Immune System Quiz

Explain how the immune system achieves all of the following.

1. Provides an immediate nonspecific immune response
2. Activates T and B cells in response to an infection
3. Responds to a later exposure to the same infectious agent
4. Distinguishes self from nonself

1. Immediate nonspecific immune responses are not geared toward one specific pathogen, but exist to stop pathogens from entering the body in the first place.  There are two different types of nonspecific defense: the first and second lines of defense.  The first line is comprised of mechanical, chemical, and biological barriers.  The mechanical barriers are physical, such as kin and mucous layers.  The mucous membranes line body opening, like the nose and mouth, and organs that have direct connection to the outside of the body, like the GI tract.  These membranes have cilia that sweep pathogens to the outside of the body.  The skin and mucous also create chemical barriers by secreting proteins.  One important chemical barrier is the lysozymes, an enzyme that destroys pathogens by opening their cell walls.  These chemical barriers can be found in body secretions, which are often acidic, and help defend against pathogens.  The biological barriers are living organisms, like bacteria that live on the skin and prevent harmful bacteria from taking root outside or inside the body by competing with them for nutrients.  The second line of defense is for when a pathogen actually enters the body.  Inflammatory responses, like when cuts swell, are reactions to tissue damage.  This damage releases cytokines and histamines, which communicate between cells and cause inflammation, respectively.  The swelling increases blood flow to the area in order to remove pathogens and start healing.  Cytokines also bring white blood cells to the wound.  White blood cells, or leukocytes, exist to find and destroy foreign objects in the body.  There are both specific, and nonspecific leukocytes.  The nonspecific leukocytes, monocytes, macrophages, and neutrophils by enveloping pathogens and digesting them, a process called phagocytosis. 

2. T and B cells are lymphocytes, meaning that they are tailored to certain pathogens.  However, it isn’t the pathogens they react to, but rather the antigens, or molecules, of the pathogens.  Their receptors recognize specific antigens and bind to them, but these lymphocytes can only recognize one antigen each.  Before they find their specific pathogen, these cells are considered naïve.  B cells are responsible for the humoral immune response, which means they are responsible for producing antibodies.  When naïve B cells first encounter an antigen, they engulf it, digest it, and display fragments of it on their surfaces.  T cells then bind to the B cells in order to release cytokines, which then help the B cells retain memory of the antigen as a plasma cell or memory cell.  Next, these plasma cells create antibodies that react to the same antigen that the B cell initially engulfed.  Whereas plasma cells are only temporary, plasma cells can last a lifetime and initiate an immediate immune response to the antigen that created them.  T cells are activated in generally the same way.  Helper T cells, on the other hand, do not actually destroy pathogens.  Instead they create chemical signals in order to help activate T cells and help them to activate B cells.  When the pathogen is cleared from the body, these helper T cells either die or become memory cells.  Cytotoxic T cells destroy all damaged cells, whether they be cancerous or infected by a virus.  These cells quickly reproduce in order to create a swarm that can combat against their specific antigen.  When the pathogen has been removed (these T cells are kamikazes, dying when they destroy their pathogens), the T cells die off or become memory cells.  Regulatory T cells mandate the entire process.

3. After T and B cells have been activated to a pathogen, their body becomes immune to it, either actively or passively.  Active immunity is when the memory cells are left behind for a specific pathogen, so the pathogen has a harder time re-infecting the body.  Immunization can also lead to active immunity by exposing the body to a dead or weakened form of the pathogen so the body can recognize it without ever getting sick.  Passive immunity is when the body receives antibodies, as opposed to creating them, for a certain pathogen.  This only lasts as long as the antibodies are alive in body fluids.

4. T cells and B cells are programmed to respond to certain antigens.  Since body, or “self” cells do not have molecules that read as foreign, the immune system does not have a problem.  When “non-self” pathogens enter the body, these cells recognize their foreign antigens and react to them as a threat.  Sometimes, T and B cells react to non-harmful foreign substances, called allergens, and create an allergic response to them.  In rarer cases, the immune system mistakes “self” for “non-self,” and begins to attack the body, in what is called an auto-immune disease.           

References:
Quick, Kevin, Holly Kiamanesh, Rosie Montague, Jean Brainard, Barbara Akre, and Douglas Wilkin. The Webb Schools Biology Textbook. N.p.: CK-12, Flexbook, 2012. Print.