Adaptive Immunity

Your adaptive immunity gets its name because it adapts and changes, or adapts, as you go through life and are exposed to specific microbes that your innate defenses can’t fight. Your body’s innate defenses are incredible, and they prevent infection by most of the microbes that you encounter in your life.

But every now and then, a microbe comes along that gets around your innate defenses and into your body. When your innate defenses are breached, it’s time for the troops of your adaptive immunity to rally and fight back.

If, for example, you’re infected with E. coli, only those white blood cells that recognize particular molecules on E. coli are activated. If you face a different infection, say the bacteria Staphylococcus aureus, only the white blood cells that recognize specific molecules on S. aureus are activated.

In other words, when your adaptive defenses come to your rescue, your body activates exactly the right team of white blood cells to fight each pathogen. That means your adaptive defenses learn to recognize specific pathogens after you encounter them.

One of the awesome features of your adaptive immunity is that it can remember a pathogen it has encountered before. This immunologic memory allows your immune system to respond much more effectively when you meet a particular pathogen again.

Certain cells of your immune system, called memory cells, remain in a semiactivated state after your first encounter with a microbe. These memory cells and their descendants hang around for a long time after they’re activated in the first battle.

When the same pathogen shows up again, these cells multiply quickly and efficiently destroy the pathogen before you even realize it came back. Memory cells are the reason why you can get some illnesses only once.

Several types of white blood cells work together to create your adaptive immunity:

  • Helper T cells: Also called CD4 cells, these cells coordinate your entire adaptive immune response. Helper T cells receive signals from the white blood cells of your innate defenses, such as dendritic cells and phagocytes, and relay those signals to the fighters of your adaptive defenses: the B cells and cytotoxic T cells.

  • B cells: These cells are activated when they detect a foreign pathogen with their B cell receptors and when they receive signals from helper T cells. They’re activated to form two types of cells: plasma cells and memory cells.

    Plasma cells produce antibodies, defensive proteins that bind specifically to antigens. Antibody proteins have a forked structure, like a letter Y, with a binding site for antigen at each tip of the Y. Your immune system releases the antibodies that plasma cells produce into the blood, where they can circulate around the body. Anything in the body that’s tagged with antibodies — such as invading pathogens — is marked for destruction by the immune system.

  • Cytotoxic T cells: Also called CD8 cells or cytotoxic T lymphocytes (CTLs), these cells come into play if microbes try to hide inside your cells so that the antibodies can’t find them. Cytotoxic T cells can detect foreign antigens on the surface of an infected host cell.

    When these cells discover an infected cell, they send signals that tell the infected cell to commit suicide — a necessary sacrifice in order to destroy the hidden microbes. CD8 T cells also tell abnormal cells within your body to die, thus preventing them from becoming cancerous.

Of all these types of white blood cells, your helper T cells are probably the most important. Antigen-presenting cells like dendritic cells and macrophages from your innate immunity activate helper T cells by showing them bits of molecules from pathogens.

After they’re activated, your helper T cells multiply and release communicating molecules called cytokines that stimulate both cytotoxic T cells and B cells. Thus, without the action of helper T cells, your entire immune system would fail.

The HIV virus infects helper T cells, slowly reducing their numbers until a person who’s infected with the virus doesn’t have enough helper T cells to activate his adaptive immunity. At this point, the person develops acquired immunodeficiency syndrome, better known as AIDS. After a person has AIDS, he’s very susceptible to infection and certain cancers, which ultimately cause the person’s death.

The activation of helper T cells and the other cells that make up your immune system involves several steps:

  1. Antigen-presenting cells attach pieces of the foreign antigen to proteins called MHC2 proteins that they display on their surface.

  2. Antigen-presenting cells also produce molecules like cytokines, signaling that they’ve detected a foreign antigen.

  3. Helper T cells bind to the displayed antigen using a receptor called a T cell receptor.

  4. After helper T cells recognize antigen and receive the signals from antigen-presenting cells, they activate; activated helper T cells multiply and then activate cytotoxic T cells and B cells. Helper T cells also send signals to phagocytes that cause them to become more efficient killers.

For each of the following questions, name the component of your adaptive defenses that’s the best match for the given description.

  1. These cells kill cells infected with viruses.

  2. These proteins stick to foreign molecules, marking them fordestruction by the immune system.

  3. These cells produce signals that activate cytotoxic T cells and B cells.

  4. These cells produce antibodies.

  5. These cells have a protein on their surface called CD8.

  6. These cells are the host cell for the HIV virus.

  7. Cells use these molecules to communicate with one another.

  8. These cells can become plasma cells and memory cells.

  9. These cells show antigens to helper T cells.

  10. These cells live a long time and help you respond quickly to pathogens when you encounter them for a second time.

  11. These molecules enter the body as part of pathogens, triggering your adaptive immune response.

For questions 12–20, use the terms that follow to label the cells and steps that occur during activation of your immune system in the figure.

image0.jpg

a. B cell

b. Cytotoxic T cell

c. Antigen

d. Antibody

e. Antigen-presenting cell

f. Helper T cell

g. Infected cell

h. Plasma cell

i. Phagocyte

The following are the answers to the practice questions.

  1. The answer is cytotoxic T cells.

  2. The answer is antibodies.

  3. The answer is helper T cells.

  4. The answer is plasma cells.

  5. The answer is cytotoxic T cells.

  6. The answer is helper T cells.

  7. The answer is cytokines.

  8. The answer is B cells.

  9. The answer is antigen-presenting cells (or dendritic cells or macrophages).

  10. The answer is memory cells.

  11. The answer is antigens.

  12. e. Antigen-presenting cell

  13. c. Antigen

  14. f. Helper T cell

  15. a. B cell

  16. h. Plasma cell

  17. d. Antibody

  18. g. Infected cell

  19. b. Cytotoxic T cell.

  20. i. Phagocyte

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