12.8.3 Immunity: Clonal Selection Theory

Question 1

Immunity: Clonal Selection Theory

Answer 1

• Review: The immune system helps an animal defend itself against pathogens such as viruses and bacteria. Four characteristics of the immune system that enable it to successfully defend the body include the ability to
· recognize self verses nonself;
· launch an attack against any and all pathogens;
· distinguish between a nearly infinite number of pathogens;
· recognize returning pathogens.
• Lymphocytes are white blood cells that develop into either B cells or T cells. Lymphocytes that mature in the bone marrow become B cells, while those that mature in the thymus become T cells. When antigens bind to antigen receptors on the surface of B or T cells, the cells become activated.
• Clonal selection is the antigen-driven cloning of lymphocytes. Once activated by an antigen, B cells will clone into plasma cells and memory B cells, while T cells will clone to produce activated T cells and memory T cells

Question 2

lymphocytes

Answer 2

• Early in their development all lymphocytes are similar, but depending on where they mature, they will develop into either B cells or T cells. Mature B and T cells have antigen receptors on their surfaces that allow them to recognize and respond to foreign molecules.
• Throughout their development, B and T cells are routinely exposed to the body’s own chemicals to prevent them from attacking normally occurring molecules in the body. If any maturing lymphocytes recognize and attack normally occurring materials in the body, they are deactivated or destroyed by apoptosis, programmed cell death.
• There are masses of different B and T cells circulating
  throughout the body, each targeted towards a specific antigen. Notice in the diagram to the left that each lymphocyte has a different antigen receptor.
• Once an antigen binds to an antigen receptor on the surface of a lymphocyte, the lymphocyte is activated. Activated B cells will divide to produce plasma cells and memory B cells, while activated T cells will divide to produce more activated T cells and memory T cells.

Question 3

clonal selection

Answer 3

• Clonal selection is the antigen-triggered cloning of
  lymphocytes. An antigen is able to activate only a small
  number from the body’s large and diverse array of
  lymphocytes. If an antigen binds to a lymphocyte’s antigen receptor, that lymphocyte will be triggered to give rise to thousands of clones, all specific to that antigen.

Question 4

primary and secondary immune response

Answer 4

• The first exposure to an antigen elicits a
  primary immune response. The primary immune response involves a gradual increase in concentration of antibodies specific to that antigen.
• Memory cells that arise from the cloning of lymphocytes allow a person’s body to respond quickly to a second exposure of the pathogen. A secondary immune response is much stronger and more rapid than a primary immune response. As shown in the diagram to the left, the antibody concentration in the secondary immune response is greater and more prolonged than in the primary immune response.

Question 5

In the figure, what is occurring at times #1 and #2?

Answer 5

• At time #1, the antigen has been identified and the body is producing antibodies to attack it. Cells specific to the antigen, memory cells, remain in the body after the primary exposure and are able to quickly suppress the second exposure to the antigen as shown at time #2.

Question 6

Which of the statements regarding humoral immunity is false?

Answer 6

• Humoral immunity is also known as cell-mediated immunity.

Question 7

True or false?

T cells produce antibodies.

Answer 7

• false

Question 8

____________________ are undifferentiated cells that will turn into ____________________ in the bone marrow and ________________ in the thymus.

Answer 8

• Lymphocytes, B cells, T cells

Question 9

Apoptosis is

Answer 9

• programmed cell death

Question 10

True or false?
Once a T cell becomes activated by an antigen, it converts into a plasma cell that then produces both antibodies against the invader as well as memory cells that will stay in the blood stream for the rest of the organism’s life.

Answer 10

• false

Question 11

How does the body prevent the immune system from attacking itself?

Answer 11

• Throughout their development, B and T cells are routinely exposed to the body’s own chemicals to prevent them from attacking normally occurring molecules in the body.