Antibodies and Immunoglobulins (Lecture Recap)

Vocabulary flashcards covering key concepts about antibodies, their structure, classes, functions, and laboratory/blood banking relevance.

Antibodies (immunoglobulins)

Immune proteins produced by plasma cells that recognize and bind antigens; classified into five classes (IgG, IgM, IgA, IgD, IgE) and consist of two heavy and two light chains.

Antigen

A molecule or part of a molecule that is recognized by antibodies; contains epitopes that antibodies bind.

Spike protein

SARS-CoV-2 protein used as the antigen in vaccines; enables viral entry by binding to host cells; antibodies against it can neutralize the virus.

Epitope

The specific part of an antigen that an antibody's binding site recognizes and binds.

Fab region

The antigen-binding fragment of an antibody; contains the variable regions that determine specificity.

Fc region

The constant region of an antibody's heavy chains; interacts with Fc receptors and complement to mediate effector functions.

Heavy chain

One of the two large polypeptide chains in an antibody; five classes (gamma, mu, alpha, delta, epsilon) determine antibody class.

Light chain

The smaller polypeptide chains in an antibody; two types: kappa and lambda; an antibody has either two kappa or two lambda light chains.

J chain

Joining chain that links antibody monomers to form polymeric IgA dimers and IgM pentamers.

Secretory component

Mucosal transport component that shields secretory IgA as it moves from plasma to secretions; aids transcytosis and protection from degradation.

IgG

The predominant circulating antibody (70–80% of serum immunoglobulins) that crosses the placenta, activates classical complement, and provides long-term immunity; has four subclasses (IgG1–IgG4) and a half-life of ~23–25 days.

IgM

First antibody produced in a primary response; secreted as a pentamer with valence 10; strong activator of the complement system; short half-life (~5–10 days); on B cells as a monomer.

IgA

Antibody found mainly in secretions (mucosal surfaces) as a dimer with J chain; secretory component helps transport; protects mucosal surfaces; can be secreted in monomer form in serum.

IgE

Antibody bound to Fcε receptors on mast cells; mediates allergic hypersensitivity reactions via histamine release.

IgD

Low-abundance immunoglobulin that mainly serves as a B-cell receptor; short serum half-life and less clear role in serum.

Isotype

The class of antibody determined by the heavy chain (IgG, IgM, IgA, IgD, IgE).

Allotype

Genetic variations in antibody constant regions between individuals within the same species.

Ideotype

The specific variable region composition of an antibody that determines its antigen/epitope specificity.

Polyclonal antibodies

A mixture of antibodies produced by multiple B-cell clones that recognize multiple epitopes on an antigen.

Monoclonal antibodies

A population of antibodies all recognizing the same epitope, produced by a single B-cell clone (often via hybridoma technology).

Heavy chain classes (names)

Gamma (IgG), Mu (IgM), Alpha (IgA), Delta (IgD), Epsilon (IgE) define antibody class.

Light chain types

Kappa and lambda light chains; each antibody has one type only (either two kappa or two lambda).

Disulfide bonds

Covalent bonds that hold heavy and light chains together and stabilize the antibody structure.

Hinge region

Flexible region between constant heavy domain 1 and 2 that allows conformational changes of the antibody.

Variable region

Amino-terminal portion of each heavy and light chain that binds antigen; contains hypervariable regions that determine specificity.

Hypervariable region

Complementarity-determining regions in the variable domains that make direct contact with the epitope; multiple hypervariable hotspots determine specificity.

Constant region

Carboxyl-terminal portion of heavy and light chains that is relatively invariant and defines the antibody class.

Fc receptors

Receptors on phagocytes and NK cells that bind the Fc portion of antibodies to trigger phagocytosis, ADCC, or complement activation.

Opsonization

Antibody coating of a pathogen that enhances phagocytosis via Fc receptor engagement.

ADCC

Antibody-dependent cellular cytotoxicity; NK cells recognize antibody-coated targets via Fc receptors and induce lysis.

Complement activation (classical pathway)

Antibody bound to antigen can trigger the classical complement cascade leading to pathogen lysis or opsonization.

Neutralization

Antibody binding blocks a pathogen's ability to attach to or enter host cells.

Valence

Number of antigen-binding sites; most antibodies are bivalent (two), while IgM pentamers have valence 10.

Secretory IgA vs IgA in serum

Secretory IgA is predominantly found in secretions (mucosa) as a dimer with a J chain and secretory component; serum IgA is mainly monomeric.

Mast cells and IgE

IgE binds Fcε receptors on mast cells; allergen binding triggers histamine release and allergic reactions.

Electrophoresis gamma region

Protein electrophoresis region where most immunoglobulins migrate (gamma globulins); used to assess antibody levels.

Blood banking antibodies

Antibodies formed against transfusion antigens (alloantibodies); important for crossmatching and preventing transfusion reactions.

ABO/RhD typing

Blood group system; ABO antigens on RBCs and RhD antigen; compatibility is essential for transfusions; O negative is often used in emergencies.

Acute transfusion reaction

Immediate hemolytic reaction due to ABO incompatibility; occurs within minutes of transfusion.

Delayed transfusion reaction

Reaction due to alloantibodies against non-ABO antigens; onset hours to days after transfusion.

Hemolytic disease of the newborn (HDN)

Maternal IgG antibodies against fetal RBC antigens cross the placenta and cause fetal hemolysis.

Enzymatic digestion: Papain

Protease that cleaves above the hinge, yielding two Fab fragments and an Fc fragment.

Enzymatic digestion: Pepsin

Protease that cleaves below the hinge, typically yielding an Fab-containing fragment and degraded Fc; lecture notes describe the Fab portion as intact.

RID and ELISA

Laboratory methods to quantify antibodies: radial immunodiffusion (RID) and enzyme-linked immunosorbent assay (ELISA).

Mucosal immunity and secretory IgA (revisited)

Secretory IgA at mucosal surfaces; secretory component and J chain facilitate transport and protection from degradation.

Monospecific IgE–mast cell interaction

IgE binds to mast cell Fc receptors and mediates hypersensitivity responses upon allergen exposure.