BPSC 71st CCE Prelims General Studies · Question 141 of 150

Correct — C, IgM. IgM is the first antibody class to appear when the body meets an antigen for the first time, and the reason is mechanical rather than a matter of convention. A naive B lymphocyte carries membrane-bound IgM (alongside IgD) as its own antigen receptor, so IgM is the only class it can secrete straight away; producing IgG, IgA or IgE first requires class-switch recombination — an irreversible cut-and-splice of the heavy-chain constant-region genes that needs T-helper cell signals (CD40–CD40L plus cytokines) and takes several days. In a primary response the antibody titre therefore rises after a lag of roughly 5–10 days, and what rises first is IgM. Secreted IgM is a pentamer: five Y-shaped monomers held together by disulphide bonds and a small J (joining) chain, about 900 kilodaltons in mass and carrying ten antigen-binding sites. That size is where the letter comes from — IgM was originally the 'macroglobulin' fraction of serum. The pentameric shape also explains what IgM is good at: each individual binding site has low affinity, because the response has not yet been sharpened by somatic hypermutation and affinity maturation, but ten sites gripping at once give very high avidity, so IgM is an outstanding agglutinator of bacteria and the most powerful activator of the classical complement pathway — one bound pentamer in its 'staple' conformation can fix C1q, whereas IgG needs several molecules sitting close together. IgM makes up only about 5–10% of serum immunoglobulin and, being far too large to leak out of capillaries, stays largely inside the bloodstream; it does not cross the placenta. Clinical practice runs on exactly this ordering: an IgM-positive serology means a recent or ongoing infection while IgG-positive means past exposure or vaccination, which is why the dengue IgM-capture ELISA (MAC-ELISA) is the standard confirmatory test in India's national programme, and why IgM detected in a newborn proves the baby's own congenital infection — maternal IgM cannot have crossed the placenta to get there.

• (a)IgG — The most tempting wrong answer, because IgG is the antibody everyone has heard of: about 75–80% of serum immunoglobulin, four subclasses (IgG1–IgG4), a serum half-life of roughly 21 days, the only class that crosses the placenta, and the one measured when a laboratory reports 'immunity'. But abundance is not sequence. IgG appears only after class switching, so it dominates the secondary (memory) response — faster, higher and longer-lasting, which is precisely why booster doses work — not the first encounter.
• (b)IgA — IgA is the specialist of mucosal surfaces, secreted as a dimer with a J chain plus a secretory component that protects it from digestive enzymes, and found in saliva, tears, nasal and gut mucus, and in colostrum and breast milk, where it gives the newborn passive protection. By sheer mass it is the antibody the body manufactures most of each day. But it guards the mucosal frontier and is itself a class-switched product; it is not the first class secreted in a systemic primary response.
• (d)IgE — IgE is the rarest immunoglobulin in serum, present in nanogram quantities. Its Fc region binds FcεRI receptors on mast cells and basophils, so that re-exposure to the antigen cross-links the bound IgE and triggers histamine release — Type I or 'immediate' hypersensitivity: allergy, asthma, urticaria and anaphylaxis, plus defence against helminth worms. The word 'immediate' is the trap here. It describes how fast the reaction unfolds on a SECOND exposure, in minutes, not which class is made first on the first exposure.

Antibodies and immunoglobulins are the same thing: the antigen-binding proteins secreted by plasma cells, which are B lymphocytes that have differentiated after meeting their antigen. The basic unit is a Y, built from two identical heavy chains and two identical light chains (kappa or lambda) held together by disulphide bonds. The two arms of the Y carry the variable regions that recognise antigen; the stem, called Fc, carries no antigen specificity at all but decides what the antibody DOES — whether it fixes complement, whether phagocytes can grab it, whether it can cross the placenta, whether it can be exported into mucus. Because the stem is built from the heavy-chain constant region, that region alone defines the class: a mu heavy chain makes IgM, gamma makes IgG, alpha makes IgA, epsilon makes IgE and delta makes IgD. There are therefore exactly five classes in humans. Class switching swaps the constant region while keeping the variable region intact, so the antibody keeps recognising the same antigen but acquires a new job — the same key on a different handle. On top of this sits the primary-versus-secondary distinction that the question is really testing: a first exposure gives a slow, low, short-lived and IgM-dominated response, while a second exposure gives a response that starts within one to three days, climbs to a far higher titre, lasts much longer and is overwhelmingly IgG. That gap between the two curves is the entire scientific basis of vaccination and of booster doses.

Read the word 'first'. The question is about a timeline, not about quantity, not about importance and not about where an antibody is found — and each of the wrong options is attractive on one of those other axes. Two options fall away simply by asking what the class is FOR: IgE is a specialist that arms mast cells for allergy and anti-worm defence, and IgA is a specialist that patrols mucous membranes; a specialist deployed to a particular tissue is not a general first responder to a new infection. That reduces the question to IgG versus IgM, which is where the item is designed to be decided. The single discriminating fact is this: the B cell's own surface receptor is IgM, so IgM is the only class it can secrete without first rearranging its heavy-chain constant-region DNA. Class switching to gamma, alpha or epsilon needs T-cell help and days of time, so IgG can never be the first thing produced in a genuine primary response — the ordering is IgM then IgG, always. A useful cross-check is the diagnostic convention every hospital uses: laboratories report IgM as the marker of recent infection and IgG as the marker of past infection or vaccination precisely because IgM comes first and fades within weeks while IgG persists for years. If you can recall why a dengue or typhoid report distinguishes IgM from IgG, you already know the answer. Memory hook: M for iMMediate and the Millionaire (macro) molecule; G for Greatest amount and Gestation, since IgG is both the most abundant and the only one crossing the placenta.

• Secreted IgM is a pentamer — five monomers plus one J (joining) chain, about 900 kDa, with ten antigen-binding sites; the 'M' comes from macroglobulin, and the ten-point grip gives high avidity even though each individual site has low affinity before affinity maturation.
• IgM is roughly 5–10% of serum immunoglobulin and stays mostly inside blood vessels because of its size; it does NOT cross the placenta, whereas IgG does — which is why IgM found in a newborn's blood proves the infant's own congenital infection rather than the mother's antibodies.
• Five human classes are defined solely by the heavy-chain constant region: mu = IgM, gamma = IgG, alpha = IgA, epsilon = IgE, delta = IgD. A naive B cell displays IgM and IgD on its surface; every other class requires class-switch recombination driven by T-helper signals.
• IgG is about 75–80% of serum immunoglobulin, has four subclasses (IgG1–IgG4) and a serum half-life of roughly 21 days, and dominates the secondary response — the reason a booster dose produces a faster and much larger antibody rise than the first dose.
• IgM is the strongest activator of the classical complement pathway, since a single bound pentamer can fix C1q; the ABO blood-group anti-A and anti-B isohaemagglutinins are IgM, which is why an ABO-incompatible transfusion causes immediate intravascular haemolysis rather than a delayed reaction.

• Answering IgG because it is the most abundant and the most familiar antibody — abundance is not sequence; IgG needs class switching and so belongs to the secondary response
• Reading 'immediate hypersensitivity' as 'immediate antibody' and choosing IgE — 'immediate' there describes minutes after a SECOND exposure, not first in the order of production
• Assuming a newborn is protected by maternal IgM — maternal IgG crosses the placenta, IgM cannot, which is exactly why IgM in a baby's blood signals the baby's own infection

BPSC asks this as a flat one-line recall — name the class, one mark, ten seconds — and recycles the same five-class list across cycles: which class crosses the placenta, which is abundant in colostrum, which mediates allergy, which activates complement best. UPSC almost never asks 'which class'; it wraps antibodies inside an application and makes you judge statements, as with hybridoma technology in 2000, why an effective malaria vaccine is hard in 2010, and monoclonal antibodies against Nipah virus in 2025. For BPSC memorise the table; for UPSC understand what the molecule does.

• practice — not a real PYQ

  Which class of immunoglobulin crosses the human placenta and provides passive immunity to the newborn?

  • (a)IgA
  • (b)IgD
  • (c)IgG
  • (d)IgM

  Answer(c) IgG — it is the only class whose Fc region is carried across the placenta by the FcRn receptor, giving the infant several months of borrowed protection. IgA reaches the baby through colostrum and breast milk, not the placenta; IgM is far too large to cross; IgD is a B-cell surface receptor.

• practice — not a real PYQ

  Antibodies in the human body are secreted by which of the following cells ?

  • (a)Plasma cells derived from B lymphocytes
  • (b)Cytotoxic T lymphocytes
  • (c)Macrophages
  • (d)Platelets

  Answer(a) Plasma cells derived from B lymphocytes — a B cell that meets its antigen differentiates into an antibody-secreting plasma cell. Cytotoxic T cells kill infected cells directly and secrete no antibody, macrophages phagocytose and present antigen, and platelets are cell fragments for clotting.