Hematologic & Immune System Overview
Blood, lymph, and the body's defenses, working as one team.
This guide covers three closely linked systems: the blood (hematologic system), the lymphatic system, and the immune system. Together they transport oxygen and nutrients, control bleeding, and defend the body against infection.
The Three Connected Parts
- Blood
- Transport (O₂, nutrients, waste), clotting, and carrying immune cells
- Lymphatic
- Returns tissue fluid to blood, filters it, and houses immune cells
- Immune
- Defends against microbes using barriers, white cells, and antibodies
Blood and immunity touch nearly every area of nursing — anemia and oxygen delivery, infection and white-cell counts, bleeding and clotting (and blood thinners), blood transfusions, and how vaccines protect clients. Understanding the normal cells and defenses lets you reason out these high-yield NCLEX topics.
Every key point in this guide fits on one free printable page. Jump to the free cheatsheet PDF →
Blood Components
A liquid (plasma) carrying three kinds of cells.
Blood is roughly 55% plasma and 45% formed elements (cells and platelets).
- Plasma
- Liquid (mostly water) with proteins (albumin, clotting factors, antibodies), electrolytes, and nutrients
- Red blood cells (RBCs)
- Carry oxygen via hemoglobin
- White blood cells (WBCs)
- Fight infection
- Platelets (thrombocytes)
- Form clots to stop bleeding
All of these blood cells are made in the red bone marrow. The percentage of blood that is red cells is the hematocrit.
Most plasma proteins — including albumin (holds fluid in vessels) and the clotting factors — are made by the liver. This is why liver disease causes both edema (low albumin) and bleeding (few clotting factors), tying the hematologic system to the liver.
Red Blood Cells & Oxygen
The body's oxygen delivery trucks.
- Red blood cells (erythrocytes) carry oxygen using hemoglobin, an iron-containing protein, and help carry CO₂ back to the lungs.
- The kidneys sense low oxygen and release erythropoietin (EPO), which tells the bone marrow to make more red cells.
- RBCs live about 120 days; old ones are broken down by the spleen and liver (producing bilirubin).
- Making RBCs requires iron, vitamin B12, and folate.
Anemia means too few red cells or too little hemoglobin, so less oxygen reaches the tissues — causing fatigue, pallor, shortness of breath, and tachycardia. Causes follow the requirements: not enough iron (iron-deficiency anemia), missing B12 (often from lack of intrinsic factor — pernicious anemia), blood loss, or kidney disease (low EPO).
White Blood Cells
The body's defenders.
White blood cells (leukocytes) fight infection. There are five types, remembered in order of how common they are with "Never Let Monkeys Eat Bananas":
- Neutrophils
- Most common; first responders to bacterial infection
- Lymphocytes
- B cells (antibodies) & T cells (specific immunity)
- Monocytes
- Become macrophages that engulf debris and microbes
- Eosinophils
- Allergic reactions and parasites
- Basophils
- Release histamine in inflammation/allergy
A very low white-cell count (neutropenia) — common with chemotherapy — leaves a client unable to fight infection. These clients need neutropenic precautions (protect them from germs), and even a low-grade fever is a serious warning sign. A high WBC count usually signals infection or inflammation.
? Which WBC type is the first responder to a bacterial infection?
Hemostasis (Clotting)
How the body stops bleeding.
When a blood vessel is injured, the body stops the bleeding in steps:
Most clotting factors are made by the liver and require vitamin K.
Because clotting factors need vitamin K and the liver, liver disease and vitamin K deficiency both cause bleeding. The blood thinner warfarin works by blocking vitamin K (monitored with PT/INR), while heparin works differently (monitored with aPTT). Too few platelets (thrombocytopenia) also causes easy bruising and bleeding.
The Lymphatic System
Drainage, filtering, and immune defense.
The lymphatic system is a network of vessels and organs that supports both fluid balance and immunity:
- Returns fluid — collects excess fluid that leaks into tissues and returns it to the bloodstream.
- Filters lymph — lymph nodes trap microbes and house immune cells (they swell when fighting infection).
- Key organs — the spleen (filters blood, stores white cells and platelets), the thymus (where T cells mature), and the tonsils.
If lymph vessels are damaged or removed (for example, lymph nodes removed during cancer surgery), fluid can't drain and builds up in the tissue — lymphedema. This is why swelling, swollen lymph nodes, and infection risk are watched closely after such surgeries.
Immunity
Two layers of defense — general and specific.
Innate vs Adaptive Immunity
- Present
- Innate: From birth · Adaptive: Develops with exposure
- Targets
- Innate: Any invader, same response · Adaptive: A specific germ
- Includes
- Innate: Skin barrier, inflammation, fever, phagocytes · Adaptive: B cells (antibodies), T cells
- Memory?
- Innate: No · Adaptive: Yes — faster next time
Active vs Passive Immunity
- Active — the body makes its own antibodies after infection or a vaccine; long-lasting.
- Passive — antibodies are given from outside (across the placenta, in breast milk, or by injection); immediate but temporary.
A vaccine exposes the body to a harmless piece of a germ so adaptive immunity makes antibodies and "memory" cells without causing the disease. If the real germ shows up later, the immune system responds fast. This is active immunity — the basis of all immunization schedules.
? Is immunity from a vaccine active or passive? What about immunity a newborn gets from the placenta?
Clinical Applications & Key Labs
Connecting the anatomy and physiology to the bedside.
A&P at the Bedside
- Anemia — watch for fatigue, pallor, and shortness of breath; check hemoglobin.
- Infection — a rising WBC count suggests infection; a very low count means infection danger.
- Bleeding risk — low platelets or blood thinners mean fall precautions and gentle care.
- Transfusions — must be ABO/Rh compatible; watch closely for reactions.
Key Hematologic Lab Values (CBC)
Ranges vary by lab, facility, and textbook — for exams, use the range given in the question. See the full NCLEX-PN lab-value reference & sources →
- Hemoglobin (Hgb)
- ~12–18 g/dL — Oxygen-carrying capacity (low = anemia)
- Hematocrit (Hct)
- ~36–55% — % of blood that is red cells
- WBC count
- 4,500–11,000/mm³ — Infection/immune activity
- Platelets
- 150,000–400,000/mm³ — Clotting ability
- INR
- ~0.8–1.1 (higher if on warfarin) — Clotting time on warfarin
You don't need to memorize every blood disorder in A&P — you need to know that red cells carry oxygen (with iron/B12/EPO), white cells fight infection, platelets and clotting factors stop bleeding (with vitamin K and the liver), the lymphatic system drains and filters, and immunity comes in innate and adaptive forms. From there, anemia, infection, bleeding, and immunity questions follow logically.
Frequently Asked Questions
Common student questions on this topic.
What are the main components of blood?
What do red blood cells and hemoglobin do?
What are the different types of white blood cells?
How does blood clot, and why does it need vitamin K?
What does the lymphatic system do?
What is the difference between innate and adaptive immunity?
Quick-Reference Cheatsheet
Every key point from this guide on a single printable page.
Hematologic & Immune System Cheatsheet
Free, printable PDF · no spam · returning visitors skip the email step.