21.3 The Adaptive Immune Response: T lymphocytes and their functional types

is the adaptive immune system nonspecific or specific?

specific

antigens

the small chemical groups often associated with pathogens

what are antigens recognized by?

receptors on the surface of B and T lymphocytes.

how can the adaptive immune system be so specific?

it has a unique way to develop 100 trillion different receptors to recognize nearly every conceivable pathogen

primary adaptive response

immune system's response to the first exposure to a pathogen

primary disease

symptoms of a first infection

why is primary disease so severe?

because it takes time for an initial adaptive immune response to a pathogen to become effective.

secondary adaptive immune response

immune response observed upon re-exposure to a pathogen, which is stronger and faster than a primary response

secondary adaptive response symptoms

no symptoms. the pathogen is often eliminated before it can cause significant tissue damage or any symptoms

immunological memory

an individual's exposure to pathogens early in life spares the person from these diseases later in life.

self recognition

the adaptive immune response is able to distinguish between self-antigens, those that are normally present in the body, and foreign antigens, those that might be on a potential pathogen.

primary cells that control the adaptive immune response

lymphocytes, the T and B cells

T cells control

not only control a multitude of immune responses directly, but also control B cell immune responses in many cases as well.

T lymphocytes recognize antigens based on what?

a two-chain protein receptor

each chain consists of how many domains?

two

variable region domain

furthest away from the T cell membrane and is so named because its amino acid sequence varies between receptors.

constant region domain

part of a lymphocyte antigen receptor that does not vary much between different receptor types

how many types of receptors does each T cell produce?

one. each T cell is specific for a single particular antigen

antigenic determinant (epitope)

one of the small regions within an antigen to which a receptor can bind, and antigenic determinants are limited by the size of the receptor itself.

where are carbohydrate antigens found?

on bacterial cell walls and on red blood cells (the ABO blood group antigens).

protein antigens

complex because of the variety of three-dimensional shapes that proteins can assume, and are especially important for the immune responses to viruses and worm parasites

Do T cells recognize free-floating or cell-bound antigens as they appear on the surface of the pathogen?

No. They only recognize antigen on the surface of specialized cells called antigen-presenting cells.

antigen processing

a mechanism that enzymatically cleaves the antigen into smaller pieces.

after the antigen is processed, where does it go?

brought to the cell's surface and associated with a MHC

major histocompatibility complex (MHC)

gene cluster whose proteins present antigens to T cells

antigen presentation

The association of the antigen fragments with an MHC molecule on the surface of a cell and results in the recognition of antigen by a T cell

peptide-binding cleft

a small indentation at the end of the MHC molecule that is furthest away from the cell membrane; it is here that the processed fragment of antigen sits.

MHC class I

found on most cells of the body, it binds to the CD8 molecule on T cells

MHC class II

found on macrophages, dendritic cells, and B cells, it binds to CD4 molecules on T cells

what do MHC class I and II have in common?

They bring processed antigen to the surface of the cell via a transport vesicle and present the antigen to the T cell and its receptor.

what are extracellular antigens brought into the endomembrane system of a cell by?

receptor-mediated endocytosis

a T cell response can only be made if

tissues must be able to express class I MHC

how are class I MHC are expressed?

many cell types express class I molecules for the presentation of intracellular antigens

How are class II MHC molecules expressed?

only on the cells of the immune system, specifically cells that affect other arms of the immune response.

molecules that bear class II MHC are aka

professional antigen-presenting cells

three types of professional antigen presenters

macrophages, dendritic cells, and B cells

Macrophages

stimulate T cells to release cytokines that enhance phagocytosis

dendritic cells

kill pathogens by phagocytosis , but their major function is to bring antigens to regional draining lymph nodes.

lymph nodes

the locations in which most T cell responses against pathogens of the interstitial tissues are mounted.

where are macrophages found?

in the skin and in the lining of mucosal surfaces, such as the nasopharynx, stomach, lungs, and intestines.

B cells

may also present antigens to T cells, which are necessary for certain types of antibody responses

T cell tolerance

process during T cell differentiation where most T cells that recognize antigens from one's own body are destroyed

thymocytes

located in the cortex of the thymus. referred to as "double negatives" because they do not bear CD4 or CD8 molecules hat you can use to follow their pathways of differentiation

positive selection

a process known as double-negative thymocytes bind to the MHC molecules they observe on the thymic epithelia, and the MHC molecules of "self" are selected. This mechanism kills many thymocytes during T cell differentiation.

what percentage of the thymocytes that enter the thymus leave it as mature, functional T cells?

2%

double positives

cells that express both CD4 and CD8 makers

where does the negative selection take place?

the junction between the cortex and medulla

negative selection

selection against thymocytes in the thymus that react with self-antigen

negative selection process

-self-antigens brouht into the thymus from other parts of the body by professional antigen-presenting cells

-the T cells that bind to these self-antigens are selected for negatively and are killed by apoptosis.

what are the only T cells left after negative selection?

the only T cells left are those that can bind to MHC molecules of the body with foreing antigens presented on their binding clefts, preventing an attack on one's own body tissues, at least under normal circumstances.

the cells that leave the thymus become single positives, expressing what?

CD4 or CD8 but not both

what do the CD4 and CD8 cells bind to

CD4 binds to class II MHC and CD8 binds to class I MHC

how do mature T cells become activated?

by recognizing processed foreign antigen in association with a self-MHC molecule and begin dividing rapidly by mitosis.

clonal expansion

growth of a clone of selected lymphocytes. is necessary to make the immune response strong enough to effectively control a pathogen

what is the specificity of a T cell based on?

the amino acid sequence and the three-dimensional shape of the antigen-binding site formed by the variable regions of the two chains of the T cell receptor

clonal selection

stimulating growth of lymphocytes that have specific receptors

T cell cloning

Each T cell that is activated has a specific receptor "hard-wired" into its DNA, and all of its progeny will have identical DNA and T cell receptors, forming clones of the original T cell.

clonal selection theory

a typical individual has a multitude (1011) of different types of T cell clones based on their receptors.

clone

a group of lymphocytes that share the same antigen receptor

antigen receptor

two-chain receptor by which lymphocytes recognize antigen

only what lymphocytes are stimulated to proliferate?

only those clones of lymphocytes whose receptors are activated by the antigen

polyclonal response

the stimulation of multiple T cell clones

what do the selected clones do once activated?

increase in number and make many copies of each cell type, each clone with its unique receptor. By the time this process is complete, the body will have large numbers of specific lymphocytes available to fight the infection

memory T cells

long-lived immune cell reserved for future exposure to an pathogen

effector T cells

immune cells with a direct, adverse effect on a pathogen

when are memory T cells and effector T cells generated?

during a primary adaptive immune response

two ways T cells and antigen-presenting cells are held together

by CD4 or CD8 attaching to MHC and by the T cell receptor binding to antigen

Helper T cells (Th)

T cells that secrete cytokines to enhance other immune responses, involved in activation of both B and T cell lymphocytes

two classes of Th cells

Th1 and Th2 cells

Th1 cells

a type of helper T cell that secretes cytokines that regulate the immunological activity and development of a variety of cells, including macrophages and other types of T cells.

Th2 cells

cytokine-secreting cells that act on B cells to drive their differentiation into plasma cells that make antibody

T cell-dependent antigens

T cell help is required for antibody responses to most protein antigens

Cytotoxic T cells (Tc)

T cells that kill target cells by inducing apoptosis using the same mechanism as NK cells.

the two things that Tc cells can do

They either express Fas ligand, which binds to the fas molecule on the target cell, or act by using perforins and granzymes contained in their cytoplasmic granules.

as more Tc cells are developed during an immune response, what happens?

they overwhelm the ability of the virus to cause disease.

can Tc cells kill more than one target cell?

yes

Regulatory T cells (Treg)

(also, suppressor T cells) class of CD4 T cells that regulates other T cell responses

T cells that secrete cytokines that help antibody responses are called ________.

a.Th1

b.Th2

c.regulatory T cells

d.thymocytes

B. Th2

The taking in of antigen and digesting it for later presentation is called ________.

a.antigen presentation

b.antigen processing

c.endocytosis

d.exocytosis

B. antigen processing

Why is clonal expansion so important?

a.to select for specific cells

b.to secrete cytokines

c.to kill target cells

d.to increase the numbers of specific cells

D. to increase the numbers of specific cells

The elimination of self-reactive thymocytes is called ________.

a.positive selection.

b.negative selection.

c.tolerance.

d.clonal selection.

B. negative selection

Which type of T cell is most effective against viruses?

a.Th1

b.Th2

c.cytotoxic T cells

d.regulatory T cells

C. cytotoxic T cells