9.17: Blood Types and Transfusions
- Page ID
- 100111
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\(\newcommand{\avec}{\mathbf a}\) \(\newcommand{\bvec}{\mathbf b}\) \(\newcommand{\cvec}{\mathbf c}\) \(\newcommand{\dvec}{\mathbf d}\) \(\newcommand{\dtil}{\widetilde{\mathbf d}}\) \(\newcommand{\evec}{\mathbf e}\) \(\newcommand{\fvec}{\mathbf f}\) \(\newcommand{\nvec}{\mathbf n}\) \(\newcommand{\pvec}{\mathbf p}\) \(\newcommand{\qvec}{\mathbf q}\) \(\newcommand{\svec}{\mathbf s}\) \(\newcommand{\tvec}{\mathbf t}\) \(\newcommand{\uvec}{\mathbf u}\) \(\newcommand{\vvec}{\mathbf v}\) \(\newcommand{\wvec}{\mathbf w}\) \(\newcommand{\xvec}{\mathbf x}\) \(\newcommand{\yvec}{\mathbf y}\) \(\newcommand{\zvec}{\mathbf z}\) \(\newcommand{\rvec}{\mathbf r}\) \(\newcommand{\mvec}{\mathbf m}\) \(\newcommand{\zerovec}{\mathbf 0}\) \(\newcommand{\onevec}{\mathbf 1}\) \(\newcommand{\real}{\mathbb R}\) \(\newcommand{\twovec}[2]{\left[\begin{array}{r}#1 \\ #2 \end{array}\right]}\) \(\newcommand{\ctwovec}[2]{\left[\begin{array}{c}#1 \\ #2 \end{array}\right]}\) \(\newcommand{\threevec}[3]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \end{array}\right]}\) \(\newcommand{\cthreevec}[3]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \end{array}\right]}\) \(\newcommand{\fourvec}[4]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \\ #4 \end{array}\right]}\) \(\newcommand{\cfourvec}[4]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \\ #4 \end{array}\right]}\) \(\newcommand{\fivevec}[5]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \\ #4 \\ #5 \\ \end{array}\right]}\) \(\newcommand{\cfivevec}[5]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \\ #4 \\ #5 \\ \end{array}\right]}\) \(\newcommand{\mattwo}[4]{\left[\begin{array}{rr}#1 \amp #2 \\ #3 \amp #4 \\ \end{array}\right]}\) \(\newcommand{\laspan}[1]{\text{Span}\{#1\}}\) \(\newcommand{\bcal}{\cal B}\) \(\newcommand{\ccal}{\cal C}\) \(\newcommand{\scal}{\cal S}\) \(\newcommand{\wcal}{\cal W}\) \(\newcommand{\ecal}{\cal E}\) \(\newcommand{\coords}[2]{\left\{#1\right\}_{#2}}\) \(\newcommand{\gray}[1]{\color{gray}{#1}}\) \(\newcommand{\lgray}[1]{\color{lightgray}{#1}}\) \(\newcommand{\rank}{\operatorname{rank}}\) \(\newcommand{\row}{\text{Row}}\) \(\newcommand{\col}{\text{Col}}\) \(\renewcommand{\row}{\text{Row}}\) \(\newcommand{\nul}{\text{Nul}}\) \(\newcommand{\var}{\text{Var}}\) \(\newcommand{\corr}{\text{corr}}\) \(\newcommand{\len}[1]{\left|#1\right|}\) \(\newcommand{\bbar}{\overline{\bvec}}\) \(\newcommand{\bhat}{\widehat{\bvec}}\) \(\newcommand{\bperp}{\bvec^\perp}\) \(\newcommand{\xhat}{\widehat{\xvec}}\) \(\newcommand{\vhat}{\widehat{\vvec}}\) \(\newcommand{\uhat}{\widehat{\uvec}}\) \(\newcommand{\what}{\widehat{\wvec}}\) \(\newcommand{\Sighat}{\widehat{\Sigma}}\) \(\newcommand{\lt}{<}\) \(\newcommand{\gt}{>}\) \(\newcommand{\amp}{&}\) \(\definecolor{fillinmathshade}{gray}{0.9}\)Blood typing and transfusion safety depend on the interaction between antigens and antibodies.
- Explain how antigens and antibodies help the body recognize self and non-self.
- Describe how the ABO and Rh systems determine blood type and compatibility.
- Discuss why blood typing and cross-matching are essential for safe transfusions.
Antigens and Antibodies: How the Body Recognizes “Self” and “Non-Self”
Your immune system is constantly on the lookout for anything that does not belong in your body. To tell friend from foe, it relies on tiny molecular markers called antigens. An antigen is any molecule — most often a protein or carbohydrate attached to a protein — that can trigger an immune response. These markers are found on the surface of all cells, bacteria, and viruses, and the immune system can usually tell whether they belong to the body (“self”) or are foreign (“non-self”).
When the immune system detects an unfamiliar antigen, it responds by making special proteins called antibodies. These Y-shaped antibodies specifically recognize and bind to that antigen, marking it for destruction or neutralization. Each antibody fits its matching antigen like a lock and key, allowing the immune system to target invaders with great precision.
The antigen–antibody system is the foundation of immune defense and is also key to understanding many biological processes — including how blood typing and transfusions work, which you will explore next.
Antigens and Antibodies: The Key to Blood Typing and Transfusions
As mentioned above, every person’s red blood cells (RBCs) carry such specific surface markers called antigens. They act like “ID tags” that the immune system recognizes as belonging to the body. If a person receives blood containing unfamiliar antigens, the immune system sees them as foreign and attacks them using antibodies — also known as "immunoglobulins", these specialized proteins that circulate in the plasma and bind to their specific antigens.
In blood typing, we identify which antigens are present on a person's RBCs. Although red blood cells carry hundreds of different antigens, only the ABO and Rh systems are routinely tested because they are the most likely to trigger strong immune reactions during transfusion. Antibodies against A, B, or Rh antigens form naturally or easily after exposure to mismatched blood, leading to rapid and dangerous clumping (agglutination) of the RBCs. In contrast, antibodies to most other RBC antigens develop only after repeated transfusions or pregnancy, so they are screened for only when a patient has a history that increases that risk.
The ABO Blood Group System
If an individual is exposed to a blood group antigen (A or B) that is not recognized as self, the individual can become sensitized to that antigen. This will cause the immune system to make specific antibodies to a particular blood group antigen and form an immunological memory against that antigen. These antibodies can bind to antigens on the surface of transfused red blood cells (or other foreign tissue cells), often leading to destruction of the cells via recruitment of other components of the immune system.
Knowing a person’s blood type is essential for matching compatible blood for transfusions or organ transplants. The four main blood types — A, B, AB, and O — differ according to which antigens are present on the surface of the red blood cells and which antibodies circulate in the plasma.
Interestingly, in the ABO system, antibodies are naturally present in the plasma from birth or early infancy without prior exposure to other blood types. This happens because harmless bacteria in the digestive tract have surface molecules that resemble A and B antigens, prompting the immune system to make anti-A or anti-B antibodies automatically.
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Example: A person with type A blood has anti-B antibodies even if they have never received a transfusion.
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This is why a transfusion mismatch (like giving type B blood to a type A person) triggers an immediate and severe reaction—the antibodies are already there, ready to attack.
The antigen determines which blood types a person can donate to, while the antibody determines which types of blood their immune system will reject as foreign.
- Blood group A individuals have the A antigen on the surface of their RBCs, and blood serum contains antibodies against the B antigen. Therefore, a group A individual can only receive blood from individuals of groups A or O types, and can donate blood to individuals of groups A or AB.
- Blood group B individuals have the B antigen on the surface of their RBCs, and blood serum containing antibodies against the A antigen. Therefore, a group B individual can only receive blood from individuals of groups B or O, and can donate blood to individuals of groups B or AB.
- Blood Group AB individuals have both A and B antigens on the surface of their RBCs, and their blood serum does not contain any antibodies against either A or B antigen. Therefore, an individual with type AB blood can receive blood from any group, but can only donate blood to another group AB individual. AB blood is also known as “universal receiver.”
- Blood group O individuals do not have either A or B antigens on the surface of their RBCs, but their blood serum contains IgM antibodies against both A and B antigens. Therefore, a group O individual can only receive blood from a group O individual, but they can donate blood to individuals of any ABO blood group (i.e. A, B, O, or AB). O blood is also known as “universal donor.”
Blood types are inherited and represent genetic contributions from both parents. The gene that codes for blood type contains three alelles: IA and IB which give type A and B blood and are dominant, and i, which is recessive and codes type O. Children will have blood types similar to their parents based on inheritance. The i allele is far more commonly expressed in the gene pool than IA and IB, which is why type O blood is the most common type despite being a recessive phenotype. Type AB is the rarest because it is the combination of less commonly expressed alleles, and is the result of codominance between IA and IB alelles.
Rhesus Factor
In addition to the ABO system, another important antigen called the Rh factor is either present (Rh-positive) or absent (Rh-negative) on the red blood cells. The Rh factor gets its name from the Rhesus monkey, in which it was first studied in the 1940s. Scientists then discovered a similar antigen on human red blood cells, calling it the Rhesus D antigen, often shortened to the Rh(D) antigen.
Together, the ABO and Rh systems form the familiar blood type labels such as A+, O–, or AB+.
In contrast to the antibodies in the ABO system, in the Rh system, people do not naturally have anti-Rh antibodies. These antibodies form only after exposure to Rh-positive blood, such as during a mismatched transfusion or pregnancy (when an Rh-negative mother carries an Rh-positive fetus).
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The first exposure usually causes no major problem because it simply “teaches” the immune system to recognize the Rh antigen.
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On later exposures, however, the newly made anti-Rh antibodies can attack Rh-positive red cells, leading to a dangerous immune reaction.
Figure \(\PageIndex{2}\): Blood Typing Test Showing Agglutination with Anti-Rh Serum.
This image shows a blood typing test using three antibody solutions: anti-A, anti-B, and anti-Rh (from left to right). Only the anti-Rh sample on the right shows visible clumping, or agglutination, indicating that this blood sample is Rh-positive. The absence of clumping in the anti-A and anti-B wells means the sample lacks both A and B antigens, so the blood type is O positive (O⁺).
Blood Transfusions
Transfusion medicine is extremely effective at treating those with severe blood loss. Transfusions are often a required component of major surgeries. Due to the different antigen blood types, blood must be cross-matched during processing to avoid potential complications, such as hemolytic reaction, renal failure, and death.
The Cross-Matching Process
Much of the routine work in a blood bank involves testing both donor and recipient blood to ensure that each patient receives blood that is compatible and safe. Several laboratory tests are used to cross-match donor and recipient samples, checking for any potential reaction between the two.
Ideally, patients receive their own blood or type-specific blood products to minimize the risk of a transfusion reaction. Cross-matching further reduces this risk, but in emergencies — when time is limited or the need for transfusion is unexpected — it may not always be possible to perform this testing in advance.
Cross-matching involves mixing a sample of the recipient’s serum with a sample of the donor’s red blood cells and checking if the mixture agglutinates, or forms clumps. These clumps are the result of antibodies binding the red blood cells together. If agglutination is not obvious by direct vision, blood bank technicians check for agglutination with a microscope. If agglutination occurs, that particular donor’s blood cannot be transfused to that particular recipient. In a blood bank, it is vital that all blood specimens are correctly identified, so labeling has been standardized using a barcode system known as ISBT 128. The blood group may be included on identification by military personnel in case they need an emergency blood transfusion.
If a patient receives blood during a transfusion that is not compatible with his or her blood type, severe problems can occur. Acute hemolytic transfusion reactions occur if donated blood cells are attacked by matching host antibodies. This can cause shock-like symptoms, such as fever, hypotension, and disseminated intravascular coagulation from immune system mediated endothelial damage. Transfusion reactions are also associated with acute renal failure. Lung injury is common as well, due to pulmonary edema from fluid overload if plasma volume becomes too high or neutrophil activation during a transfusion reaction. If the donated blood is contaminated with bacteria, it may induce septic shock in the patient.
Key Terms
- antibodies: Also known as an immunoglobulin (Ig), a large Y-shaped protein produced by B-cells that is used by the immune system to identify and neutralize foreign objects such as bacteria and viruses.
- antigen: A substance that induces an immune response, usually foreign.
- hemolysis: The destruction of red blood cells from pathological causes, such as infection or immune system mediated damage.
- agglutinate: The act of red blood cells clumping together due to antibody reactivity.