and clump to form a clot.
  4. Blood PLASMA is the liquid compartment of blood, where all the blood cells are suspended. It is a yellow-colored liquid (obtained after removal of blood cells). Blood SERUM is a part of the plasma without clotting factors.
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One might recall that during a blood transplant, the hospital staff is particular about the ‘type’ of blood given to the patient. This is ensured that the blood is not REJECTED by the recipient. This property is attributed to the presence of ANTIGENS on the surface of RBCs.

Blood types are of largely four types – A, B, AB, and O. These names are given according to the antigens present on the RBCs. Antigens present on the surface of RBCs are of two types – A and B. Antigens are typically foreign substances that may enter our body and cause an immunological reaction (usually in form of generation of ANTIBODIES, which combat antigens to defend the body).

Blood group antigens are LIPOPOLYSACCHARIDES – which are carbohydrates attached to lipids. The presence of a specific antigen confers to the individual having that blood type.  

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  1. A Group

Blood group A is the oldest blood type found, which may have existed in our biological ancestors as well. The presence of A antigens means that the antibodies produced by the body are of the anti-B type, hence it will be compatible with A antigens. So, if an individual with blood group A is transplanted with the blood of group B, the antibodies of the recipient will be sought to attack and destroy the newly transplanted cells.

  1. B Group

Blood group A is the opposite of A – the presence of B antigens on the surface of RBCs and the production of anti-A type antibodies by the body. B group was a result of a MUTATION in the genes coding for the A antigen about 3.5 million years ago. An individual with the B blood group cannot receive the blood of A-type.

  1. AB Group

AB group is an example of CODOMINANCE, wherein both A and B antigens are present on the surface of the RBCs. In an individual with AB blood type, no antibodies against the blood antigens are produced – making these people UNIVERSAL RECIPIENTS. People with AB blood type can receive blood from A, B, AB as well as O blood group people.

  1. O Group

Blood group O is denoted when there are no antigens on the RBCs. This was also caused due to a mutation that occurred 2.5 million years ago, which caused inactivation of the protein which makes up the antigens. Such a person would possess antibodies for both A and B antigens – thus this person can only be supplied with O type blood but would be able to donate blood to all the types, making them the UNIVERSAL DONOR. Genetically, O type blood is RECESSIVE, meaning it is not expressed in the presence of genes coding for type A or B.

blood group

The positive or negative annotation along with the blood type refers to the RH FACTOR, which was observed in and named after the Rhesus monkey. It refers to a protein that may or may not be present in conjugation with the blood group antigen. The presence of the Rh Factor makes a person positive, whereas lack thereof makes the person negative to it. 85% of the people are Rh-positive, making the Rh-negative trait a relatively rare one.

O negative blood type is the universal donor, while the AB positive individuals are the universal recipients.

Some hypotheses about the existence of different blood types refer to the selectivity of pathogens to a particular blood type – but this has not been conclusively stated. As an evolutionary measure to ensure survival, the blood type most susceptible to disease (which is considered to be the A-type) has become the rarest, while those not that susceptible, such as the other three types, have become somewhat common. Those with A-type blood have been found to be more prone (than people with the other blood types) to malaria, which has caused them to die out – making B and AB blood types most common.

A reason states for the susceptibility of a specific blood type to disease is due to the causative agents having antibodies or antigens like the blood antigens. Remember that antigens and their specific antibodies can cause a variety of reactions, so a pathogen possessing an antigen similar in structure to a blood antigen or antibody can trigger an immunological reaction, which can prove to be fatal to the individual. For example, assume a person with blood type A. When they are infected with malaria, who produced an antigen complementary to the antibody B (remember people with blood type A have B antibodies) will cause an immunological reaction and harm the host.

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Keeping all this in mind, blood transfusions are performed with the utmost care. Once blood is donated, it is ‘typed’ using blood typing kits, checked for diseases and only then bagged in a blood bank. A person in need of blood is also subjected to a blood typing test and accordingly, blood transfusion is carried out and further treatment is administered. Although blood donation and transfusion are routine and simple procedures in the medical field, appropriate protocols must be followed to ensure no error to ensure the health of the individual.

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