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What Are Leukocytes? - Definition, Types & Function

Lesson Transcript
Instructor: Lynee Carter
Leukocytes are white blood cells and are the body's greatest defense against infection and other diseases. Explore the definition and function of leukocytes, as well as the different types: neutrophils, basophils, eosinophils, monocytes, and lymphocytes. Updated: 09/23/2021


Did you know our bodies are a battlefield? Every day, 'enemy' germs in the environment try to enter our bodies, and when they are successful, we sometimes get sick. So our bodies have defending 'soldiers' that work to fight off these harmful invaders. These soldiers are leukocytes, also called white blood cells.

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  • 0:01 What Are Leukocytes?
  • 1:09 Neutrophils
  • 1:34 Basophils
  • 2:12 Eosinophils
  • 2:40 Monocytes
  • 3:10 Lymphocytes
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immune system

What are Leukocytes?

Leukocytes -- a body's army of soldiers -- are a part of our immune system. They not only fight the germs that cause disease and infections, they strive to protect us against any foreign agent that appears to be a threat. Some produce weapons in the form of antibodies while others make direct attacks. Some kill by completely devouring certain invaders. Then there are those who notify other leukocytes that an invasion has taken place.



There are five different leukocytes that accomplish specific tasks based on their abilities and the type of invaders they are fighting. They are called neutrophils, basophils, eosinophils, monocytes, and lymphocytes. Let's explore each of these in detail.


Neutrophils make up the largest amount of leukocytes in the body. They travel around in the blood looking for their main target. When the body puts out a chemical signal indicating that bacteria is trying to enter, the neutrophils are one of the first ones to arrive at the site. They then kill the invaders by consuming them. The pus that you see in an infection is made up of these neutrophils that have died during this process.


Basophils make up the smallest amount of leukocytes in the body but can have a big effect when activated. They are responsible for the allergic reactions that cause some people to have asthma attacks, swelling, hives, and difficulty breathing. Although these symptoms are not pleasant to experience, they are actually how the body responds to specific invaders. These can be things like mold, dust, animal hair, grass, and even sometimes medications. Basophils detect these invaders in the body and make antibodies that help destroy their foreign substances. They also call other leukocytes to come help in the fight.



Eosinophils have many powerful functions even though their targets are only bacteria and parasites. They carry out their duties by trapping invaders, disrupting foreign activities, and being a participant in allergic reactions. Unlike other leukocytes, they kill without having to know the specific invader. They have the ability to recognize and attack any intruder that is harmful. Eosinophils have also been known to help out in some organ formation and tissue repair.


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