Immunology, Chapter 4: Antigen-Recognition by B-cell, and T-cell Receptors

Each B-cell produces an immmunoglobulin of __________ specificity?

.... of SINGLE specificity?

How many types of light chains can an immunoglobulin posses, and how does it affect their functioning?

A given immunoglobulin has either lambda (l) chains and kappa (k) chains, never one of each. No functional difference has been found between antibodies having (l) chains and kappa (k) chains, and either type of light chain can be found in antibodies of any of the five major classes.

Define the term affinity?

Affinity is the strength of the interaction between a single antigen-binding site and its antigen.

What structural components constitute an Immunoglobulin molecule?

All antibodies are constructed in the same way from paired heavy and light polypeptide chains, and the generic term immunoglobulin is used for all such proteins.

How are the T-cell receptors and B-cell receptors similar?

Although B cells and T cells recog nize foreign molecules in distinct fashions, the receptor molecules they use for this task are very similar in structure. These T-cell receptors (TCRs) are related to immunoglobulins both in their protein structure—having both V and C regions—and in the genetic mechanism that produces their great variability.

Define an antigen?

An antigen is any molecule or part of a molecule that is specifically recognized by the highly specialized recognition proteins of lymphocytes.

What is antibody?

Antibodies are the secreted form of the B-cell receptor, immunoglobulin of the same antigen specificity as the B-cell's BCR is secreted as antibody by terminally differentiated B cells—the plasma cells.

Describe the shape of an antibody molecule and the functional purpose of its structure

Antibody molecules are roughly Y-shaped molecules consisting of three equal-sized portions connected by a flexible tether. The ends of the two arms of the Y—the V regions—vary in their detailed structure between different antibody molecules. These are involved in antigen binding. The stem of the Y—the C region—is far less variable and is the part that interacts with effector cells and molecules.

Define the term avidity?

Avidity is the strength of the interaction between two identical antigens binding simultaneously to two antigen binding sites.

MHC molecules are highly polymorphic, what does that mean?

Each type of MHC molecule occurs in many different versions within the population. Most people are therefore heterozygous for the MHC mole cules: that is, they express two different forms of each type of MHC molecule.

List the five classes of Immunoglobulins?

Five different classes of immunoglobulins—IgM, IgD, IgG, IgA, IgE—can be distinguished by their C regions. regions. More subtle differences con-fined to the V region account for the specificity of antigen binding.

IgG antibodies consist of four polypeptide chains, list and describe them

IgG antibodies are composed of two different kinds of polypeptide chains. One, of approximately 50 kDa, is called the heavy or H chain , and the other, of 25 kDa, is the light or L chain. Each IgG molecule consists of two heavy chains and two light chains.

Which antibody is the most abundant isotope in the plasma?

IgG is by far the most abundant immunoglobulin and has several subclasses (IgG1, 2, 3, and 4 in humans). The distinctive functional properties of the different classes and subclasses of antibodies are conferred by the carboxy-terminal part of the heavy chain, where it is not associated with the light chain.

Name the highly specialized recognition proteins of B cells are called what?

Immunoglobulins (Ig) which are produced by B cells in a vast range of antigen specificities.

How does the structure and arrangement of the antibody molecule facilitate its ability to bind up to two identical antigens at a time?

In any given immunoglobulin molecule, the two heavy chains and the two light chains are identical, giving an antibody molecule two identical antigen-binding sites (see Fig. 4.1). This gives it the ability to bind simultaneously to two identical antigens on a surface and hence increase the total strength of the interaction

The antibody molecule has two separate functions, describe them?

One function is to bind specifically to the pathogen or its products that elicited the immune response; the other function is to recruit other cells and molecules to destroy the pathogen once antibody has bound.

How does the structure of the antibody molecule facilitate its recognition and effector functions?

Recognition and effector functions are structurally separated in the antibody molecule, one part of which specifically binds to the antigen whereas the other engages the elimination mechanisms.

What is the term for the additional dimension of antigen recognition provided by MHC interactions in T-cell receptor-antigen-recognition?

T-cell receptors recognize features of both the peptide antigen and the MHC molecule to which it is bound. This introduces an extra dimension to antigen recognition by T cells, known as *MHC Restriction , because any given T-cell receptor is specific for a unique combination of a particular peptide and a particular MHC molecule.

What are MHC molecules?

The MHC molecules are transmembrane glycoproteins encoded in the large cluster of genes known as the major histocompatibility complex (MHC).

How do the T-cell receptors and B-cell receptors differ?

The T-cell receptor differs from the B-cell receptor in an important way, however: it does not recognize and bind antigen by itself, but instead recognizes short peptide fragments of protein antigens, which are presented by proteins known as MHC molecules on the surfaces of host cells.

Name and describe the region of the antibody molecule responsible for antigen recognition?

The antigen-binding region varies extensively between antibody molecules and is known as the variable region or V-region.

How does the primary function of T-cell receptors differ from that of B-cell receptors?

The antigen-recognition molecules of T cells are made solely as membrane-bound proteins, which are associated with an intracellular signaling complex and function only to signal T cells for activation.

What determines the ultimate function and class of an antibody molecule?

The class, and thus the effector function, of an antibody is defined by the struc-ture of its heavy chain.

What is the primary function of the B-cell receptor?

The function of the B-cell receptor is to recognize and bind antigen via the V regions exposed on the surface of the cell, thus transmitting a signal that activates the B cell, leading to clonal expansion and antibody production.

What might an overabundance of lambda (l) chains be indicative of?

The ratio of the two types of light chains varies from species to species. In humans it is 2:1. The reason for this variation is unknown. Distortions of this ratio can sometimes be used to detect the abnormal proliferation of a clone of B-cells. These will all express the identical light chain, and thus an excess of lambda (l) chains in a person might indicate the presence of a B-cell tumor producing lambda (l) chains.

Name and describe the region of the antibody molecule responsible for its effector functions?

The region of the antibody molecule that engages the effector functions of the immune system does not vary in the same way and is known as the constant region or C region. It comes in FIVE main forms, each of which is specialized for activating different effector mechanisms. The membrane-bound B-cell receptor does not have these effector functions, because the C-region remains inserted in the membrane of the B cell.

What is the main effector function B-cells in adaptive immunity?

The secretion of antibodies, which bind pathogens or their toxic products in the extracellular spaces of the body, is the main effector function of B cells in adaptive immunity.

Describe the similarity and difference in the structure of a B-cell receptor and its corresponding antibody?

The structure of a B-cell receptor is identical to that of its corresponding anti-body except for a small portion of the carboxy terminus of the heavy-chain C region. In the B-cell receptor, the carboxy terminus is a hydrophobic amino acid sequence that anchors the molecule in the membrane, and in the anti-body it is a hydrophilic sequence that allows secretion.

What is the most striking feature of MHC molecules?

Their most striking structural feature is a cleft in the extracellular face of the molecule, in which peptides can be bound.

How many classes/types of heavy chains are there, and what is the general term for each class of heavy chain? (hint: the heavy chain determines the class of antibody molecules, and there are 5 classes of immunoglobulins)

There are five main heavy-chain classes or isotypes , some of which have several subtypes, and these determine the functional activity of an antibody molecule. The five major classes of immunoglobulin are immunoglobulin-M (IgM), immunoglobulin-D (IgD), immunoglobulin-G (IgG), immunoglobulin-A (IgA), and immunoglobulin-E (IgE). Their heavy chains are denoted by the corresponding lower-case Greek letter (m, d, g, a, and e, respectively).

What type of B-cells secrete antibody?

Those whom have been stimulated by their Immunoglobulin-BCRs to differentiate into plasma cells (the effector B-cells).

How many types of light chains are found in antibodies, list them?

Two types of light chains, lambda (l) and kappa (k), are found in antibodies.

Membrane-bound immunoglobulin on the B-cell surface serves as the cell's receptor for antigen, and is known as ________...(WHAT)?

the B-cell receptor ( BCR ).

What type of antigen recognition molecule constitutes the B-cell receptor (BCR)?

the recognition proteins known as Immunoglobulins (Ig)