Chapter 43 (3)

Acquired immunity is based on both a __ and a __.
- The __ involves hte activation and clonal selection of effector B cells, which secrete antibodies that circulate in the blood and lymph. The __ is so named because blood and lymph were long ago called body humors. It is also called hte __ because of hte key role in antibodies.

The predominant __ involves the activation and clonal selection of cytotoxic T cells, which identify and destroy the target cells. A third population of lymphocytes, the helper T cells, aids both responses.

Activated by encounteres with antigen-presenting cells, helper T cells play a central roll in enhancing __ and __. The helper T cell proliferates after interacting with antigen fragments displayed by antigen- presenting cells (usually dendritic). The resulting clone of cells differentiates into activated helper T cells and memory helper T cells. Activated helper T cells secrete cytokines that stimulate the activation of nearby B cells and cytotoxic T cells.

A helper T cell and the antigen-presenting cell displaying its specific epitope have a complex interaction. THe T cell receptors on the surface of the helper T cell bind to the antigen fragment that is held by a __ on the antigen-presenting cell. At the same time, a protein called __, found on the surface of most helper T cells, binds to the class II MHC molecule.

__ helps keep the helper T cell and antigen-presenting cell joined. As the two cells interat, signals in the form of cytokinesare exchanged in oth directions.

The three principal types of antigen-presenting cells- __, __ and __ interact with helper T cells ind iffernet contexts. _ are particularly important in triggering a primary immune response. They serve as sentinels in the epidermis and other tisseus frequently exposed to foreign antigens.

After __ capture antigens, they migrate from the infection site to lymphoid tissues. THere they present antigens, via class II MHC molecuels to helper T cells. macrophages play the key role in initiating a secondary immune response by presenting antigens to memory helper T cells, while the humoral response relies mainly on B cells to present antigens to helper T cells.

__ are the effector cells in a cell-mediated immune respose. To become active, they require signaling molecules from helper T cells as well as interaction with an antigen-presenting cell. Once activated, they can eliminate cancerous body cells and body cells infected by viruses or other intracellular pathogens.

Fragments of nonself proteins synthesized in such target cells associate with __ and are displayed on the cell surface, where they can be recognized by __. A surface protein called __, found on most cytotoxic T cells, enhances the interactions between a target cell and a __. Binding of __ to a class I MHC molecule helps keep the two cells in contact while the __ is activated. Thus, the roles of class I MHC molecules and __ are similar to those of Class II MHC molecules and __.

THe targeted destruction of an infected cell b y a __ involves the secretion of proteins that cause cell rupture and cell death. The death of the infected cell not only deprives the pathogen of a place to reproduce but also exposes it to circulating antibodies, which mark it for disposal.

After destroying an infected cell, the __ may move on and kill other cells infected with the same pathogen.

The secretion of antibodies by clonally selected B cells is the hallmark of teh humoral response. Activation of this response typically involves B cells and helper T cells, as well as proteins on the surface of bacteria.
- __ activation by an antigen is aided by cytokines secreted from helper T cells that have encountered the same antigen. Stimulated by both an antigen and cytokines, the B cell proliferates and differentiatses into a clone of antibody-secreting plasma cells and a clone of memory B cells.

The pathway for antigen processing and display in B cells differ from that in other antigen-presenting cells. A __ or __ can present fragments from a wide variety of protein antigens, whereas a B cell presents only the antigen to which it specifically binds. When an antigen first binds to receptors on the surface of a B cell, the cell takes in a few of the foreign molecules by __.
-- The B cell then presents an __ to a helper T cell. This achieves the direct cell-to-cell contact that is usually critical to B cell activation.

B cell activation leads to a robust humoral response: An activated B cell gives rise to a clone of thousands of plasma cells,e ach of which secretes approx 2000 antibody molecules every second of the cell's 4- to- 5 day life span. Furthermore, most antigens recognized by B cells contain multiple __. An exposure to a signle antigen therefore normally activates a variety of B cells, with different clones of plasma cells directed against different __ on the common antigen.

For __, including polysaccharides, that contact multiple receptors on a single cell, a Bcell response can occr w/o the involvement of cytokines or helper T cells. Although such responses generate no memory B cells, they play an important role in defending against many bacteria.

For a given B cell, the antibodies produced differ from the B cell receptor only in the __ region of the heavy chain. In place of a transmembrane region and cytoplasmic tail, the heavy chain contains sequences that determine where the antibody is distributed and how it mediates antigen disposal.

The five major types of heavy-chain C regions determines five major classes of antibodies. Changes in the heavy-chain gene that switch B cells from production of one antibody class to another occur only in response to antigen stimulation and to specific regulatory signals from T cells.
What are the five?

The power of antibody specificity and antigen-antibody binding has been harnessed in lab research and clinical diagnosis. Some antibody tools are __: they are the products of many different clones of B cells,e ach specific for a dif epitope. Antibodies produced following exposure toa microbian antigen are __.

In contrast, other antibody tools are __: they are prepared from a single clone of B cells grown in clutture. All the __ produced by such a culture are identical and specific for the same epitope on an antigen. __ are particularly useful for tagging specific moleucles.

The bidning of antibodies to antigens can interfere with pathogen function in many ways. In teh simplest of htese, __, antibodies bind to surface proteins of a virus or bacterium, thereby blocking the pathogen's ability to infect a host cell.

Similarly, antibodies sometimes bind to and neutralize toxins released in body fluids. In a process called __, the antibodies bound to antigens present a readily recognized structure for macrophages and therefore increase phagocytosis. Because each antibody has two antigen-binding sites, antibodies can also facilitate phagocytosis by linking bacterial cells, virus particles, or antigens into aggregates.

Antibodies sometimes work together with the proteins of the complement system to dispose of pathogens. Binding of antigen-antibody complexes on a microbe or foreign cell to one of the complement proteins triggers a cascade in which each protein of hte complement system activates the next. ultimately, activated complement proteins generate a __ taht forms a pore in the membrane of the foreign cell. Ions and water rush into the cell, causing it to swell and lyse. Whether activated as part of innate or acquired defenses, this cascade of complement protein activity results in the lysis of microbes and produces factors that promote inflammation or stimulate phagocytosis.

When antibodies facilitate phagocytosis, they also help fine-tune the humoral immune response. Recall that phagocytosis enables macrophages and dendritic cells to present antigens to and stimulate helper T cells, which in turn stimulate the very B cells whose antibodies contribute to phagocytosis. This positive feedback betweent he __ and __ immune systems contributes to a coordinated, effective response to infection.

Although antibodies are the cornerstoones of the response in body fluids, there is also a mechanism by which they can bring about the death of infected body cells. When a virus uses a cell's biosynthetic machinery to produce viral proteins, these viral products can appear on the cell surface. If antibodies specific for epitopes on these viral proteins bind the exposed proteins, the presence of bound antibody at the cell surface can recruit a __. The __ then releases proteins that cause the infected cell to undergo apoptosis.

Inr esponse to infection, clones of memory cells form, providing __. In contrast, a distinct type of immunity results when the __ antibodies of a pregmant women cross the placenta to her fetus. The transferred antibodies are poised to immediately help destroy any pathogens for which they are specific. This protection is called __ because the antibodies providied by the mother guard against microbes that have never infected the newborn. Because __ does not involve the recipient's B and T cells, it persists only as long as the transferred antibodies last. However, __ antibodies are passed from a mother to her infant in breast milk. These antibodies provide additional protection against infection while the infant's immune system develops.

Both __ and __ immunity can be induced artificially. Active immunity can develop from the introduction of antigens into the bdoy through __, often called __.
- Today, many sources of antigen are used to make vaccines, including inactivated bacterial toxins, killed microbes, parts of microbes, weakened microbes that generally do not cause illness, and even genes encoding microbial proteins. Because all of these agents induce a primary immune respoonse and immunological memory, an encounter with the pathogen from which the vaccine was derived triggers a rapid and strong secondary response.

In __, antibodies from an immune animal are injected into a nonimmune animal.

True or False:
Like pathogens, cells from another person ccan be recognized and attacked by immune defenses.
True False

To avoid harmful immune reacitons in human blood transfusions, ABO blood groups must be taken into account.
- It turns out that certain bacterial normally present in the body have __ very similar to the A and B blood group antigens. By responding to the bacterial __ similar to B antigen, a person with type A blood makes antibodies that can react with B antigen. No antibodies are made against the bacterial epitope similar to A antigen, since lymphocytes reactive with self antigens are inactivated or eliminated during development.

In the case of tissue and organ transplants, or __, it is __ taht stimulate the immune response that leads to rejection. Each vertebrate species has many different alleles for each class I and class II MHC gene, enabling presnetation of antigen fragments that vary in shape and charge.

True or False:
This diversity of MHC moleucles almost guarantees that no two people will have exactly the same set. Thus, int eh vast majority of graft and transplant recipients, some MHC moleucels on the donated tissue are foreign to the recipient. To minimize rejection, physicians try to use donor tissue bearing MHC molecuels that match those of the recipient as closely as possible. In addition, the recipient takes medicines that suppress immune responses. However, these medicines can leave the recipient more susceptible to infections during the course of treatment.
True False

In a bone marrow transplant between individuals, the problem of rejection is reversed: The donor tissue can reject the recipient's body tissues. Bone marrow transplants are used to treat leukemia and other cancers as well as various __ (blood cell) diseases. Prior to recieiving transplanted bone marrow, the recipient is typically treated with radiation to eliminate his or her own bone marrow cells, thus destroying the sourc e of abnormal cells. This treatment effectively obliterates the recipient's immune system, leaving little chance of graft rejection. Howver, lymphocytes in the donated marrow may react against the recipient. The __ is limited if the MHC molecules of the donor and recipient are well matched.