DOC PREVIEW
MIT 7 012 - Study Notes

This preview shows page 1-2-3 out of 9 pages.

Save
View full document
View full document
Premium Document
Do you want full access? Go Premium and unlock all 9 pages.
Access to all documents
Download any document
Ad free experience
View full document
Premium Document
Do you want full access? Go Premium and unlock all 9 pages.
Access to all documents
Download any document
Ad free experience
View full document
Premium Document
Do you want full access? Go Premium and unlock all 9 pages.
Access to all documents
Download any document
Ad free experience
Premium Document
Do you want full access? Go Premium and unlock all 9 pages.
Access to all documents
Download any document
Ad free experience

Unformatted text preview:

AIDS and the Immune System In order to understand AIDS (acquired immunodeficiency syndrome), we need to discuss two major biological phenomena: the virus that causes AIDS, known as human immunodeficiency virus, HIV, and the cells in the immune system that are by HIV. Only when both are described does the disease begin to make sense. We begin by extending our earlier description of how the immune system works and then examining the details of the life cycle of . In our earlier discussion of immune cells, we listed two major classes of lymphocytes that together carry out the bulk of the functions needed for effective immunity - the B lymphocytes, whose descendant make antibodies, and the T lymphocytes. The disease of AIDS is ultimately about T lymphocytes, or rather their loss. The T lymphocytes (so named because they begin their development and diversification in the thymus, the gland near the thyroid gland in the neck) can be further divided into two subclasses: the cytotoxic or killer lymphocytes (TC cells) and the helper T cells (TH cells). The TC cells are specialized to recognizeand kill infected or otherwise defective cells throughout the body. By killing a cell recently infected by a virus, for example, the TC cells can abort the viral infectious cycle that may have started inside the cell befor the virus has a chance to multiply significantly. This is a highly effective way by which the immune system prevents the extensive spread of viruses throughout the body. B cell activation The other class of T lymphocytes, the TH cells, are the central targets of attack by HIV, the causal agent of AIDS. For that reason, we will now elaborate in more detail TH cell function. TH cells are not responsible for directly making antibody (the job of the B cells) or for killing infected or defective cells (the job of the TC cells). Their role is instead largely regulatory: the TH cells tell the B cells when they should begin clonal expansion and antibody diversification. Thus, they control the B cells. Such control is vital to a well-functioning immune system that might otherwise make inadequate or excess antibody in response to antigenic challenge. An outline of how this occurs is pictured schematically in part (a) of Fig. 19.18 in Purves. In our previous discussion of antibody production, the implication was that foreign protein molecules serve as the antigenic stimulus to provoke B cells. In fact, the provoking antigenic stimuli in the immune system are never entire protein molecules, but only fragments thereof. These oligopeptide fragments are generated by and displayed on the surface macrophages and the B cells. The macrophages and the B cells can ingest foreign antigenic proteins and cleave them into small pieces. The resulting protein fragments are positioned on the external surface of the cell where they serve as the immediate antigenic stimuli that cause B cell clonal expansion and antibody production. A macrophage cell itself has no immunological specificity. In the case of the poliovirus particles discussed in a previous lecture, the macrophage will ingested poliovirus, break down the capsid protein molecules into a number of small oligopeptide fragments and then export these to its plasma membrane, displaying them on its extracellular surface AIDS and the Immune System 1 MIT Biology Department7.012: Introductory Biology - Fall 2004Instructors: Professor Eric Lander, Professor Robert A. Weinberg, Dr. Claudette GardelNow many TH cells come by and look over the oligopeptides that the macrophage is displaying. Each of the T helper cells that passes by and examines the macrophage's oligopeptide "wares" (as would a shopper in a store) has a very narrow, circumscribed interest in only one particular antigen. When, by rare coincidence, an antigenic fragment displayed by the macrophage is recognized by the T helper, the TH cell becomes physiologically activated, and begins a program of rapid clonal expansion. Of course, in the great majority of cases, encounters between the helper T cell and macrophages will not result in clonal expansion because the TH cell will not happen across the antigenic fragment in which it has a special interest. Once the TH cell has undergone physiologic activation and clonal expansion, it will go on to its next phase of life, its next job. It will now seek out B lymphocytes that happen to be interested in the very same antigenic fragment. The B cell's “specialized interest” in one or another antigen arises, as described in the immunology lectures, through the unique antibody on the B cell surface. The TH cell, having found a B cell which shares interest in the same oligopeptide antigen, will then bind to the B cell and send a signal (via growth factors termed cytokines) to the B cell that induces the B cell to become activated and undergo clonal expansion. The B cell will then proliferate in response and produce descendant plasma cells that eventually begin to secrete antibody. In effect, the helper TH cell is saying to the B cell "I have recognized an oligopeptide antigen fragment displayed by a macrophage. I see that you’re interested in the same oligopeptide antigen. The fact that we have both encountered this antigen on separate occasions suggests that this is an important antigen against which you must mount a response. For that reason, I am now urging you proliferate and make antibody against this antigenic fragment." Without this command coming from the TH cell, the B cell will not begin its program of clonal expansion and antibody production. Only now do we begin to sense the vital (albeit indirect) role played by the TH cell in antibody production. AIDS We have all read about the discovery of the AIDS virus, both in France and in the United States in 1981. In the U.S., a cluster of 5 young, homosexual men were discovered who suffered from Pneumocystis carinii pneumonia infections, a disease that was otherwise only known in severely immuno-suppressed individuals. Soon thereafter, it was realized that their immuno-suppression was more far-reaching, involving mucosal yeast infections, widespread, disseminated cytomegalovirus infections, and chronic peri-anal herpes simplex virual infections. In addition to these debilitating infections, these individuals showed swollen lymph glands and night sweats. They also frequently contracted an unusual malignancy - Kaposi's sarcoma (a proliferation


View Full Document

MIT 7 012 - Study Notes

Documents in this Course
Quiz 2

Quiz 2

14 pages

Viruses

Viruses

7 pages

Exam 1

Exam 1

10 pages

Exam One

Exam One

11 pages

Load more
Download Study Notes
Our administrator received your request to download this document. We will send you the file to your email shortly.
Loading Unlocking...
Login

Join to view Study Notes and access 3M+ class-specific study document.

or
We will never post anything without your permission.
Don't have an account?
Sign Up

Join to view Study Notes 2 2 and access 3M+ class-specific study document.

or

By creating an account you agree to our Privacy Policy and Terms Of Use

Already a member?