Adaptive Immunity I: B-Cells, B-Cell Receptors and Antibodies

Vocabulary flashcards covering key terms, molecules and functions related to B-cell receptors and antibodies in adaptive immunity.

Adaptive Immunity

The slow, highly specific second line of defence that uses variable recognition molecules and generates immunological memory.

Severe Combined Immunodeficiency (SCID)

A life-threatening disorder (often X-linked IL-2R mutation) characterised by few T & NK cells and absent B cells, leading to recurrent viral and fungal infections and death if untreated.

Antigen

Any molecular determinant (usually protein, but also carbohydrate, lipid or nucleic acid) that can be recognised by antibodies, B-cell receptors or T-cell receptors and can induce immune responses.

Immunogen

A molecule that elicits an innate and/or adaptive immune response; all antigens are immunogens, but not all immunogens are antigens.

B-Cell Receptor (BCR)

The membrane-bound form of an antibody with a transmembrane domain and short cytoplasmic tail that initiates signalling, survival, proliferation and differentiation upon antigen binding.

Antibody (Immunoglobulin, Ig)

A glycoprotein composed of two identical heavy chains and two identical light chains that recognises antigen and mediates effector functions when secreted.

Heavy Chain

The larger polypeptide of an antibody that determines class (μ, δ, γ, α, ε) and contains constant and variable regions.

Light Chain

The smaller polypeptide (κ or λ) of an antibody that pairs with a heavy chain and contributes to antigen binding.

Variable Region

The N-terminal portion of heavy and light chains that forms the antigen-binding site and shows sequence diversity.

Constant Region

The C-terminal portion of heavy and light chains that mediates effector functions and defines antibody class.

Fab Fragment

The "fragment antigen binding" portion of an antibody that contains the antigen-binding site; generated by papain digestion.

Fc Fragment

The "fragment crystallisable" portion lacking antigen-binding activity but capable of binding Fc receptors and activating complement.

Neutralisation

Antibody function whereby high-affinity antibodies bind viruses, bacteria or toxins to block their interaction with host targets.

Opsonisation

The coating of pathogens with antibodies whose Fc regions engage Fcγ/ Fcα receptors on phagocytes, enhancing phagocytosis.

Classical Complement Activation

An antibody-mediated pathway, initiated most efficiently by IgM and also IgG, leading to complement cascade and pathogen lysis.

Antibody-Dependent Cell-Mediated Cytotoxicity (ADCC)

Killing of antibody-coated target cells by NK cells (via FcγRIII) and other leukocytes, resulting in apoptosis of infected or malignant cells.

Degranulation

Release of inflammatory mediators (e.g., histamine) from granulocytes triggered by IgE binding to FcεR on mast cells, basophils or eosinophils.

J-Chain

A small polypeptide that links IgA dimers and IgM pentamers and facilitates their transepithelial transport via the poly-Ig receptor.

Poly-Ig Receptor (pIgR)

Epithelial cell receptor that binds J-chain-containing IgA/IgM and mediates their transcytosis into mucosal secretions.

IgM

The first antibody produced in an immune response; exists as a pentamer, potent complement activator and good opsoniser.

IgD

A monomeric antibody that, with IgM, functions mainly as a BCR on mature naïve B cells to regulate activation and suppression.

IgG

A monomeric antibody (four subclasses) that crosses the placenta, neutralises pathogens, activates complement and mediates ADCC.

IgA

An antibody existing as monomer or dimer; the dimer is secreted across epithelia to protect mucosal surfaces and is protease-resistant.

IgE

A monomeric antibody that binds FcεR on mast cells, basophils and eosinophils, triggering degranulation for allergy and parasite defence.

Plasma Cell

A differentiated B cell that secretes large quantities of antibodies.

Memory B Cell

A long-lived B cell generated after antigen exposure that responds rapidly upon re-encounter with the same antigen.

Opsonin

A molecule (e.g., antibody or complement component) that enhances phagocytosis by marking a pathogen for ingestion.

Fc Receptor (FcR)

A cell-surface receptor that binds the Fc region of antibodies, mediating effector functions such as phagocytosis, ADCC or degranulation.

Naïve B Cell

A mature B cell that has left the bone marrow, expresses IgM and IgD BCRs, but has not yet encountered its specific antigen.

Complement

A system of plasma proteins that can be activated by antibodies (classical pathway) to lyse pathogens, opsonise them and promote inflammation.