(USMLE topics) Cellular basis of blood groups including ABO, Rh (Rhesus) and other less known systems, why blood typing is important in blood transfusion.
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A blood type refers to the PRESENCE or ABSENCE of a certain marker, or ANTIGEN, on the surface of a person’s red blood cells. For example, in the ABO system, presence of A or B antigen gives type A or B, presence of both antigens gives type AB, while their ABSENCE gives type O.
Blood typing is critical for blood transfusion, as there are very SPECIFIC ways in which blood types must be MATCHED between the donor and recipient for a safe transfusion. The rule is simple: patients should NOT be given antigens that their own blood does NOT have. This is because the recipient’s immune system may recognize any “NEW” antigen as “FOREIGN” and develop antibodies to target it for destruction. Depending on the scale of the triggered immune response, the reaction can be serious or fatal.
Applying the rule, a type A patient, who is NEGATIVE for B antigen, can only receive blood from type A and type O donors, whose blood does NOT contain B antigen. A type AB patient, having both antigens, can receive blood from anyone, while a type O person, being NEGATIVE for both A and B, can only receive from type O donors, but can give blood to anyone.
Another important system is the Rh system, for which, D antigen, or Rh factor, is best known. The blood type for this antigen can be either Rh-positive or Rh-negative. By the same rule, a Rh-negative patient canNOT receive blood from a Rh-positive donor, while the reverse direction is fine.
Each of the 4 types of the ABO system can be Rh-positive or negative. This gives 8 possible combinations – the 8 basic blood types everyone knows about.
But ABO and Rh are only a FRACTION of the 35 currently known blood group systems, many of which can cause serious reactions during transfusion if mismatched. Altogether there are HUNDREDS of antigens, giving rise to a gigantic number of possible blood types. A fully specified blood type should describe the COMPLETE SET of antigens that a person has. In theory, this list must be determined for both donor and recipient before a transfusion can take place. In reality, however, most people only need to care about their ABO type and Rh factor.
The ABO and Rh systems are the most important in blood transfusion for 2 reasons. First, most people can produce ROBUST antibodies against A, B and D antigens, which may NOT be the case for other antigens. In fact, anti-A and anti-B antibodies are usually developed during the first year of life. Second, the 8 basic blood types are distributed in comparable proportions that make mismatching a likely event. Most other antigens occur at such frequencies that ONLY a VERY SMALL subset of patients is potentially at risk. For example, if 99.99% of a population is positive for a certain antigen and only 0.01% is negative, only that tiny fraction of negative patients is at risk regarding that antigen. To account for possible INcompatibility OUTSIDE ABO and Rh, an ADDITIONAL test is usually made before transfusion. A blood sample from the patient is mixed with a sample of donor blood and the mixture is examined for CLUMPS. No clumping means a compatible match.

In this video, Dr Mike explains the different ABO blood types and discusses who can give and receive blood in transfusions.

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Diabetes refers to a group of conditions characterized by a high level of blood glucose, commonly referred to as blood sugar. Too much sugar in the blood can cause serious, sometimes life-threatening health problems.
There are two types of chronic diabetic conditions: type 1 diabetes and type 2 diabetes. Pregnant women may acquire a transient form of the disease called “gestational diabetes” which usually resolves after the birth of baby. Pre-diabetes is when the blood sugar level is at the borderline: higher than normal, but lower than in diabetics. Prediabetes may or may not progress to diabetes.
During food digestion, carbohydrates – or carb – break down into glucose which is carried by the bloodstream to various organs of the body. Here, it is either consumed as an energy source – in muscles for example – or is stored for later use in the liver. Insulin is a hormone produced by beta cells of the pancreas and is necessary for glucose intake by target cells. In other words, when insulin is deficient, muscle or liver cells are unable to use or store glucose, and as a result, glucose accumulates in the blood.
In healthy people, beta cells of the pancreas produce insulin; insulin binds to its receptor on target cells and induces glucose intake.
In type 1 diabetes, beta cells of the pancreas are destroyed by the immune system by mistake. The reason why this happens is unclear, but genetic factors are believed to play a major role. Insulin production is reduced; less insulin binds to its receptor on target cells; less glucose is taken into the cells, more glucose stays in the blood. Type 1 is characterized by early onset, symptoms commonly start suddenly and before the age of 20. Type 1 diabetes is normally managed with insulin injection. Type 1 diabetics are therefore “insulin dependent”.
In type 2 diabetes, the pancreas produces enough insulin but something goes wrong either with receptor binding or insulin signaling inside the target cells. The cells are not responsive to insulin and therefore cannot import glucose; glucose stays in the blood. In other words, type 2 diabetics are “insulin resistant”. Here again, genetic factors predispose susceptibility to the disease, but it is believed that lifestyle plays a very important role in type 2. Typically, obesity, inactive lifestyle, and unhealthy diet are associated with higher risk of type 2 diabetes. Type 2 is characterized by adult onset; symptoms usually appear gradually and start after the age of 30. Type 2 diabetes accounts for about 80 to 90% of all diabetics. Management focuses on weight loss and includes a low-carb diet.
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(USMLE topics) Structure, functions of the BBB. Physical, transport and metabolic barriers. Non-barrier areas. Strategies to overcome the barrier, to deliver therapeutic drugs into the brain. This video is available for instant download licensing here : https://www.alilamedicalmedia.com/-/galleries/narrated-videos-by-topics/basic-neurobiology/-/medias/d278c4e0-c462-4a43-991c-ff769ed3cf6d-blood-brain-barrier-narrated-animation
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Voice by: Ashley Fleming
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The blood-brain barrier refers to the highly selective permeability of blood vessels within the central nervous system. The barrier controls substances that can enter or leave the nervous tissue. It helps maintain the stable state, or homeostasis, of brain tissue, amid the fluctuations of circulating substances in the blood, many of which can act as neurotransmitters and could create chaos in neuronal activities if allowed to diffuse freely into the brain. The barrier also protects the brain from blood-borne pathogens and toxins.
The blood-brain barrier is composed of several cell types, including:
– Endothelial cells that form the wall of blood vessels;
– Mural cells, namely pericytes, partially covering the outside of endothelial cells;
– And glial cells astrocytes, whose extended processes, called end-feet, wrap around the vessels.
The endothelial cells alone can fulfill the functions of the blood-brain barrier, but their interactions with the adjacent cells seem to be required for its formation, maintenance and regulation.
The brain endothelial cells, unlike their counterparts in other tissues, possess unique properties that allow them to tightly control the passage of substances between the blood and brain. These properties can be classified into physical, transport, and metabolic categories:
– The brain endothelial cells are held together by tight junctions, which serve as physical barriers, preventing movements of substances through the space between cells.
– They have very low rates of vesicle-mediated transcellular transport.
– They control the movement of ions and substances with specific transporters, of which there are two major types: efflux transporters and nutrient transporters:
+ Efflux transporters use cellular energy to move substances against their concentration gradient. These transporters are usually located on the blood side of endothelial cells. They transport lipophilic molecules, which have passively diffused through the cell membrane, back to the blood.
+ Nutrient transporters, on the other hand, facilitate the movement of nutrients, such as glucose and essential amino acids, into the brain, down their concentration gradient.
– The brain endothelial cells also contains a number of enzymes that metabolize, and thus inactivate, certain neurotransmitters, drugs and toxins, preventing them from entering the brain.
An intact blood-brain barrier is critical for normal brain functions. Neurological diseases such as encephalitis, multiple sclerosis, brain traumas, Alzheimer’s disease, epilepsy, strokes and tumors, can breach the barrier, and this, in turn, contributes to disease pathology and further progression.
But not all areas of the brain have the blood-brain barrier. Some brain structures are involved in hormonal control and require better access to systemic blood, so they can detect changes in circulating signals and respond accordingly. These non-barrier areas are located around the midline of the ventricular system, and are known as circumventricular organs. Some of their bordering regions have a leaky barrier.
The blood-brain barrier also has its downside. While it protects the brain from unwanted drugs and toxins, it also prevents therapeutic drugs from entering the central nervous system to treat diseases. Several strategies are developed to overcome this obstacle.
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