Third Line of Defense in the Body: Definition & Overview

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  • 0:04 What Is The Immune System?
  • 1:31 Specific Immunities
  • 2:46 T-Cells
  • 4:46 B-Cells
  • 5:47 Memory Cells
  • 6:18 Lesson Summary
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Lesson Transcript
Instructor: Amanda Robb

Amanda holds a Masters in Science from Tufts Medical School in Cellular and Molecular Physiology. She has taught high school Biology and Physics for 8 years.

In this lesson we'll be learning about the specific immune system, which is the third line of defense in the body against pathogens. In this lesson, we'll explore what cells are involved and how they do their jobs.

What Is the Immune System?

Think of your body as a fortress. Inside, precious cells work day and night to keep you alive. Outside the castle gates lurk dangers, like bacteria and parasites, waiting to come in and take advantage of your resources. The castle walls keep most invaders out, but some sneak through. The ones that penetrate the outer walls might get caught in the moat. However, in the unlikely event something makes it through the outer defense system, there are soldiers on guard ready to fight.

These systems are analogous to the immune system in your body. Your immune system is a collection of cells and tissues that defend your body against invaders. Like the outer walls and moat, your body has non-specific defenses that protect your body against all invaders, including your skin, certain chemicals, and cells that are on guard patrolling your body.

If pathogens do get through, your body activates the third line of defense, which is your specific immune system. Your specific immune system is made of white blood cells called lymphocytes: B-cells and T-cells. These cells recognize the pathogen invading specifically, compared to the second line of defense, which is activated by any pathogen. Your lymphocytes mount a defense system that is catered to that particular invader.

Specific Immunities

When a pathogen comes in contact with your body, it has to breach the first line of defense to get inside. Your skin and mucus membranes are the main barrier here. Mucus traps the pathogens, and then is forced out of your body when you cough or blow your nose. Your skin also secretes chemicals that have antiviral properties, killing viruses on contact. If the pathogens get through that defense, the next line is non-specific immunity cells that patrol your tissues engulfing pathogens. There are other cells that do this, like macrophages, but the dendritic cells are most important for activating the third line of defense in your body.

Dendritic cells reside in your tissues, waiting for an invader to arrive. When they do find one, they engulf it and digest it. After they do this, they select pieces of the invader called antigens and put them on their surfaces. The dendritic cells migrate back to lymph nodes, key locations in your body filled with immune cells. There, they show the antigens, called antigen presentation, to two types of lymphocytes, T-cells and B-cells, activating them for a full immune response.


Your body has many T-cells, which circulate through your lymphatic system. Each T-cell is unique and only will recognize one particular antigen. Therefore, when the dendritic cell shows the antigen to lymphocytes in the lymph nodes, it is really waiting for the right T-cell to come along, which is fit specifically to the antigen.

Although this may seem inefficient, that T-cell will be a perfect fit for the pathogen, allowing for a strong immune response. Imagine you are the boss of a big company. You can have thousands of workers. Would you rather have thousands of workers that are okay at everything, or a thousand different experts, ready to crush their daily tasks? I would rather have the experts, and your immune system is no different.

There are two types of T-cells, helper T-cells and cytotoxic T-cells. When either T-cell is a match to the antigen presented by the dendritic cell, it divides rapidly in a process called clonal expansion. Helper T-cells, like the name sounds, go on to help activate the rest of the immune system. The helper T-cell secretes chemical messages called cytokines, which recruit other cells, like macrophages, B-cells, and T-cells to mount an attack.

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