NCERT grounding
NCERT Class 12 Biology, Chapter 7 (Human Health and Disease), §7.2.2 introduces acquired immunity as the second tier of human defence, distinguishing it from innate immunity by two defining properties: antigen specificity and immunological memory. §7.2.3 extends this into the active–passive classification, while §7.2.4 builds vaccination on the memory principle. NIOS Biology Chapter 31 (Immunobiology — An Introduction) supplies the supporting framework: B- and T-cell origin, antibody structure, the five immunoglobulin classes (IgA, IgD, IgE, IgG, IgM) and the table contrasting humoral with cell-mediated responses. Together these two sources fix the NEET examinable surface for this subtopic.
"Acquired immunity, on the other hand, is pathogen specific. It is characterised by memory."
NCERT Class 12, §7.2.2
Acquired immunity in depth
Acquired immunity — also called adaptive or specific immunity — is the body's tailored response to particular foreign molecules called antigens. It emerges only after exposure (natural infection, vaccination or transferred antibodies) and is therefore absent at birth except where the mother has passed antibodies across the placenta. Two functional features make it irreplaceable: it can distinguish almost any conceivable antigen from "self", and it remembers what it has seen.
The cellular machinery is built from two specialised lymphocyte lineages. Both descend from haemopoietic stem cells in the bone marrow. B-lymphocytes complete their maturation in the bone marrow itself and, when activated, transform into plasma cells that secrete antibodies. T-lymphocytes leave the bone marrow as immature precursors, migrate to the thymus for maturation, and then populate peripheral lymphoid organs where they orchestrate both arms of the response. T-cells do not themselves secrete antibodies but help B-cells produce them and also directly kill infected or graft cells.
Out of this architecture emerge two effector arms — the antibody-driven humoral response and the lymphocyte-driven cell-mediated response — and two modes of acquisition — active and passive. NEET questions repeatedly probe the junction of these two classifications, so the rest of this article walks each axis in detail.
The two arms — humoral and cell-mediated
The acquired response routes through two effector arms that are anatomically distinct but functionally cooperative. Helper T-cells stand at the junction, signalling to both. NCERT introduces them in §7.2.2 as the "two types of our acquired immune response" — antibody-mediated and cell-mediated.
Humoral (antibody-mediated)
B-lymphocyte
EFFECTOR CELL
- B-cells activate, become plasma cells, secrete antibodies
- Antibodies circulate in blood, lymph and body fluids
- Acts mainly on extracellular pathogens, bacteria, toxins
- Combines with antigen to neutralise, agglutinate or opsonise
- Examples — most bacterial diseases, soluble toxins
Cell-mediated (CMI)
T-lymphocyte
EFFECTOR CELL
- Cytotoxic T-cells kill infected or abnormal cells directly
- No antibodies secreted — contact-dependent killing
- Acts on intracellular viruses, tumour cells, transplants
- Responsible for graft rejection (kidney, heart, liver)
- Mediates delayed-type hypersensitivity
The cell-mediated arm explains why transplants require careful tissue matching. NCERT §7.2.2 frames it directly: when an organ from "any source — an animal, another primate, or any human being" is grafted, the recipient's T-cells recognise the foreign surface markers as non-self and reject the graft. This is why blood-group and HLA matching are mandatory before transplantation and why patients must continue immunosuppressants for life. NEET 2017 Q.81 and NEET 2019 Q.72 both tested exactly this point.
Antibody structure and the five classes
An antibody (immunoglobulin, Ig) is a Y-shaped glycoprotein. Four polypeptide chains are linked by disulphide bonds: two identical heavy (H) chains of roughly 50 kDa each and two identical light (L) chains of roughly 25 kDa each. NCERT therefore writes the formula as H2L2. The Y has two antigen-binding tips ("Fab" prongs) formed by the variable N-terminal ends of one heavy and one light chain, and a constant stem ("Fc") composed of the lower halves of the two heavy chains. The variable region confers antigen specificity; the constant region determines the antibody's class and effector function.
Figure 1. Cartoon of the Y-shaped antibody. Two heavy chains (dark) and two light chains (teal) are joined by disulphide bonds (dashed). Each Fab arm carries an antigen-binding tip composed of the variable N-terminal portions of one H and one L chain. The Fc stem determines class.
Humans produce five immunoglobulin classes, each adapted to a particular task:
IgG
~75%
of serum Ig
Main antibody of the secondary response. Only class that crosses the placenta — confers passive immunity to foetus.
IgA
Mucosal
secretions, colostrum
Found in saliva, tears, gut and respiratory mucus. Abundant in colostrum — passes to the newborn via breast milk.
IgM
First
Ig produced
Pentameric in serum. The earliest antibody made during a primary response; potent agglutinator.
IgE
Allergy
& parasite defence
Binds mast cells. Mediates Type-I hypersensitivity (allergens, pollen, dust mites) and helminth response.
IgD
B-cell
surface receptor
Functions chiefly as an antigen receptor on the surface of mature, naïve B-lymphocytes.
Primary and secondary (anamnestic) responses
The first time the body meets a given antigen, the small population of B-cells whose surface receptors happen to fit it must be selected, activated and clonally expanded. This takes time, and the resulting antibody output is modest, dominated by IgM, and peaks only after a lag of several days. This is the primary immune response. A subset of activated B- and T-cells does not differentiate into short-lived effectors; instead they persist as long-lived memory cells.
On re-exposure to the same antigen, the memory pool proliferates rapidly. Antibody output rises after a much shorter lag, reaches a higher peak, lasts longer, and is dominated by high-affinity IgG. NCERT terms this the secondary or anamnestic response. It is the reason vaccination works — the vaccine triggers the primary response so that any future natural exposure encounters a primed memory system. NEET 2022 Q.189 explicitly tested this point.
From first encounter to memory recall
-
Day 0
Antigen entry
Pathogen breaches innate defences; antigen taken up by antigen-presenting cells.
First exposure -
Day 3–10
Primary response
Antigen-specific B-cells clonally expand. IgM dominates the early antibody output; intensity low.
Slow, modest -
Resolution
Memory pool laid down
A subset of B- and T-cells becomes long-lived memory cells circulating in lymphoid tissue.
Years to decades -
Re-exposure
Secondary response
Memory cells proliferate quickly. IgG-rich output peaks faster, higher, and persists longer.
Anamnestic
Active vs passive immunity
Acquired immunity is sliced a second way — by who makes the antibodies. When the host's own immune system manufactures antibodies in response to antigen exposure, the result is active immunity. The exposure can be natural (recovering from chickenpox, for instance) or artificial (deliberate immunisation with a vaccine). Active immunity is slow to develop because it requires the primary response, but it carries memory and so is long-lasting.
When ready-made antibodies are transferred from another source into the body, the result is passive immunity. No primary response is mounted by the recipient, no memory cells are formed, and the borrowed antibodies decay within weeks. The trade-off is speed: passive immunity acts the moment the antibodies arrive, which is decisive when there is no time to wait for a primary response — for example, tetanus or snake-venom exposure where antitoxin must be injected immediately.
Active immunity
Self-made
SOURCE OF ANTIBODY
- Antibodies produced by host's own B-cells
- Slow to develop (lag of days to weeks)
- Long-lasting — memory cells persist
- Natural: recovery from infection
- Artificial: vaccination (e.g. BCG, MMR)
Passive immunity
Borrowed
SOURCE OF ANTIBODY
- Ready-made antibodies transferred in
- Immediate effect, no lag
- Short-lived — antibodies decay; no memory
- Natural: IgG via placenta, IgA via colostrum
- Artificial: antitoxin for tetanus, anti-snake-venom
Two natural passive transfers are NEET staples. The first occurs during pregnancy, when maternal IgG crosses the placenta to the foetus. The second occurs after birth: the yellowish colostrum secreted in the first few days of lactation is rich in IgA, which coats and protects the neonate's gut mucosa. The colostrum–IgA pairing was tested directly in NEET 2019 Q.2. On the artificial side, NEET 2016 Q.75 distinguished antivenom (preformed antibodies → passive) from oral polio drops (live attenuated organism → active).
Worked examples
A patient receives a kidney transplant. Despite a close tissue-match, the graft begins to be rejected after a few days. Which arm of acquired immunity is principally responsible, and which lymphocyte mediates the rejection?
Solution. Graft rejection is a cell-mediated immune response. The donor kidney carries surface antigens (notably MHC molecules) that the recipient's immune system flags as "non-self". Cytotoxic T-lymphocytes recognise these markers and kill the graft cells by direct contact. No antibody-mediated mechanism is required. This is exactly why immunosuppressants are continued for life after transplantation — they damp down CMI. NEET 2017 Q.81 and NEET 2019 Q.72 are direct tests of this fact.
A newborn's blood contains antibodies against measles even though the infant has never been vaccinated or exposed. Explain the origin and category of this immunity.
Solution. The antibodies are maternal IgG that crossed the placenta during pregnancy. The infant did not manufacture them, so this is passive immunity — and because it occurred naturally rather than by injection, it is naturally acquired passive immunity. IgG is the only immunoglobulin class that crosses the placenta in humans; it confers protection for the first few months of life until the infant's own immune system matures. Note: colostrum-derived IgA protects the gut via breastfeeding — also naturally acquired passive immunity, but a separate transfer.
An antibody is described as H2L2. Identify (i) what the symbols mean, (ii) how many antigen-binding sites the molecule has, and (iii) which immunoglobulin is present in the highest concentration in human serum.
Solution. (i) H2L2 means the antibody is built from two identical heavy chains and two identical light chains, linked by disulphide bonds. (ii) The molecule carries two antigen-binding sites, one at each tip of the Y, formed by the variable portions of one H and one L chain. (iii) IgG is the most abundant immunoglobulin in serum (~75% of total Ig) and is the principal antibody of the secondary response.
Why is the secondary immune response described as "anamnestic"? Sketch the qualitative difference between primary and secondary antibody curves.
Solution. "Anamnestic" derives from the Greek anamnēsis, meaning recollection — it underlines that the response is driven by memory cells formed during the primary encounter. On re-exposure to the same antigen the memory pool divides faster than naïve cells, so antibodies appear after a shorter lag, reach a higher peak titre, persist longer, and are predominantly high-affinity IgG. The primary curve, by contrast, has a long lag, a low peak, and is initially dominated by IgM. NEET 2022 Q.189 tested this concept directly.