Question

Part I – The Options The students in Dr. Stanley’s course were very excited about their upcoming trip to Nepal to take samples from villagers to study Chlamydia trachomatis eye disease in native populations. The tickets were purchased and the accommodations booked. They had gone to the Centers for Disease Control and Prevention (CDC) website and looked at the travelers’ health page for Nepal and discovered they needed two vaccines that weren’t routine, one for hepatitis A and the other for typhoid fever. There was only one hepatitis A vaccine, a formalin-inactivated version of hepatitis A virus (a “killed” vaccine) given by subcutaneous injection. But there were two recommended vaccine options for typhoid fever. One of the vaccine options was a subunit vaccine, consisting of a capsular polysaccharide called Vi, from the causative agent of typhoid fever, the bacterium Salmonella enterica serovar Typhi (hereafter designated S. Typhi). This vaccine is given by intramuscular injection. The second vaccine option was a live, attenuated version of S. Typhi. The “modified live” vaccine is a strain of S. Typhi that has multiple mutations, including ones in genes encoding the biosynthetic pathway for production of the Vi capsule. This vaccine is given as enteric-coated capsules. The students with a fear of needles thought that an oral vaccine wounded better, but the idea of eating live bacteria on purpose even as a part of a vaccine, didn’t sound so great either. Dr. Stanley told the students they should get out their textbooks to help them understand the kind of immunity each vaccine would give them so they could decide which vaccine would be best for a pathogen like S. Typhi, which has the following properties:

• The organism is spread by the fecal-oral route (i.e., it is acquired by ingesting food or water contaminated with the bacteria) so it enters via a mucosal site.

• The organism enters host cells and lives within these cells so that it is “hidden” from extracellular defenses.

• The organism enters the bloodstream where it multiplies and spreads to other tissues.

Before they looked up the different kinds of vaccines, he suggested they answer the following questions.

Questions

1. What is an antigen?

2. What are the two different types of adaptive immunity?

3. Why does it take one to two weeks for adaptive immunity to be induced the first time the body sees an antigen?

4. Why does it take a much shorter time for adaptive immunity to be induced on subsequent encounters with the antigen?

5. What are the three types of antigen-presenting cells?

6. What is meant by “antigen presentation,” and how do antigen-presenting cells present antigens?

7. How do T cells recognize antigen?

8. What type of antigens do T cells recognize (i.e., what types of molecules and what is their molecular nature)?

9. What do helper T cells help?

10. How do cytotoxic T lymphocytes recognize cells to kill?

11. How do cytotoxic T lymphocytes kill cells?

12. What is the relationship between antibodies and B cells?

13. How do B cells recognize antigens?

14. What types of antigens do B cells recognize (i.e., what types of molecules and what is their

      molecular nature)?

15. What are the functions of IgG antibodies?

16. What is the main function of secretory IgA (sIgA)?

17. What feature of antibodies results in their ability to cross-link antigens?

Answer 1

1. An antigen is a molecule that can get recognized as a foreign particle by the immune system, which in turn, gets stimulated and produces a response to eliminate it from the body.

2. The two branches of adaptive immunity includes- Humoral and cellular. In case of humoral immunity, the B cells produces antibodies against the foreign antigen, whereas, in cell mediated immunity, the T cells are involved.

3. When the body encounters a foreign antigen for the first time, it generates a primary immune response. During this process antigen specific T cells and B cells are selected, which become activated in response to the antigen and proliferate to yield highly specific active T cells, plasma cells and memory cells, which then function to eliminate the antigen from the body. This activation and proliferation of lymphocytes during primary response makes the process slow, and it may take one to two weeks for the body to mount a primary response.

4. When the same antigen enters the body for the second time, a rapid and more effective secondary immune response is generated. In this case, the memory cells (that were generated against the antigen during primary response) are involved. Since, memory cells against the antigen are already present in the body, secondary immune response is rapid. Also, number of lymphocytes generated during secondary response is much greater compared to the primary response and they have a greater affinity for the antigen. Thus, secondary immune response is faster and stronger than the primary response.

5. The three types of antigen presenting cells include- B cells, dendritic cells and macrophages. They function to process and present an antigen on the surface, so that they can get recognized by the T cells.

6. The T cells can not recognize an antigen directly. In order for an antigen to get recognized by the T cells, they need to be processed and presented by the Antigen Presenting Cells (APCs). This is known as 'Antigen Presentation'. The APCs digest the antigen into smaller pieces, and these fragments are bound by the Major Histocompatibility Complex (MHC). The MHC together with the bound antigen is presented on the cell surface and is recognized by the T cells.

7. T cells recognize only those antigens, that are processed by the antigen presenting cells and presented on the cell surface together with the MHC. Two different types of MHCs are there. MHC type I interacts with CD8 marker on Tc cells, whereas MHC type II interacts with CD4 marker on Th cells. Thus, the receptor on T cells bind to the antigen fragment presented by the MHC, whereas, the coreceptor (CD4 or CD8) binds with the MHC, to strengthen the overall interaction.