Which class of immunoglobulin is the first to be produced in response to an infection?

Question

UnlockIAS · Accepted Answer

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.

Why the other options are wrong — (a) 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 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 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.

Concept: 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.

State PCS 2025 Q141 — Science & Technology PYQ with Answer

State PCS 2025 Q141 — Science & Technology PYQ with Answer