Cell-mediated Immunity: Definition & Response

Instructor: Catherine Paul

Catherine has taught high school science and has a master's degree in biology.

Learn about cell-mediated immunity by investigating its function and process in the immune system. We will cover how T lymphocytes and major histocompatibility complex molecules play an important role in carrying out this immune response.

The Cell-Mediated Response - T Cells on Patrol

We might not think of ourselves as having our own private security force on hand, but we actually do have our own force against invaders - our immune system! It's a biological system with many specialized agents in place to protect us from disease. Our immune system is trained to get the 'bad guys' (pathogens) with two security systems, the humoral response and the cell-mediated immune response. In this lesson, we'll go over the basics of cell-mediated immunity.

One type of security system in our immune system is called cell-mediated immunity. This response enlists the help of T lymphocytes (T cells). The cell-mediated immune response specializes in recognizing and attacking invaders located within our own cells. Wait a minute, how does the T cell know if a cell is infected with a bacteria or virus if it's hiding inside the cell?

Imagine you were throwing a big event, and your private security was told to make sure everyone who entered wore a specific badge. Your security dutifully checked everyone to ensure they had the correct badge. Anyone without the proper credentials, or correct badge, is obviously an intruder.

This is precisely how cell-mediated immunity works. Think of our T cells as the patrolling security and other cells, such as B cells, are guests at the event. These B cells wear badges to let the patrolling T cells know that they are healthy and not intruders. These badges are called major histocompatibility complex molecules, or MHC molecules for short.

But, if a cell is infected, say with a pathogen, it needs a way to let the immune system know that it is not healthy. Infected cells do this by breaking down small bits of this pathogen that has replicated itself within the cell. They then present these pieces of pathogen with an MHC molecule on its surface membrane. This is the way a cell can wear a 'badge' that says 'I'm infected with an intruder!' A patrolling T cell will recognize this MHC molecule, or 'badge'. Then the T cell will 'sound the alarm,' or in other words, stimulate an immune response.

T Cells Are Specific

Not every T cell can recognize every cell that has been infected. Imagine each T cell and each MHC molecule is like a unique puzzle piece. Just like fitting two puzzle pieces together, the infected cell must come in contact with its specific T cell match. Also, there are two main types of 'badges' that a cell can display, called class I and class II MHC molecules. Almost all cells with a nucleus can present class I MHC molecules. This is in contrast to class II MHC molecules, which can only be presented by specialized immune cells called antigen-presenting cells. These are limited to macrophages, B cells, and dendritic cells.

T cytotoxic (Tc) cells are typically identified by how they present CD8 on their membrane. CD8 helps the Tc cells recognize class I MHC molecules and are the task force assigned to check on cells displaying these specific types of 'badges'. T helper (Th) cells present CD4 on their membrane, which helps bind to class II MHC molecules making them the task force on duty to identify MHC II molecules.

T Cell Activation
T Cell Activation

Immune System Activation

The T cell has found its target - an infected cell that is presenting a specific MHC complex. An activated Th cell will radio for backup by secreting cytokines. These cytokines tell various other cells in the immune system to attack the infected cells. Th cells use cytokines to activate the other main type of security in our immune system, the humoral response.

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