Immunity

• Complementary, not the same shape — and selected, not made to fit. Few cells fit at first, so the primary response is slow; memory cells make the fit many, so the secondary response is fast.
• Complementary, not the same shape; selected, not made to fit. Memory cells make the second response sooner, faster, larger and longer. Vaccines contain antigens that stimulate immune responses to provide long-term immunity.

• 11.1.1 I can describe — describe the mode of action of phagocytes (macrophages and neutrophils)
• 11.1.2 I can explain — explain what is meant by an antigen (see 4.1.3) and state the difference between self antigens and non-self antigens
• 11.1.3 I can describe — describe the sequence of events that occurs during a primary immune response with reference to the roles of: • macrophages • B-lymphocytes, including plasma cells • T-lymphocytes, limited to T-helper cells and T-killer cells
• 11.1.4 I can explain — explain the role of memory cells in the secondary immune response and in long-term immunity
• 11.2.1 I can relate — relate the molecular structure of antibodies to their functions
• 11.2.2 I can outline — outline the hybridoma method for the production of monoclonal antibodies
• 11.2.3 I can outline — outline the principles of using monoclonal antibodies in the diagnosis of disease and in the treatment of disease
• 11.2.4 I can describe — describe the differences between active immunity and passive immunity and between natural immunity and artificial immunity
• 11.2.5 I can explain — explain that vaccines contain antigens that stimulate immune responses to provide long-term immunity
• 11.2.6 I can explain — explain how vaccination programmes can help to control the spread of infectious diseases

Why each kind is used where it is. A snakebite or a tetanus toxin acts within hours, far faster than a primary response can develop, so ready-made antibodies are given: protection now, but no memory. A vaccine is given before any exposure, so the slow start does not matter, and it leaves memory cells for years. A newborn baby’s own immune response is still developing; its mother’s antibodies cover the first months.

• “The antibody is the same shape as the antigen, and a B cell makes an antibody to fit whatever antigen it meets.” Correct Complementary, not the same shape — and selected, not made to fit. The binding site is complementary to one region of the antigen. Every B cell’s receptor shape exists before the infection; the antigen selects the few cells whose receptors are already complementary (clonal selection), and those cells divide by mitosis.
• “Phagocytes produce antibodies.” Correct Phagocytes engulf and digest. Plasma cells, formed from B-lymphocytes, secrete antibodies.
• “T-killer cells kill bacteria.” Correct T-killer cells destroy infected body cells, which display the pathogen’s antigen. Bacteria in blood and tissue fluid are dealt with by antibodies and phagocytes.
• “The second response is faster because the antibodies from the first infection are still in the blood.” Correct Those antibodies have almost gone. The second response is faster because memory cells are present in large numbers, and they divide into plasma cells that make new antibody.
• “An antibody has one binding site.” Correct Two identical antigen-binding sites, one at the tip of each arm. That is what lets one antibody link two pathogens (agglutination).
• “Antibodies from the mother give the baby active immunity.” Correct Antibodies across the placenta or in breast milk give natural passive immunity: the baby makes no memory cells, so it lasts only weeks to months.
• “Vaccines contain antibodies.” Correct Vaccines contain antigens that stimulate immune responses to provide long-term immunity. An injection of antibodies is passive immunity, not a vaccine.
• “Herd immunity means everybody is vaccinated.” Correct Enough of the population is immune that the pathogen cannot spread, which protects the people who are not immune.
• “Phagocytes produce antibodies.” Repair Phagocytes engulf and digest; plasma cells, from B-lymphocytes, secrete antibodies.
• “An antigen is a foreign cell.” Repair An antigen is a molecule, usually a protein or glycoprotein, that stimulates an immune response. Cells carry antigens.
• “The antibody has the same shape as the antigen.” Repair Its binding site is complementary to a region of the antigen.
• “B cells make antibodies to fit any antigen they meet.” Repair Each B cell’s receptor shape already exists; the antigen selects the few cells whose receptors are complementary (clonal selection), and they divide by mitosis.
• “The macrophage presents the antigen to the B cell, which then makes antibodies straight away.” Repair The macrophage presents to T-helper cells. The selected B cell is activated by their cytokines, divides by mitosis, and the clone differentiates into plasma cells, which secrete the antibodies.
• “T-killer cells kill bacteria.” Repair T-killer cells destroy infected body cells; bacteria outside cells are dealt with by antibodies and phagocytes.
• “In the secondary response the antibodies from the first infection are still there.” Repair Those antibodies have almost gone; memory cells make new ones quickly.
• “An antibody has one binding site.” Repair Two identical antigen-binding sites, one on each arm.
• “Antibodies punch holes in bacteria.” Repair Antibodies bound to a bacterium trigger its lysis. Background, not examined: the holes are made by other blood proteins that the bound antibodies activate, not by the antibody itself. Some textbooks state the shortcut; this chapter does not.
• “Monoclonal antibodies come from one antibody.” Repair From one clone of cells, all descended from one hybridoma.
• “A mother’s antibodies across the placenta give the baby active immunity.” Repair Natural passive immunity: the baby makes no memory cells.
• “Vaccines contain antibodies.” Repair Vaccines contain antigens; a person given antibodies has passive immunity.
• “Herd immunity means everyone is vaccinated.” Repair A high enough proportion is immune that the pathogen cannot spread, protecting those who are not vaccinated.

• Read the command word before you decide how much to write. This syllabus has seventeen of them: assess, calculate, comment, compare, contrast, define, describe, determine, discuss, explain, give, identify, outline, predict, sketch, state and suggest. The four that carry topic 11’s outcomes, in the syllabus’s own words: describe — “state the points of a topic / give characteristics and main features”; explain — “set out purposes or reasons / make the relationships between things clear / say why and/or how and support with relevant evidence”; outline — “set out the main points”; state — “express in clear terms”. Questions on this topic also use compare (“identify/comment on similarities and/or differences”), calculate (“work out from given facts, figures or information”) and suggest (“apply knowledge and understanding to situations where there are a range of valid responses to make proposals / put forward considerations”). Relate, in 11.2.1, is the outcome’s verb, not a command word: it asks you to pair each structural feature with the function it serves. Check any other word against the syllabus’s table.
• Interleave with the chapters that use this one. Topic 10 (infectious diseases) is the other half of this chapter: when you revise it, explain why vaccination has controlled TB and cholera only partly, and why HIV is so damaging to the immune response. Topic 19 (genetic technology) returns to antibodies: when you reach it, explain why a therapeutic monoclonal antibody is humanised. Recalling a topic inside a new context is worth more than another pass over this chapter on its own; at A Level, Paper 4 assumes the whole of the AS content, so nothing here is ever finished with.

• Cambridge International AS & A Level Biology 9700 has five components. Topic 11 is AS Level content, so it is examined in Papers 1, 2 and 3. AS Level content: examined in Paper 1 (multiple choice), Paper 2 (AS structured) and, as practical context, Paper 3. Assumed knowledge for Papers 4 and 5. AS Level candidates take Papers 1, 2 and 3; A Level candidates take all five, either staged over two years (Papers 1–3 in year one, Papers 4 and 5 in year two) or together in one series. Examinations are available in the June and November series, and in March in India.
• Across both the AS Level and the A Level the assessment objectives are weighted AO1 40% (knowledge and understanding), AO2 40% (handling, applying and evaluating information) and AO3 20% (experimental skills and investigations). AS candidates are graded a–e; A Level candidates A*–E. There is no data booklet in Biology. At A Level, the statistical formulae (Hardy–Weinberg, the Lincoln index, Simpson’s index, standard deviation, standard error, 95% confidence intervals, the χ² test, the t-test, and Pearson’s and Spearman’s correlation) are printed in a question when it needs them, and so are the tables of critical values; degrees of freedom you must work out yourself. Everything else — magnification, surface area to volume, RQ, Rf, rates — you must recall, and this chapter says which is which.
• A Paper 1 item on this topic can turn on one exact distinction: which cell secretes antibodies (a plasma cell, never a phagocyte), what a T-killer cell destroys (infected body cells), how many binding sites an antibody has (two), or which kind of immunity a case gives (antibodies across the placenta are natural passive). Wrong options can be the near-misses listed in the danger zones below. In Paper 2 you may be asked to describe the mode of action of phagocytes or the sequence of the primary response, explain the secondary response, how a vaccine works or how each part of the antibody serves its function, and outline the hybridoma method.
• A question can give you a graph of antibody concentration against time after two exposures to read lag, peak and duration from; a diagram of an antibody to label and explain; an electron micrograph of a plasma cell to measure and relate to its function; a test strip to interpret; or vaccination figures to turn into percentages. The mathematics is AS: magnification = image size ÷ actual size, a percentage, a rate from a graph, and eyepiece graticule calibration. You recall all of them; none is printed. Every concentration, count and coverage figure in this chapter is a fictional learning value.
• The syllabus names no practical for topic 11. Its microscope work is a prepared, stained blood smear: calibrate the eyepiece graticule, measure each white cell, count 200 white cells along a fixed path, and draw a neutrophil and a lymphocyte to the syllabus’s rules. Its planning context is an antibody titre: serum diluted by serial dilution (independent variable), clumping of antigen-coated beads judged in each well (dependent variable), saline and uncoated beads as controls, and judging clumping by eye as the main source of error.
• Read the command word before you decide how much to write. This syllabus has seventeen of them: assess, calculate, comment, compare, contrast, define, describe, determine, discuss, explain, give, identify, outline, predict, sketch, state and suggest. The four that carry topic 11’s outcomes, in the syllabus’s own words: describe — “state the points of a topic / give characteristics and main features”; explain — “set out purposes or reasons / make the relationships between things clear / say why and/or how and support with relevant evidence”; outline — “set out the main points”; state — “express in clear terms”. Questions on this topic also use compare (“identify/comment on similarities and/or differences”), calculate (“work out from given facts, figures or information”) and suggest (“apply knowledge and understanding to situations where there are a range of valid responses to make proposals / put forward considerations”). Relate, in 11.2.1, is the outcome’s verb, not a command word: it asks you to pair each structural feature with the function it serves. Check any other word against the syllabus’s table.