21.3 The Adaptive Immune Response: T lymphocytes and Their Functional Types

Can the innate immune response usually eliminate a pathogen completely?

No, the innate immune response (and early induced responses) often cannot completely control pathogen growth.

If the innate immune system can't eliminate pathogens, what does it achieve?

It slows pathogen growth and allows time for the adaptive immune system to respond.

Besides slowing pathogen growth, what important role does the innate immune system play?

It sends signals to adaptive immune system cells, guiding them on how to attack the pathogen.

What are the two main "arms" of the immune response?

The innate immune response and the adaptive immune response.

What is the greatest strength of the adaptive immune response?

Its specificity—the ability to specifically recognize and respond to a wide variety of pathogens.

What is an antigen?

An antigen is a small chemical group, often associated with pathogens, recognized by receptors on B and T lymphocytes.

Which types of cells recognize antigens?

B lymphocytes (B cells) and T lymphocytes (T cells).

How versatile is the adaptive immune system's ability to recognize pathogens?

It can recognize nearly any pathogen.

How many different receptors can the adaptive immune system create to recognize pathogens?

As many as 10¹² (100 trillion) different receptors.

Why was it surprising that the adaptive immune system could make so many different receptors?

Because there is not enough DNA in a cell to have a separate gene for each receptor type.

What is the immune system’s first exposure to a pathogen called? (Define it)

The primary adaptive response is the immune system’s first exposure and reaction to a pathogen.

Why are symptoms usually worse during a primary adaptive response?

Because it takes time for the initial adaptive immune response to become effective.

What is primary disease?

Primary disease refers to the relatively severe symptoms that occur during a first infection, before the adaptive immune system can fully respond.

What happens when the immune system encounters the same pathogen again? (Define it)

A secondary adaptive response is generated, which is stronger and faster than the primary response.

Why is the secondary adaptive response important?

It often eliminates the pathogen before significant tissue damage or symptoms occur.

What happens if the secondary adaptive response works well enough?

The individual may not experience any symptoms and might not even know they were infected.

What is immunological memory? (Define it)

Immunological memory is the immune system’s ability to remember a pathogen after the first exposure, providing protection against future infections by the same pathogen.

How does immunological memory benefit a person over time?

Early exposure to pathogens helps protect the person from developing the same diseases later in life.

What important feature allows the adaptive immune system to prevent attacking the body's own tissues?

The ability to distinguish between self-antigens (normal body components) and foreign antigens (from pathogens).

What happens to T and B cells as they mature to prevent attacks on self-antigens?

Mechanisms exist during their maturation that eliminate or deactivate cells that recognize self-antigens.

Are these self-recognition mechanisms perfect?

No, they are not 100% effective.

What can happen if self-recognition mechanisms break down?

The individual can develop autoimmune diseases, where the immune system attacks the body's own tissues.

what are the primary cells that control the adaptive immune response

lymphoctyes t and b cells

t cells not only control the immune response, but aloso

the b cells

y is t cell important

many of the decisions about how to attack a pathogen are made at the T cell level, and knowledge of their functional types is crucial to understanding the functioning and regulation of adaptive immune responses as a whole.

how do t lymphocytes recognize antigens

based on a 2 chain protein receptor, alpha beta t cell receptors

what receptor is the most common and important

alpha beta t cell receptor

what does the 2 chain protein receptor contain

variable region domain, constatant region domain, and transmembrane region

what are the 2 domains of the 2 chain protein receptor

variable region domain and constant region domain

what is the variable region domiain

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

wbat is the constant region domain

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

What causes the diversity of antigens that a receptor can recognize?

Differences in the amino acid sequences of the variable domains of the receptor chains.

What forms the antigen-binding site of a receptor?

The terminal ends of both receptor chains combine to form the antigen-binding site.

How many types of receptors does each T cell produce?

Each T cell produces only one type of receptor, making it specific to a single particular antigen.

How is the antigenic specificity of a receptor determined?

By the combined amino acid sequences at the ends of the receptor chains.

What are antigens typically like in size and complexity?

Antigens are usually large and complex.

What is an antigenic determinant? (Define it)

An antigenic determinant (also called an epitope) is a small region within an antigen that a receptor can bind to.

How big is an antigenic determinant usually?

It typically consists of six or fewer amino acid residues (for proteins) or one or two sugar moieties (for carbohydrates).

Why are antigenic determinants limited in size?

Because their size must match the size of the receptor's binding site.

Where are carbohydrate antigens commonly found?

On bacterial cell walls and red blood cells (e.g., ABO blood group antigens).

Are carbohydrate antigens more or less diverse than protein antigens?

They are usually less diverse than protein antigens.

Why are protein antigens especially important in immune responses?

Because proteins have a wide variety of three-dimensional shapes, making them complex and crucial for immune responses against viruses and worm parasites.

What accounts for the chemical basis of antigen specificity?

The interaction between the shape of the antigen and the complementary shape of the amino acids in the receptor’s antigen-binding site.

Can T cells bind to different parts of the same antigen?

Yes, different T cells can bind to different antigenic determinants on the same antigen.

Do T cells recognize free-floating antigens directly?

No, T cells recognize antigens that are processed and presented by specialized cells called antigen-presenting cells.

What is antigen processing? (Define it)

Antigen processing is the mechanism by which an antigen-presenting cell enzymatically digests an antigen into smaller fragments.

What happens to antigen fragments after processing?

They are brought to the cell’s surface and attached to special proteins called MHC molecules.

What is a major histocompatibility complex (MHC)? (Define it)

The major histocompatibility complex (MHC) is a cluster of genes that encode proteins responsible for displaying processed antigen fragments on the surface of cells.

What is antigen presentation? (Define it)

Antigen presentation is the process where antigen fragments, combined with MHC molecules, are displayed on the surface of an antigen-presenting cell for recognition by T cells.

What part of the MHC molecule holds the processed antigen fragment?

The peptide-binding cleft at the far end of the MHC molecule holds the fragment.

What actually binds to the T cell receptor during antigen recognition?

The combination of the MHC molecule and the processed antigen fragment.

What happens first during antigen presentation?

pathogen or extracellular antigen is phagocytized by an antigen-presenting cell (like a dendritic cell) and enclosed in a vesicle.

ow are antigens extracted from the ingested pathogen?

lysosomes digest the pathogen to release antigens.

What happens after antigens are extracted?

he antigens bind with MHC proteins that enter the vesicle.

What happens next to the MHC-antigen complex?

The complex moves to the outer surface of the cell membrane.

hat is the result of antigen presentation by a dendritic cell?

he dendritic cell presents antigens to T cells, which recognize and bind to the MHC-antigen complex to activate the immune response.

What are the two types of MHC molecules?

MHC class I and MHC class II.

What is MHC class I? (Define it)

MHC class I molecules present antigens from intracellular pathogens (like viruses) to T cells.

How are intracellular antigens processed for MHC class I presentation?

They are digested by the proteasome, transported into the endoplasmic reticulum by the TAP system, bound to MHC class I, and sent to the cell surface.

What is MHC class II? (Define it)

MHC class II molecules present antigens from extracellular pathogens (like bacteria, fungi, and parasites) to T cells.

How are extracellular antigens processed for MHC class II presentation?

They are brought into the cell by receptor-mediated endocytosis, fused with vesicles containing MHC class II molecules from the Golgi apparatus, and transported to the cell surface.

despite differences, what basic mechanism do MHC class I and class II both follow?

Antigens are digested, associated with MHC molecules, and presented on the cell surface for T cell recognition.

Which MHC molecules are expressed by many cell types for presenting intracellular antigens?

Class I MHC molecules

What kind of immune response do Class I MHC molecules stimulate?

They stimulate a cytotoxic T cell response, leading to destruction of the infected cell and the pathogen inside.

Why is Class I MHC expression important for fighting viruses?

Viruses infect almost every tissue in the body, so nearly all tissues must express Class I MHC molecules to allow a T cell immune response.

Which MHC molecules are expressed only by certain immune cells?

Class II MHC molecules.

What are professional antigen-presenting cells? (Define it)

Professional antigen-presenting cells (APCs) are immune cells that express Class II MHC molecules and are specialized in presenting antigens to help coordinate the immune response.

Why are these cells called "professional" APCs?

To distinguish them from regular cells that only express Class I MHC but do not help coordinate wider immune responses.

What are the three types of professional antigen-presenting cells?

Macrophages, dendritic cells, and B cells.

What is the main role of macrophages as professional APCs?

Macrophages stimulate T cells to release cytokines that enhance phagocytosis.

What do dendritic cells do?

Dendritic cells kill pathogens by phagocytosis but primarily bring antigens to lymph nodes to activate T cells.

Where do dendritic cells carry antigens?

To regional draining lymph nodes.

Where are most T cell responses against interstitial tissue pathogens initiated?

In the lymph nodes.

Where are macrophages commonly found?

In the skin and mucosal linings such as the nasopharynx, stomach, lungs, and intestines.

What is the role of B cells as professional APCs?

B cells present antigens to T cells, which is crucial for initiating certain antibody responses.

What is T cell tolerance? (Define it)

T cell tolerance is the process of eliminating T cells that might mistakenly attack the body's own cells.

Where does T cell development and tolerance occur?

In the thymus.

What are "double-negative" thymocytes?

Thymocytes that do not yet express CD4 or CD8 surface markers.

What happens to thymocytes when they first enter the cortex of the thymus?

They interact with cortical epithelial cells and undergo positive selection.

What is positive selection? (Define it)

Positive selection is the process where thymocytes that can bind to self-MHC molecules on thymic epithelial cells are allowed to survive, while those that cannot bind undergo apoptosis.

What percentage of thymocytes survive the process of T cell differentiation?

Only about 2% survive to become mature, functional T cells.

What happens to thymocytes that successfully bind to MHC molecules during positive selection?

They become "double-positive" cells, expressing both CD4 and CD8 receptors.

Where do thymocytes move after positive selection?

They move to the junction between the thymic cortex and medulla.

hat is negative selection? (Define it)

Negative selection is the process where thymocytes that bind strongly to self-antigens presented by antigen-presenting cells (APCs) are eliminated through apoptosis to prevent autoimmunity.

What kind of antigens are presented during negative selection?

Self-antigens from other parts of the body.

What happens to thymocytes that bind to self-antigens during negative selection?

They undergo apoptosis and are removed.

What happens to thymocytes that survive both positive and negative selection?

They become single-positive cells, expressing either CD4 or CD8, but not both.

What do CD4+ T cells do?

CD4+ T cells bind to Class II MHC molecules and function as helper T cells.

What do CD8+ T cells do?

CD8+ T cells bind to Class I MHC molecules and function as cytotoxic T cells.

What is the purpose of T cell differentiation and selection overall?

To ensure T cells can recognize self-MHC presenting foreign antigens without attacking the body’s own tissues.

Can T cell tolerance ever fail?

Yes, if tolerance mechanisms break down, it can lead to autoimmune diseases.

How do mature T cells become activated?

By recognizing processed foreign antigens presented together with a self-MHC molecule.

What happens after a T cell becomes activated?

It begins dividing rapidly by mitosis, a process called clonal expansion.

What is clonal expansion? (Define it)

Clonal expansion is the rapid multiplication of activated T cells to strengthen the immune response against a pathogen.

What determines the specificity of a T cell for an antigen?

The amino acid sequence and three-dimensional shape of the antigen-binding site on the T cell receptor.

What is an antigen receptor? (Define it)

An antigen receptor is a protein on the surface of T cells (or B cells) that specifically binds to a particular antigen.

What is clonal selection? (Define it)

Clonal selection is the process where only T cells with receptors specific to a given antigen are activated and expanded.

What happens to the DNA and receptors of activated T cells and their progeny?

All progeny (descendant cells) have identical DNA and T cell receptors as the original activated T cell.

What is a clone? (Define it)

A clone is a group of lymphocytes that all share the same antigen receptor and originate from a single activated T cell.