BPSC 70th CCE Re-exam Prelims General Studies · Question 7 of 142

Correct — C, By only B cells. Antibodies are immunoglobulins, and in the human body exactly one cell type secretes them: the plasma cell, which is what a B lymphocyte becomes once an antigen has activated it. A single plasma cell can pour out thousands of identical antibody molecules a second, all of them specific to the one antigen that triggered it. No T lymphocyte, of any subtype, ever secretes an antibody. NCERT Class 12 Biology puts the division of labour in one sentence in the chapter on Human Health and Disease: B-lymphocytes produce an army of proteins in response to pathogens, called antibodies, while the T-lymphocytes themselves do not secrete antibodies but help the B cells produce them. That sentence is the whole question. When a vaccine introduces an antigen — a weakened or killed pathogen, a purified surface protein, a viral vector or an mRNA instruction to make one — the antigen is taken up and displayed, helper T cells recognise the display and release signals, and the B cells whose receptors fit that antigen divide, mature into plasma cells and start manufacturing antibody. Some of them instead become memory B cells, which is why the protection lasts. Every step of that chain except the last needs T cells; only the last step, the manufacture, is what the stem asks about. So (a) and (b) both misattribute production, and since (c) is right, the escape option (d) cannot stand. The response as a whole is a team effort; the antibody factory is not.

• (a)By only T cells — The reverse of the truth. T lymphocytes carry out cell-mediated immunity — cytotoxic T cells kill the body's own cells once a virus has got inside them, and helper T cells coordinate the response — but they secrete cytokines, not immunoglobulins. This is the arm of immunity responsible for rejecting a transplanted organ, which is why tissue matching and immunosuppressants are needed after a graft.
• (b)By both B and T cells — The trap, and a genuinely well-informed candidate's answer. Helper T cells ARE required for the B-cell response to most protein antigens — without their signals the B cell does not switch antibody class or form good memory. But helping is not producing. The stem asks which cells the antibodies are produced BY, and the answer to that is only the B cell line. Read the verb carefully; the option would be correct if the stem asked which cells take part in the response.
• (d)None of the above — Fails because option (c) is exactly right. An escape option can only win when every named alternative can be shown to fail, and here the standard textbook attribution of antibody production to B lymphocytes and their plasma cells is squarely on the list.

Adaptive immunity has two arms, and telling them apart is the single most examinable idea in this chapter. Humoral, or antibody-mediated, immunity is run by B lymphocytes: on meeting an antigen a matching B cell proliferates and its daughters become plasma cells that secrete antibody into the blood and lymph, where it neutralises toxins, blocks viruses from entering cells and tags bacteria for destruction. It works on pathogens while they are outside cells. Cell-mediated immunity is run by T lymphocytes, which do not secrete antibody at all: cytotoxic T cells destroy the body's own infected cells, and helper T cells release signalling molecules that switch the rest of the response on. It works on pathogens that have already got inside. The letters record where the cells mature — T for the thymus, and B originally for the bursa of Fabricius in birds, the bone marrow in mammals. Both types arise from stem cells in the bone marrow.

Two habits get this right in seconds. First, read the operative verb. 'Produced by' is a manufacturing question, and only the B lineage manufactures antibody; a stem that instead said 'which cells participate in the immune response to a vaccine' would take option (b). BPSC does this deliberately — the paper is full of stems where the wrong answer is a true statement about a slightly different question. Second, hold the two-arm picture rather than a list of cells: antibody in the fluid is B-cell work, killing infected cells is T-cell work. That also fixes a related exam fact, that vaccination confers ACTIVE immunity — the body makes its own antibodies and keeps memory cells — whereas injecting ready-made antibodies, as in tetanus antitoxin or snake antivenom, confers passive immunity, which acts at once and fades quickly because no memory is laid down.

• An antibody is an immunoglobulin of structure H₂L₂ — two identical heavy chains and two identical light chains — secreted by plasma cells derived from activated B lymphocytes.
• Five classes of immunoglobulin exist: IgG, IgM, IgA, IgE and IgD. IgM is the first class produced in a primary response; IgG is the most abundant in serum and the only class that crosses the placenta.
• T lymphocytes mature in the thymus and B lymphocytes in the bone marrow, though both originate from bone-marrow stem cells; T cells secrete cytokines, never antibodies.
• Vaccination gives active immunity through memory B and T cells, so a second encounter produces a faster and much larger antibody response; injected antiserum such as tetanus antitoxin or anti-snake-venom gives passive immunity with no memory.
• Cell-mediated immunity by T lymphocytes is what causes rejection of a transplanted organ, which is why tissue typing and immunosuppressive drugs are needed after a graft.

• Choosing 'both B and T cells' because helper T cells are genuinely required. They enable the response; they do not manufacture the antibody.
• Confusing antigen with antibody. An antigen is the foreign molecule that provokes the response; the antibody is the protein produced against it.
• Assuming a vaccine must contain a live pathogen. Subunit, toxoid, viral-vector and mRNA vaccines introduce only an antigen or the instructions to make one.

BPSC asks immunology as one-line attribution — which cell makes antibody, which immunoglobulin appears first, how a particular vaccine platform works — and rewards a candidate who has the NCERT sentences precisely rather than approximately. UPSC frames the same material as a best-description or statement-evaluation question, as in its 2022 item on what B cells and T cells do, so the same knowledge has to survive being restated in unfamiliar words.

• practice — not a real PYQ

  Antibodies in the human body are secreted by

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

  Answer(b) Plasma cells derived from B lymphocytes — the effector cells that an activated B lymphocyte becomes. Macrophages present antigen and engulf pathogens but secrete no immunoglobulin.

• practice — not a real PYQ

  Injection of anti-snake-venom serum into a snakebite victim is an example of

  • (a)Active immunity
  • (b)Passive immunity
  • (c)Innate immunity
  • (d)Autoimmunity

  Answer(b) Passive immunity — ready-made antibodies are supplied from outside, so protection begins at once but fades quickly, because the patient's own B cells were never activated and no memory cells were formed.