The immune response — Edexcel International A Level Biology
Non-specific defences, antigens and antibodies, B and T cells, natural and artificial immunity, and the evolutionary race between pathogens and hosts.
Revision notes
5 short notes, in the order of the specification. Each one in short:
Non-specific responses act against any pathogen in the same way. Inflammation increases blood flow and brings phagocytes to damaged tissue, lysozyme digests bacterial cell walls, interferon stops viruses replicating in neighbouring cells, and phagocytosis engulfs and digests pathogens in phagocytes.
An antigen is a molecule, usually a protein or glycoprotein, that the immune system recognises as foreign and that triggers an immune response. Antibodies are Y-shaped proteins made by plasma cells that bind a specific antigen. Macrophages present antigens to T cells to start the response.
B cells produce antibodies: when activated they become plasma cells (B effector cells) that secrete antibody, or B memory cells. T helper cells release cytokines that activate B cells and killer T cells. T killer cells destroy infected cells, and T memory cells give a faster secondary response.
In active immunity the body makes its own antibodies and memory cells, after infection (natural) or vaccination (artificial). In passive immunity ready-made antibodies are received, across the placenta or in breast milk (natural) or by injection (artificial). Passive immunity is immediate but short-lived.
The evolutionary race theory states that hosts and pathogens are in a continuing cycle of selection: mutations that help a pathogen evade defences are selected, then hosts with improved resistance are selected. Evasion mechanisms such as antigenic variation, hiding inside cells and suppressing immunity support the theory.
Exam questions
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Quick check questions
How does lysozyme kill bacteria?
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It hydrolyses peptidoglycan in the cell wall, so the bacterium bursts by osmosis.
How does interferon protect a neighbouring cell?
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It switches on genes for enzymes that break down viral mRNA and block protein synthesis, so the virus cannot replicate.
Which cell secretes antibodies?
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The plasma cell (B effector cell).
What is the difference between natural passive and artificial passive immunity?
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Natural passive: antibodies pass from mother to baby. Artificial passive: ready-made antibodies are injected.
Why does antigenic variation help a pathogen survive?
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Memory cells and antibodies for the old antigen do not recognise the new antigen, so there is no fast secondary response.
Frequently asked questions
What is the difference between innate and adaptive immunity?
Innate (non-specific) responses such as inflammation, lysozyme, interferon and phagocytosis act against any pathogen, quickly and in the same way. The adaptive (specific) response uses B and T cells whose receptors are complementary to one antigen, takes days at first, and produces memory cells for faster future responses.
How do antibodies work?
Antibodies are Y-shaped proteins made by plasma cells. The variable region on each arm has a shape complementary to one antigen. They bind to pathogens and toxins, clump pathogens together (agglutination), make them easier for phagocytes to engulf, and block attachment to host cells.
What do T helper cells and T killer cells do?
T helper cells are activated by antigen-presenting cells and release cytokines that stimulate B cells and T killer cells. T killer cells destroy body cells that display the antigen, such as virus-infected cells, by releasing perforin and enzymes. Both types also form T memory cells.
What is the difference between active and passive immunity?
In active immunity the person makes their own antibodies and memory cells, after infection or vaccination, so protection is slow to start but long-lasting. In passive immunity antibodies come from outside, through the placenta, breast milk or injection. Protection is immediate but short-lived, with no memory cells.
What is the evolutionary race between pathogens and hosts?
It is the theory that pathogens and hosts continually evolve in response to each other. Mutations that help a pathogen avoid host defences, such as antigenic variation or hiding in cells, are selected. Hosts with improved immunity are then selected, so neither side gains a lasting advantage.
Written and checked against the Edexcel International A Level Biology (YBI11) specification · Updated October 2026