Cell-Mediated Immune Response: Definition, Steps & Features

Cell-Mediated Immune Response: Definition, Steps & Features
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  • 0:01 Cell-Mediated Immune Response
  • 1:00 T Cell Activation
  • 2:38 Fighting Pathogens
  • 4:35 Lesson Summary
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Lesson Transcript
Instructor: Christopher Muscato

Chris has a master's degree in history and teaches at the University of Northern Colorado.

Our immune systems are pretty good, but sometimes pathogens can sneak in and infect living cells. Luckily, we've got a back up system. Let's explore cell-mediated response and see how this protects our bodies from infections that may otherwise go untreated.

Cell-Mediated Immune Response

Your body is a war zone, constantly fighting off diseases and infections and all sorts of harmful things. Luckily, we've got some pretty good defense systems. Now, generally, when a pathogen enters the body, it's targeted by white blood cells, in response to antibodies, which we call humoral immunity. The only issue is that if the pathogen manages to actually infect a living cell within the body, the antibodies are ineffective. The body won't recognize a pathogen within a living cell and the humoral immune response passes it by. But luckily we've got a backup system.

The cell-mediated immune response identifies and destroys infected cells, preventing the bacteria or virus from spreading any further. Cell-mediated immunity provides a double layer of security, keeping us safe in the continual battle of existence.

T Cell Activation

Normal white blood cells, often called b cells, need antibodies to help them identify pathogens, which is why they can't find pathogens hiding inside an infected cell. This means that cell mediated immunity relies on a different cast of characters. These are special white blood cells that target pathogens within a cell, called T lymphocytes, or T cells for short. The T cells are activated in response to specific antigens, substances indicating the need for an immune response that are created by the infected cell.

The process begins with what is called an antigen-presenting cell, a kind of white blood cell that produces antigens. This APC eats a pathogen, digests it, and creates hundreds of antigens to help the immune system recognize the intruding disease. The APC presents these antigens on its surface using a protein called MHCII. The antigen on the MHCII alerts T cells that there is a pathogen within the body and tells them what they are looking for. T cells are activated when they receive these instructions from APCs or infected cells via a molecule on the T cell surface responsible for recognizing MHC antigens, called the T cell receptor.

So, quick recap. Pathogens enter the body; APC creates antigens, which they display with the protein MHCII; T cells recognize this antigen using the T cell receptor and set off to find and destroy the pathogen.

Fighting Pathogens

Now that the T cell knows what it's looking for, it is able to identify the pathogen hiding within existing cells. Now, finding and eradicating the infection actually requires a few different kinds of T cells, each with a distinct focus. The two main types are cytotoxic T cells and helper T cells.

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