Q2 LONG ANSWER 10M ★ High-yield

Define antigen and hapten. Describe the structure and classes of immunoglobulins and the primary and secondary immune response.

Microbiology · 2nd Professional — Para-Clinical

Antigen — Definitions

Term	Meaning
Antigen	A substance that reacts specifically with antibody or with a lymphocyte receptor
Immunogen	A substance that can induce an immune response. All immunogens are antigens, but not all antigens are immunogens
HAPTEN	An incomplete antigen — too small to induce a response alone, but becomes immunogenic when coupled TO A carrier protein. It can still react with preformed antibody. This is how penicillin causes allergy: the drug binds host protein and the complex is recognised
Epitope (antigenic determinant)	The smallest part of the antigen that actually binds the antibody or receptor; one antigen carries many epitopes
Adjuvant	A substance that enhances the response to an antigen without being antigenically related — alum, Freund adjuvant; it works by provoking innate signals and by depot formation
Superantigen	Binds MHC class II and the T-cell receptor outside the antigen groove, activating up to 20% of all T cells non-specifically — a massive cytokine storm. Examples: staphylococcal TSST-1 and enterotoxins, streptococcal pyrogenic exotoxins

Determinants of immunogenicity — foreignness (the greater the phylogenetic distance, the stronger the response), large molecular size (above about 10 kDa), chemical complexity (proteins are the best immunogens, then polysaccharides; lipids and nucleic acids are poor unless conjugated), degradability, and the dose, route and genetic constitution of the host

Immunoglobulin Structure

The basic unit is a Y-shaped glycoprotein of four polypeptide chains → two identical heavy chains (about 50 kDa) and two identical light chains (about 25 kDa), joined by disulphide bonds → Light chains are either kappa or lambda — never both in one molecule → Each chain has a variable (V) region at the amino end and a constant (C) region → The variable regions of one heavy and one light chain together form the antigen-binding site, containing three hypervariable regions (complementarity-determining regions) that make the actual contact → The hinge region gives flexibility so the two arms can grip epitopes at varying distances → The heavy chain constant region forms the Fc portion, which determines the class and all the effector functions

Fragment	Produced by	Properties
Fab (2 per molecule)	PAPAIN digestion	"Fragment antigen binding" — each binds antigen but is monovalent, so it cannot cross-link and cannot precipitate or agglutinate
Fc (1 per molecule)	Papain digestion	"Fragment crystallisable" — no antigen binding; carries complement fixation, placental transfer, opsonisation and cell binding
F(ab')2	Pepsin digestion	Bivalent, so it can cross-link and precipitate, but has NO Fc — therefore no complement activation and less immunogenicity. This is why F(ab')2 antivenom causes fewer reactions than whole antiserum

CLINICAL PEARL

Splitting the molecule separates recognition from action, and that is exploited therapeutically. The Fab arms decide what is bound; the Fc stem decides what happens next. Removing the Fc, as in F(ab’)2 antivenom or digoxin-specific Fab fragments, keeps the binding and discards the complement activation and immunogenicity — which is exactly what is wanted when the object is to mop up a toxin rather than to provoke inflammation.

Classes of Immunoglobulin

Class	Heavy chain	Form and %	Key features
IgG	Gamma	Monomer; 75–80% — the most abundant	The only class crossing the placenta (giving passive immunity for 3–6 months); predominant in the secondary response; fixes complement; opsonises; neutralises toxins; longest half-life (about 23 days); four subclasses
IgM	Mu	Pentamer with a J chain; 5–10%; the largest ("millionaire molecule")	First antibody in the primary response and first made by the fetus; the most efficient complement activator (10 binding sites); best agglutinator; cannot cross the placenta — so IgM in cord blood means intrauterine infection; it is the natural isotype of ABO isohaemagglutinins
IgA	Alpha	Monomer in serum; dimer with J chain and secretory component in secretions; 10–15%	The chief mucosal antibody — in saliva, tears, respiratory, gastrointestinal and genitourinary secretions, and colostrum. The secretory component protects it from proteolysis. Does not fix complement by the classical pathway. Produced in the greatest daily amount of any class
IgE	Epsilon	Monomer; lowest serum concentration of all	Binds with high affinity to mast cells and basophils; mediates type I hypersensitivity; defence against helminths. Raised in atopy and parasitic infection
IgD	Delta	Monomer; trace	Chiefly a surface receptor on mature naive B cells; function otherwise unclear

Primary and Secondary Immune Response

Feature	Primary response	Secondary (anamnestic) response
Lag phase	Long — 5 to 10 days	Short — 1 to 3 days
Antibody class	IgM predominates	IgG predominates (class switching)
Peak level	Low	Much higher
Duration	Short	Prolonged
Affinity	Lower	Higher — affinity maturation by somatic hypermutation in germinal centres
Antigen dose needed	Large, often with adjuvant	Small
Cells involved	Naive B cells	Memory B cells

The secondary response is the entire basis of booster doses and of why a vaccinated person is protected: on real exposure the response is fast enough to prevent disease

Monoclonal Antibodies

Produced by hybridoma technology — a B cell from an immunised animal is fused with a myeloma cell, giving an immortal clone secreting a single antibody of one specificity (Köhler and Milstein)
Nomenclature indicates origin and immunogenicity — "-omab" murine (most immunogenic, provoking human anti-mouse antibodies), "-ximab" chimeric, "-zumab" humanised, "-umab" fully human (least immunogenic)
Diagnostic uses — ELISA and rapid tests, immunohistochemistry, blood grouping, flow cytometry, and tumour marker assays
Therapeutic uses — rituximab (anti-CD20), trastuzumab (anti-HER2), infliximab and adalimumab (anti-TNF), tocilizumab (anti-IL-6R), omalizumab (anti-IgE), eculizumab (anti-C5), and the checkpoint inhibitors in cancer
Passive immunisation — palivizumab against respiratory syncytial virus, and monoclonal antibodies developed against SARS-CoV-2 and rabies
Limitations — very high cost, parenteral administration, infusion reactions, immunogenicity, and infection risk with immunosuppressive agents; biosimilars produced in India have substantially improved access

Applied Aspects

IgM in the newborn indicates intrauterine infection, since maternal IgM cannot cross the placenta — the practical basis of diagnosing congenital rubella, toxoplasmosis, syphilis and cytomegalovirus
A fourfold rise in antibody titre between acute and convalescent sera is diagnostic; a single titre is hard to interpret where the population has high background seropositivity, as in India
IgM indicates recent infection and IgG past exposure or immunity, which is why both are measured in dengue, hepatitis and torch screening
Secretory IgA in colostrum protects the newborn gut, which is a concrete immunological argument for breastfeeding and for feeding through diarrhoeal illness
Selective IgA deficiency is the commonest primary immunodeficiency, and these patients may develop anaphylaxis to blood products containing IgA
Monoclonal antibodies are now major therapeutic agents — rituximab, trastuzumab, infliximab and the checkpoint inhibitors
Hapten-carrier conjugation explains drug allergy — penicillin is far too small to be immunogenic alone, and becomes so only by binding host protein; this is also why cross-reactivity occurs within a drug class
Superantigens explain toxic shock syndrome — a small amount of TSST-1 activates a huge fraction of T cells at once, and the resulting cytokine storm causes the shock rather than any direct toxicity
Adjuvants are what make a purified protein vaccine work; without an innate danger signal the adaptive system responds poorly, which is why alum is added to toxoid and recombinant vaccines
A booster dose exploits the secondary response, producing higher-affinity IgG within days; the interval between doses matters because memory cells need time to develop
Immunoglobulin levels vary with age — a physiological trough at 3 to 6 months as maternal IgG wanes before the infant’s own production rises, which is when antibody deficiencies first declare themselves
Serum protein electrophoresis is a useful screen — a monoclonal band suggests myeloma, a diffuse polyclonal rise suggests chronic infection or liver disease, and a reduced gamma region suggests hypogammaglobulinaemia
Antibody neutralises toxin but cannot reach an intracellular organism, which is why humoral immunity protects against diphtheria and tetanus and cell-mediated immunity is required against tuberculosis
Passive antibody interferes with live vaccines, so measles vaccination is deferred for some months after immunoglobulin or blood transfusion
Affinity maturation takes weeks, which is why the antibody of a primary response is less protective than that of a secondary one even at the same titre

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