Blood
Blood is a specialized connective tissue, part liquid and part cells, that carries oxygen, nutrients, waste, hormones, and immune defenses through the cardiovascular system.
Blood is a specialized connective tissue, part liquid and part cells, that carries oxygen, nutrients, waste, hormones, and immune defenses through the cardiovascular system. This page covers what blood is made of, the three cellular components and where they come from, and the surface antigens that determine blood type.
Key Takeaways
By volume, blood is about 55% plasma and 45% cells.
Plasma's three major proteins are albumin (osmotic pressure), globulins (antibodies), and fibrinogen (clotting).
Red blood cells lack a nucleus, carry hemoglobin, have a biconcave disc shape, and live about 120 days.
White blood cells split into granulocytes (neutrophils, eosinophils, basophils) and agranulocytes (lymphocytes, monocytes).
Platelets are megakaryocyte fragments that initiate clotting; production is regulated by thrombopoietin.
All blood cells arise from common bone marrow stem cells through hematopoiesis.
ABO blood type depends on A/B surface antigens; antibodies form against whichever antigens a person lacks.
Rh status depends on the D antigen; O negative is the universal donor, AB positive the universal recipient.
Rh incompatibility between an Rh-negative mother and Rh-positive fetus risks hemolytic disease of the newborn in later pregnancies, prevented with Rho(D) immune globulin.
Blood Composition Overview
By volume, blood is about 55% liquid and 45% cells. The liquid portion is called plasma; the cellular portion includes red blood cells, white blood cells, and platelets.
Plasma
Plasma is a pale yellow fluid that makes up just over half of blood volume. It's mostly water, but it also carries proteins, hormones, nutrients, waste products, and clotting factors.
The three major plasma proteins are:
Albumin — helps maintain osmotic pressure.
Globulins — include antibodies.
Fibrinogen — important for clot formation.
The Cellular Components of Blood
Red Blood Cells (Erythrocytes)
Red blood cells have one main job: transporting oxygen from the lungs to the tissues, and carrying carbon dioxide and waste from the tissues back to the lungs. To do this efficiently, they're packed with hemoglobin, a protein that binds oxygen — each hemoglobin molecule can carry up to four oxygen molecules at a time.
To maximize space for hemoglobin, mature red blood cells lack a nucleus and other organelles. They also have a distinctive biconcave disc shape, which increases surface area for gas exchange and gives the cells flexibility to squeeze through tiny capillaries.
Red blood cells have a lifespan of about 120 days, after which they're removed and broken down by the spleen or liver. The percentage of blood volume made up by red blood cells is called hematocrit, often used clinically as a measure of oxygen-carrying capacity.
White Blood Cells (Leukocytes)
White blood cells make up less than 1% of total blood volume but are essential for immune defense. They fall into two categories based on whether they contain visible granules in their cytoplasm.
MCAT Callout — Granulocytes vs. Agranulocytes:
Granulocytes (visible granules): neutrophils, eosinophils, basophils.
Agranulocytes (no visible granules): lymphocytes, monocytes.
Granulocytes:
Neutrophils — the most abundant type; first responders to infection, specializing in fighting bacteria.
Eosinophils — target parasites and play a role in mediating allergic responses.
Basophils — the least common type; release histamine and contribute to inflammatory reactions, especially during allergies.
Agranulocytes:
Lymphocytes — include B cells, T cells, and natural killer cells; target specific pathogens and provide long-term immunity.
Monocytes — circulate in the blood and, once they migrate into tissue, become macrophages, which digest pathogens, clear debris, and contribute to chronic inflammation and immune signaling.
Even though white blood cells are few in number relative to red blood cells, they provide diverse and powerful immune responses against infections and other threats.
Platelets (Thrombocytes)
Platelets aren't full cells — they're fragments of large bone marrow cells called megakaryocytes. Their main role is to initiate blood clotting. When a blood vessel is injured, platelets stick to exposed collagen, become activated, and release chemicals that recruit more platelets to the site, forming a temporary plug that begins the process of coagulation.
Platelet production is regulated by a hormone called thrombopoietin, made primarily in the liver and kidneys.
All three cellular components — red blood cells, white blood cells, and platelets — originate from common stem cells in the bone marrow, through a process called hematopoiesis.
Red Blood Cell Surface Antigens and Blood Typing
Red blood cells carry specific proteins or molecules on their surface, called antigens, that the immune system can recognize. An antigen is any substance capable of triggering an immune response. For blood typing, two antigen systems matter most: the ABO system and the Rh factor.
The ABO System
A person's ABO blood type depends on which antigens are present on their red blood cells:
A antigens only → type A
B antigens only → type B
Both antigens → type AB
Neither antigen → type O
The immune system also produces antibodies against whichever antigens a person's own red blood cells don't carry:
Type A blood → anti-B antibodies
Type B blood → anti-A antibodies
Type AB blood → no antibodies
Type O blood → anti-A and anti-B antibodies
The Rh Factor
The Rh factor is a second antigen system, based on a protein found on red blood cells — the most important version is the D antigen. Red blood cells with the D antigen make a person Rh-positive; without it, Rh-negative.
Combining ABO type with Rh status gives eight possible blood types: A positive, A negative, B positive, B negative, AB positive, AB negative, O positive, and O negative.
Blood Type | Antigens Present | Antibodies Present |
|---|---|---|
A positive | A, Rh(D) | anti-B |
A negative | A | anti-B |
B positive | B, Rh(D) | anti-A |
B negative | B | anti-A |
AB positive | A, B, Rh(D) | none |
AB negative | A, B | none |
O positive | Rh(D) | anti-A, anti-B |
O negative | none | anti-A, anti-B |
Transfusion Compatibility
Understanding blood groups is essential for transfusions: when someone receives donor blood, their immune system attacks any red blood cells carrying unfamiliar antigens. For example, giving type A blood to a type B person triggers an attack from the recipient's anti-A antibodies. That's why transfusion compatibility depends on both ABO and Rh status.
O negative is considered the universal donor for red blood cells, since it carries no A, B, or Rh antigens. AB positive is the universal recipient, since those individuals have no antibodies against A, B, or Rh antigens.
Rh Factor and Pregnancy
Rh status is also important during pregnancy. If an Rh-negative mother carries an Rh-positive fetus, fetal blood can mix with maternal circulation during delivery. The mother's immune system may then recognize the Rh antigen as foreign and begin producing anti-Rh antibodies — this usually doesn't affect the first pregnancy, but becomes a risk in future pregnancies if the fetus is again Rh-positive.
Maternal anti-Rh antibodies can cross the placenta and attack the fetus's red blood cells, potentially causing hemolytic disease of the newborn. To prevent this, Rh-negative mothers are given an injection of Rho(D) immune globulin, which blocks the immune response and protects future pregnancies.
While ABO and Rh are the most clinically important and most heavily emphasized blood group systems, other recognized blood group systems and hundreds of distinct blood types exist based on additional surface antigens — though these fall outside standard MCAT-level detail.
Common MCAT Mistakes
Reversing ABO antibody logic. Antibodies form against the antigens a person's red blood cells lack, not the ones they carry. Type A blood has anti-B antibodies (not anti-A); type O has both anti-A and anti-B, since it carries neither antigen.
Mixing up universal donor and universal recipient. O negative is the universal donor because its cells carry no A, B, or Rh antigens for a recipient's immune system to attack. AB positive is the universal recipient because those individuals produce no anti-A, anti-B, or anti-Rh antibodies.
Confusing granulocytes with agranulocytes. Neutrophils, eosinophils, and basophils are granulocytes (visible cytoplasmic granules); lymphocytes and monocytes are agranulocytes. Neutrophils are the most abundant white blood cell, not basophils — basophils are the least common.
Misplacing when Rh sensitization causes harm. An Rh-negative mother isn't typically sensitized until fetal blood mixes with hers during delivery, so a first Rh-incompatible pregnancy is usually unaffected — the risk of hemolytic disease of the newborn appears in subsequent Rh-positive pregnancies, which is why Rho(D) immune globulin is given.
MCAT-Style Concept Check
Question: A person's plasma contains anti-A antibodies but no anti-B antibodies. Based on the ABO system, what is this person's blood type?
A) Type A
B) Type B
C) Type AB
D) Type O
Answer: B
Explanation: The immune system produces antibodies against whichever ABO antigens a person's own red blood cells don't carry. Having anti-A antibodies means this person's cells lack the A antigen; having no anti-B antibodies means their cells do carry the B antigen. Cells carrying only the B antigen define type B blood, which is exactly the pattern (anti-A present, anti-B absent) described here — type A would show the reverse (anti-B present, anti-A absent), type AB would have no antibodies, and type O would have both.
FAQ
What percentage of blood is plasma versus cells?
Blood is about 55% plasma (liquid) and 45% cells (red blood cells, white blood cells, and platelets) by volume.
Who is the universal blood donor and who is the universal blood recipient?
O negative is the universal donor, since its red blood cells carry no A, B, or Rh antigens. AB positive is the universal recipient, since those individuals have no antibodies against A, B, or Rh antigens.
What's the difference between granulocytes and agranulocytes?
Granulocytes (neutrophils, eosinophils, basophils) have visible granules in their cytoplasm; agranulocytes (lymphocytes, monocytes) don't. Both groups are types of white blood cells involved in immune defense.
Why do Rh-negative mothers receive Rho(D) immune globulin during pregnancy?
If an Rh-negative mother carries an Rh-positive fetus, exposure to fetal blood during delivery can trigger her immune system to make anti-Rh antibodies, which could attack the red blood cells of a future Rh-positive fetus and cause hemolytic disease of the newborn. Rho(D) immune globulin blocks this immune response, protecting future pregnancies.
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