Antibodies: Structure, Classes, Functions and Clinical Interpretation

Antibodies (immunoglobulins) are antigen-binding glycoproteins made by B cells and plasma cells. Their variable regions recognise specific epitopes, while constant regions determine what happens next—neutralisation, complement activation, phagocytosis, mucosal transport, mast-cell activation or placental transfer. Antibody structure, class and affinity explain serology, vaccination, allergy, autoimmunity, transfusion reactions and many antibody-based therapies.

Learning outcomes

The learner should be able to draw and label an immunoglobulin, distinguish variable and constant regions, explain heavy-chain classes and light chains, compare IgM, IgG, IgA, IgE and IgD, describe antibody effector functions, interpret affinity and avidity, and recognise antibody-related disease and treatment complications.

1. Basic immunoglobulin structure

A conventional antibody has a Y-shaped structure formed by two identical heavy chains and two identical light chains. Disulphide bonds hold the chains together, and non-covalent forces maintain their folded domains.

Region	Structure	Function
Variable region	Variable domains of one heavy and one light chain form each binding site	Recognises a specific epitope
Complementarity-determining regions	Hypervariable loops within the variable region	Make direct contact with antigen
Framework region	Conserved scaffold around CDRs	Maintains antibody shape
Fab	Antigen-binding arms	Neutralisation and specificity
Hinge	Flexible segment between arms and Fc in some classes	Allows different binding angles and distances
Fc	Constant heavy-chain region	Fc-receptor binding, complement, transport and effector function

2. Light and heavy chains

2.1 Light chains

Light chains are either kappa or lambda. One antibody uses only one type; both may be present in a person, but an individual B-cell clone expresses one light-chain type. Excess free light chains can occur in plasma-cell disorders and are measured clinically.

2.2 Heavy chains

Heavy-chain constant regions define immunoglobulin class: mu for IgM, delta for IgD, gamma for IgG, alpha for IgA and epsilon for IgE. Class switching changes the heavy-chain constant region while retaining the same antigen-binding variable region.

3. Antigen–antibody binding

Binding depends on shape, charge, hydrophobic forces and hydrogen bonds. An epitope is the part of an antigen recognised by an antibody; an antigen may contain many epitopes. Antibodies can recognise linear sequences or conformational structures that disappear when a protein is denatured.

3.1 Affinity and avidity

• Affinity: strength of one binding site–epitope interaction.
• Avidity: combined strength of all interactions. IgM has moderate affinity per site but high avidity because it binds multivalently.
• Affinity maturation in germinal centres selects B cells with stronger binding.

3.2 Cross-reactivity

An antibody can bind a similar epitope on a different antigen. Cross-reactivity helps some immune protection but can cause false-positive serology or molecular mimicry in autoimmunity.

4. Immunoglobulin classes

Class	Typical form/location	Major functions	Clinical points
IgM	Surface monomer; secreted pentamer in serum	Early response, agglutination and strong classical complement activation	Large size limits tissue penetration; high avidity; recent infection interpretation requires caution
IgG	Monomer; serum and extravascular spaces	Neutralisation, opsonisation, complement, ADCC and placental transfer	Four subclasses have different Fc functions; dominant long-term systemic antibody
IgA	Serum monomer; secretory dimer in mucosa and milk	Neutralisation and immune exclusion at mucosal surfaces	Selective deficiency can cause mucosal infection and transfusion reactions
IgE	Low free serum level; Fc bound to mast cells/basophils	Immediate hypersensitivity and helminth defence	Cross-linking causes degranulation and anaphylaxis
IgD	Surface receptor on naive B cells	B-cell activation and antigen sensing	Little routine serum diagnostic use

5. IgG subclasses

• IgG1: abundant; responds well to protein antigens and crosses placenta effectively.
• IgG2: important for polysaccharide responses.
• IgG3: strong complement activation and a flexible hinge, but shorter half-life.
• IgG4: limited complement activation and associated with immune tolerance; abnormal levels occur in selected disease.

Subclass levels must be interpreted with age, vaccine response and recurrent infection; an isolated low value is not automatically clinically significant.

6. IgA and mucosal protection

Plasma cells beneath mucosal epithelium produce dimeric IgA joined by a J chain. The polymeric immunoglobulin receptor transports it across epithelial cells, and the secretory component protects it from proteolysis. Secretory IgA blocks attachment and neutralises toxins with relatively little complement-mediated inflammation.

IgA in breast milk supports infant mucosal protection. Selective IgA deficiency can be asymptomatic, but some patients develop recurrent respiratory or gastrointestinal infection, allergy, autoimmunity or reactions to blood products containing IgA.

7. Fc receptors and effector functions

Effector mechanism	How antibody participates	Outcome
Neutralisation	Fab blocks toxin, viral receptor or microbial attachment	Prevents entry or tissue injury
Opsonisation	IgG Fc is recognised by phagocyte Fc receptors	Enhanced phagocytosis and killing
Complement activation	Antigen-bound IgM/IgG recruits C1	C3b coating, inflammation and membrane attack
ADCC	NK-cell Fc receptors bind antibody-coated targets	Target-cell apoptosis
Mast-cell activation	IgE Fc binds high-affinity Fc-epsilon receptors	Histamine, leukotriene and cytokine release
Placental transfer	FcRn transports IgG across placenta	Temporary neonatal systemic protection

8. Antibody production and class switching

1. B-cell receptor binds antigen.
2. Antigen is internalised and peptide presented on MHC II.
3. T follicular helper cells provide CD40L and cytokines.
4. Activated B cells proliferate in follicles and germinal centres.
5. Somatic hypermutation and selection improve affinity.
6. Class switching selects the Fc function suited to the pathogen and tissue.
7. Plasma cells secrete antibody; memory B cells remain.

Class switching is irreversible at the DNA level for an individual B-cell clone but preserves antigen specificity. A defect in CD40–CD40L signalling or activation-induced cytidine deaminase can produce hyper-IgM syndromes.

9. Monoclonal and polyclonal antibodies

• Polyclonal response: many B-cell clones recognise different epitopes on the same antigen; broad but heterogeneous.
• Monoclonal antibody: one clone recognises one epitope; highly specific and used in diagnostics and treatment.
• Human, humanised and chimeric products: differ in immunogenicity and pharmacology; product-specific monitoring is required.
• Antibody fragments: Fab or single-chain fragments may penetrate tissues differently and lack some Fc functions.

10. Clinical laboratory use

Test	What it detects	Clinical use
Quantitative immunoglobulins	Serum IgG, IgA, IgM and sometimes subclasses	Evaluate recurrent infection or immune deficiency
Specific antibody titres	Response to vaccines or pathogens	Assess functional humoral immunity
Electrophoresis/immunofixation	Monoclonal immunoglobulin or light chain	Myeloma and monoclonal gammopathy assessment
Direct antiglobulin test	IgG/complement attached to patient red cells	Immune haemolysis and transfusion reaction
Indirect antiglobulin test	Serum antibodies against red-cell antigens	Crossmatching and antenatal antibody screening
Serology	Pathogen-specific IgM/IgG or other antibodies	Exposure, infection timing or immunity support
Immunohistochemistry/flow cytometry	Antibody-defined tissue or cell markers	Classify tumours, lymphoma and receptor status

11. Antibody-mediated disorders

11.1 Allergy and anaphylaxis

Allergen-specific IgE binds mast cells. Re-exposure cross-links IgE and releases histamine, tryptase, leukotrienes and cytokines. Urticaria, angioedema, wheeze, vomiting, hypotension or collapse require emergency treatment, not delayed antibody testing.

11.2 Autoantibodies

Autoantibodies may stimulate receptors (Graves disease), block receptors (myasthenia), destroy cells (autoimmune haemolytic anaemia) or form immune complexes (lupus). A positive autoantibody is not always equivalent to active disease.

11.3 Antibody deficiency

Agammaglobulinaemia, common variable immunodeficiency, selective IgA deficiency, protein loss, HIV, lymphoma and B-cell-depleting medicines can impair antibody protection. Recurrent encapsulated-bacterial infection is a key clue.

12. Passive antibodies and therapeutic applications

Immunoglobulin, antitoxin, antivenom and monoclonal antibodies provide immediate or targeted effects without requiring the patient to generate a new immune response. Duration depends on half-life and clearance. Risks include infusion reactions, serum sickness-like illness, thrombosis, haemolysis, renal injury, infection susceptibility and target-specific toxicity.

Always record product name, dose, batch when required, indication, timing and observed reaction. Passive antibodies can interfere with some live vaccines or serological tests; follow immunisation guidance.

13. Emergencies involving antibodies

14. Interpretation principles

• Match the antibody class to timing, tissue and pathogen.
• Check whether the assay detects total antibody, a specific isotype, neutralising activity or a tissue-bound antibody.
• Use the laboratory reference range and method; do not compare values from different assays casually.
• Interpret low immunoglobulin with infection history and vaccine responses.
• Do not use an isolated positive antibody to diagnose active disease without clinical correlation.

15. Cases

Case 1: Positive IgG after vaccination

A patient has pathogen-specific IgG but no symptoms. This may represent vaccination or past infection. It should not be interpreted as proof of current disease without compatible clinical and direct pathogen testing.

Case 2: Recurrent bacterial infection

An adult has repeated sinusitis and pneumonia. Order quantitative immunoglobulins and functional vaccine-response testing while evaluating HIV, protein loss, malignancy and medication effects.

Case 3: Acute haemolytic transfusion reaction

Fever, flank pain, dark urine and hypotension develop during transfusion. Stop the transfusion immediately, assess ABCDE, maintain intravenous access and activate the transfusion-reaction protocol; antibody testing follows urgent stabilisation.

16. Exam pearls

• Variable regions determine specificity; constant Fc regions determine effector function.
• IgM is pentameric in serum and has high avidity; IgG crosses placenta; IgA protects mucosa; IgE activates mast cells; IgD is mainly a B-cell receptor.
• Class switching changes heavy-chain constant region, not antigen specificity.
• Affinity is one-site strength; avidity is total multivalent strength.
• Antibodies can neutralise, opsonise, activate complement and trigger ADCC.
• A positive antibody test may represent past exposure, vaccination, cross-reactivity or active disease; context is essential.

References and further reading

• NCBI Bookshelf: B cells and antibodies.
• NCBI Bookshelf: Immunoglobulin structure and function.
• NCBI Bookshelf: Humoral immune response.
• Follow local transfusion, immunology, anaphylaxis, immunisation and therapeutic-antibody protocols.