CHAPTER 4 Antigen Recognition in the Adaptive Immune System

Comprehensive vocabulary terms and definitions covering antigen receptor structure, genetic recombination, diversity mechanisms, and lymphocyte maturation processes.

BCR

Membrane-bound antibody found on B cells that serves as a receptor for antigens.

TCR

The T cell receptor found on T cells used for recognizing antigens.

Clonal Distribution

A principle where each lymphocyte clone is specific for a distinct antigen and possesses a unique receptor different from all other clones.

Repertoire

The term used to describe the total collection of distinct lymphocyte clones within an individual.

Hypervariable Regions

Three short segments within the variable (V) regions also known as complementarity-determining regions ($\text{CDRs}$) that specifically bind to the antigen.

Framework Regions

The less variable stretches located between the hypervariable regions, consisting of four segments per V region.

Constant (C) Domain

The conserved portion of the receptor; in secreted antibodies, this region engages effector functions such as binding complement proteins or $Fc$ receptors.

Antibody Monomer

A structure comprising two identical heavy ($H$) chains and two identical light ($L$) chains held together by disulfide bonds.

Isotype

The class of an antibody ($IgG$, $IgM$, $IgA$, $IgD$, or $IgE$) determined by the specific type of heavy chain present.

Fab

The Fragment, antigen-binding region of an antibody containing the variable sequences that form the target-binding site.

Fc

The Fragment, crystallizable region of the heavy chain that interacts with complement proteins and host immune cells like macrophages.

Idiotype

The specific antigen-binding surface of an antibody formed by six $CDRs$ that determine its specificity.

Hinge Region

A part of the antibody that allows the arms to move independently to bind antigens with different spacing.

BCR Complex

A signaling unit formed by the association of membrane-bound immunoglobulin with Ig\text{̑} and Ig\text{̒} proteins.

TCR Complex

A signaling unit formed by the association of the TCR with $CD3$ and $\zeta$ chains.

Dual Form of Antibodies

Antibodies can be expressed as membrane receptors or as proteins secreted by plasma cells, whereas TCRs only function as membrane receptors.

Epitope

The specific structure or antigenic determinant on an antigen that is recognized by an antibody.

Linear Epitope

An epitope composed of a single continuous segment of a polypeptide chain.

Conformational Epitope

An epitope formed by amino acids from different parts of a polypeptide chain that are brought together by protein folding.

Affinity

The strength of the interaction between a single antigen-binding site of an antibody and a single epitope.

Avidity

The overall strength of binding between a multivalent antibody and an antigen with multiple antigenic determinants.

Affinity Maturation

The process by which the antigen-binding strength of antibodies increases during secondary and subsequent immune responses.

Cross-reactivity

The phenomenon where an antibody produced against one antigen binds to a different but structurally similar antigen.

Monoclonal Antibodies

Identical antibodies produced by a single clone of B lymphocytes, often generated in labs using hybridoma technology.

Hybridomas

Fused cells created from a mouse splenocyte and an immortal myeloma cell line to produce monoclonal antibodies.

̓̔ TCR

A type of TCR expressed by $5\text{--}10\%$ of T cells, primarily located in the epithelium, which recognizes various protein and nonprotein antigens.

NK-T Cells

A subpopulation of T cells expressing $\alpha\beta$ TCRs with limited diversity that recognize lipid antigens displayed by $CD1$ molecules.

MAIT Cells

Mucosal associated invariant T cells that recognize vitamin B metabolites bound to the MHC-like protein $MRI$.

Lymphopoiesis

The production of new lymphocytes from stem cells in central lymphoid tissues like the bone marrow and thymus.

$$IL\text{-}7$$

A growth factor that stimulates the proliferation and maintenance of early lymphocyte progenitors before the expression of antigen receptors.

V(D)J Recombinase

An enzyme complex composed of $RAG\text{-}1$ and $RAG\text{-}2$ proteins that mediates the somatic recombination of antigen receptor genes.

Combinatorial Diversity

The diversity created by using different combinations of $V$, $D$, and $J$ gene segments in different clones.

Junctional Diversity

The diversity created at the coding joints during recombination through the addition ($TdT$ or $P\text{-nucleotides}$) or removal ($exonucleases$) of nucleotides.

$$TdT$$

Terminal deoxyribonucleotidyl transferase; an enzyme that adds random N-nucleotides to the joints of rearranged genes.

Allelic Exclusion

A process ensuring that each B cell expresses only one heavy chain and one light chain, resulting in a single antigen specificity.

Receptor Editing

A mechanism where self-reactive immature B cells undergo further light chain gene recombination to change their specificity and avoid self-reactivity.

Anergy

A state of functional non-responsiveness to an antigen, often leading to the ultimate death of the lymphocyte.

Positive Selection

A process in the thymus where immature T cells survive only if their TCRs can recognize self-MHC molecules.

Negative Selection

A process where immature lymphocytes with high affinity for self-antigens are eliminated via apoptosis to ensure central tolerance.

AIRE

The autoimmune regulator protein that allows thymic epithelial cells to express tissue-specific self-peptides for negative selection of T cells.

Pax-5

A transcription factor essential for B cell development that opens chromatin at the immunoglobulin loci.

Notch-1

A major regulator of T cell development that, when bound to its ligand, activates genes necessary for T cell lineage commitment.

Pre-BCR

A complex composed of a rearranged $\mu$ heavy chain and a surrogate light chain that signals the cell to move to the next stage of B cell development.

Checkpoints

Specific stages during lymphocyte maturation where cells are tested for the expression of functional receptor components; failure leads to apoptosis.