Cell-Mediated Immunity

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Cell-Mediated Objectives

Define what an antigen is and explain how it relates to defending against pathogens.
Describe the steps of cell-mediated immunity, including the cells that are involved in the process.
Explain how HIV and the Yersinia bacteria that cause the plague negatively impact the immune response.

Specific Immunity

Now it gets personal!

To recognize a pathogen, specific types of lymphocytes have to be able to recognize antigens, surface molecules that are distinctly different than the molecules on our own individual cells.

Two Specific (Acquired) Lines of Defense

Cell-Mediated Immunity

A type of lymphocyte called the “cytotoxic T cell” learns what antigens on a pathogen look like and kills cells infected with that pathogen. We are covering this specific defense in this section.

Antibody-Mediated Immunity

A type of lymphocyte called the “plasma B cell” learns to produce antibodies that stick to the antigens on the surface of pathogens. This will be covered in the next section.

In this section we are focusing on cell-mediated immunity.

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This poster summarizes the cell-mediated immune response.

Specific Cell-Mediated Immunity

Here are the details of how lymphocytes can kill cells infected by a specific pathogen.

Helper T cells that have communicated with the antigen-presenting macrophages tell naive (uneducated) T cells what to to seek out and destroy.

Most of the naive T cells that are educated become cytotoxic T cells that seek out cells infected with the target pathogen and destroy them.

The cytotoxic T cells have chemicals that dissolve the infected cell’s plasma membrane. This is similar to how natural killer cells destroy damaged cells, but more specific in targeting cells that harbor a certain pathogen.

Some of the educated T cells hold back and are called memory T cells. If the pathogen attacks again, the memory T cells start undergoing mitosis and become large numbers of cytotoxic T cells that attack cells infected by the pathogen.

The macrophage while it was eating up debris during non-specific inflammation, ripped up pathogens and presented the pathogen’s surface markers (called antigens) on their membranes. The macrophages become antigen-presenting cells.

The macrophages communicate chemically with a type of lymphocyte, the helper T cell, which will tell other B cells and T cells what to kill.

T cells are lymphocytes that left the bone marrow and matured in the thymus gland, located near the heart. B cells are lymphocytes that spent extra time in the bone marrow maturing.

So the last thing you would want are diseases that harm either the macrophage (monocyte) or the helper T cell (lymphocyte). Your body would lose the ability to recognize when it is under attack from pathogens.

But we have both. The “plague” bacteria Yersinia pestis attacks monocytes and was responsible for killing approximately 1/3 of Europeans in the 14th century and probably many more people on the Asian and African continents. Luckily we now have antibiotics that kill these bacteria.

This is an iconic image of a plague doctor wearing garb to avoid infection.

Unfortunately, HIV (human immunodeficiency virus) attacks a group of helper T cells, interfering with the white blood cells’ abilities to recognize and attack pathogens. Individuals succumb to pathogens their bodies would have otherwise have been able to fight off.

The next section introduces the other acquired and specific defense: antibody-mediated immunity.

Check your knowledge. Can you:

define what an antigen is and explain how it relates to defending against pathogens?
describe the steps of cell-mediated immunity, including the cells that are involved in the process?
explain how HIV and the Yersinia bacteria that cause the plague negatively impact the immune response?