Antigens & Antibodies

Antigen

A substance that stimulates antibody formation and can bind that antibody (anti-body generating factor).

Antigenicity vs. immunogenicity

Antigenicity = ability to be recognized/bind; Immunogenicity = ability to elicit an immune response.

Immune system distinction

Self vs. non-self (not good vs bad); responses to harmless antigens can cause hypersensitivity/autoimmunity.

Factors for good immunogen

Foreignness, size, chemical complexity, plus host factors (age, health, adjuvants, route, genetics).

Rank of immune responses by antigen foreignness

Heteroantigen > Alloantigen > Autoantigen.

Size effect on immunogenicity

<1000 Da: non-immunogenic; 1000-6000 Da: sometimes; >6000 Da: immunogenic.

Hapten

A small molecule (<~1000 Da) that is non-immunogenic alone; becomes immunogenic when linked to a large carrier.

Strongest immunogens

High molecular-weight proteins (including conjugates) > large polysaccharides; LPS is extremely antigenic; lipids/nucleic acids are usually poor (with autoimmune exceptions).

Host factors that decrease responses

Newborns/elderly, illness (e.g., HIV); mucosal routes favor IgA; IM/IV favor IgG and IgM; adjuvants and genetics (MHC/HLA) influence responses.

Epitope

The minimal part of an antigen that binds antibody or lymphocyte receptor; antigens usually have several epitopes.

Linear vs. conformational epitopes

Linear = sequence-based; Conformational = depends on 3-D structure.

Typical epitope sizes for B and T cells

B-cell epitopes are ~7-10 amino acids or 2-7 sugars; T-cell epitopes are similar in size but must be presented by MHC/HLA.

MHC vs. HLA

MHC is the genetic region; HLA are the human MHC genes/proteins. Class I and II directly mediate immune responses; Class III encodes some cytokines/complement/heat-shock proteins.

Core HLA function

Not provided in the notes.

Class I pathway

Expressed on all nucleated cells; presents intracellular (endogenous) peptides to CD8+ T cells (self no response; foreign activation).

Class II pathway

Expressed on antigen-presenting cells (dendritic cells, monocytes/macrophages, B cells, some epithelium); presents extracellular (phagocytosed) peptides to CD4+ T-helper cells, typically driving humoral responses.

Importance of antibodies clinically

They fight infection and are crucial in diagnostics (infection, autoimmunity, allergy), immunophenotyping (CD markers/flow), and detecting many analytes with high specificity and affinity.

Location and detection of antibodies

Primarily in serum and secretions; comprise ~15-20% of serum proteins; migrate with gamma globulins on serum protein electrophoresis.

Basic immunoglobulin structure

Two heavy and two light chains linked by disulfide bonds; variable (V) and constant (C) regions; hinge; Fab (antigen-binding) and Fc (effector) regions.

Definition of Ig class and light-chain types

The heavy chain defines class (IgM, IgD, IgG, IgA, IgE). Light chains are kappa or lambda; a given antibody has either two kappa or two lambda (never one of each).

Fab vs Fc

Fab binds antigen and contains the entire light chain (kappa or lambda) plus part of the heavy chain. Fc is heavy-chain only and mediates effector functions. Papain digestion yields 2 Fab + 1 Fc.

Variable vs constant regions

Variable regions (especially CDRs) form diverse antigen-binding sites; the heavy-chain constant region determines class and effector functions.

Antigen binding and bonds

Binding occurs in the Fab region and is held by non-covalent bonds (hydrogen bonds, electrostatic, Van der Waals, hydrophobic). Multiple weak bonds ensure specificity.

Specificity and cross-reactivity

Specificity: binding to one determinant. Cross-reactivity: binding to a different antigen that shares a similar epitope (usually at lower affinity).

Affinity

The strength of one binding site for its epitope.

Avidity

The overall strength of binding, calculated as affinity multiplied by the number of binding sites.

Complement fixation

An effector function that follows antigen binding, involving the binding of antibodies to complement components.

Antibody-dependent cell-mediated cytotoxicity (ADCC)

A process where effector cells kill antibody-coated targets via Fc receptor engagement.

Complement-dependent cytotoxicity (CDC)

The process where the Fc region recruits the first complement component to activate the classical cascade and lyse targets.

IgG

Major serum immunoglobulin; monomer; ~3-week half-life; crosses placenta; activates classical complement; opsonin; neutralizes toxins/viruses; has four subclasses (IgG1, IgG2, IgG3, IgG4), with IgG1 and IgG3 fixing complement well.

IgM

First in the primary response; pentamer with J chain giving 10 binding sites; best at classical complement activation; strong agglutination; also opsonization and toxin neutralization.

IgA

Second most common in serum; predominant in secretions; IgA1 (serum, monomer) and secretory IgA/IgA2 (dimer with J chain); opsonin; mucosal protection; present in breast milk.

IgE

Least abundant in serum; heat-labile; binds mast cells/basophils leading to degranulation; mediates Type I hypersensitivity; important in anti-helminth responses.

IgD

Primarily on mature B-cell surfaces; scarce in serum; regulates B-cell maturation/differentiation.

Primary antibody response

The immune response during the first exposure to an antigen, characterized by the production of IgM followed by IgG.

Secondary antibody response

The immune response upon re-exposure to an antigen, which is faster, larger, and IgG-dominant due to memory B cells.

Four phases of the primary antibody response

Lag, Log, Plateau, Decline.

Isotype (class) switching

The process where a B cell replaces the heavy-chain constant region after activation, changing class and effector function.

Somatic hypermutation

Rapid mutations in immunoglobulin variable regions.

Affinity maturation

The process of selecting for higher-affinity clones, increasing antibody effectiveness over time.