Ch 1 · 11.1 The immune system · AS & A Level Biology 9700
Topic 1 of 2 in Immunity — AS & A Level Biology 9700 — 68 lessons.
Introduction
The immune system defends the body against pathogens using two main types of white blood cell, phagocytes and lymphocytes, which respond to infection in different but complementary ways. (Syllabus section 11.1.)
Key ideas — Phagocytes and lymphocytes
Phagocytes, including neutrophils and macrophages, are white blood cells with a lobed nucleus, in neutrophils, and many lysosomes containing digestive enzymes; they engulf and destroy pathogens as part of the non-specific immune response. Lymphocytes are white blood cells with a large, rounded nucleus that fills most of the cell and relatively little cytoplasm; they are responsible for the specific immune response and are of two main types, B-lymphocytes and T-lymphocytes, which mature in the bone marrow and thymus gland respectively.
🧑🏫 Teacher's explanation
Think of your immune system as a highly organized security force for a high-security building. Phagocytes are like the patrol guards; they roam the hallways and, if they see anything that looks suspicious (a pathogen), they grab it and swallow it to destroy it immediately. This is a 'one-size-fits-all' approach—they don't care if it's a thief or a stray cat, if it shouldn't be there, they attack.
📖 Definitions
🔍 Worked example — Easy
A student observes a neutrophil under a microscope. Describe two structural features of this cell that are relevant to its function in the immune response.
🔍 Worked example — Standard
Compare the role of B-lymphocytes and T-helper cells during a primary immune response.
🔍 Worked example — Exam-style
A patient is exposed to a virus for the first time. Explain why the secondary immune response is faster and more effective than the primary response.
⚠️ Common errors
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🎯 Exam question
Key ideas — The mechanism of phagocytosis
A phagocyte is attracted to a site of infection by chemicals released by pathogens or damaged tissue, chemotaxis. It recognises molecules on the surface of the pathogen and engulfs it by endocytosis, enclosing it within a vesicle called a phagosome. Lysosomes fuse with the phagosome, releasing hydrolytic enzymes such as lysozyme that break down the pathogen. Some phagocytes, particularly macrophages, then display fragments of the digested pathogen's antigens on their own cell-surface membrane; these are now antigen-presenting cells, which activate lymphocytes and link the non-specific and specific immune responses.
🧑🏫 Teacher's explanation
Think of your immune system like a high-security building. Phagocytes are the security guards patrolling the halls. When they spot an intruder (a pathogen), they don't just stand there; they physically grab the intruder, pull them into a private room (the phagosome), and use chemical cleaners (lysosomal enzymes) to dissolve them.
📖 Definitions
🔍 Worked example — Easy
A student identifies a pathogen in a tissue sample. Describe the first two steps of the phagocytic process once the phagocyte detects the pathogen.
🔍 Worked example — Standard
Compare the role of a macrophage in the non-specific immune response versus its role in the specific immune response.
🔍 Worked example — Exam-style
A patient is exposed to a virus for the first time. Calculate the relative speed of the immune response if the primary response takes 14 days to produce significant antibodies, whereas a secondary response takes 2 days. What is the percentage reduction in time for the secondary response?
⚠️ Common errors
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🎯 Exam question
(a) Describe the process of phagocytosis from the moment the phagocyte detects the bacterium until the bacterium is digested. [4] (b) Explain how the macrophage contributes to the activation of the specific immune response. [2] [6 marks]
Key ideas — B-lymphocytes and T-lymphocytes
Each lymphocyte has receptors on its surface for one specific antigen; there are millions of different lymphocytes, each matching a different antigen.
🧑🏫 Teacher's explanation
Think of your immune system as a highly specialized security force for a high-tech building. The antigens are the 'ID badges' worn by everyone. 'Self-antigens' are the official badges that say 'I belong here,' while 'non-self antigens' are the fake badges worn by intruders like bacteria or viruses.
📖 Definitions
🔍 Worked example — Easy
A patient is injected with a vaccine containing a weakened version of a pathogen. Explain why the patient does not show symptoms of the disease but develops long-term immunity.
🔍 Worked example — Standard
Compare the roles of T-helper cells and T-killer cells in the immune response.
🔍 Worked example — Exam-style
Calculate the percentage increase in antibody concentration if the concentration rises from 2 × 10^2 units to 1 × 10^5 units during a secondary immune response.
⚠️ Common errors
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🎯 Exam question
(a) Describe the role of T-helper cells and B-lymphocytes in the primary immune response. [3] (b) Explain why the secondary immune response is faster and more effective than the primary response. [2] [5 marks]
Key ideas — Antigens: self and non-self
An antigen is a molecule, usually a protein or glycoprotein on the surface of a cell or virus, that stimulates an immune response and the production of a specific antibody. Self antigens are on the body's own cells; lymphocytes that would react to them are destroyed or switched off during development, so the body does not attack itself. Non-self antigens come from outside: on pathogens, transplanted organs, blood of the wrong group, or toxins. These trigger an immune response.
🧑🏫 Teacher's explanation
Think of your immune system as a highly trained security team for a high-end hotel. The hotel has a specific 'uniform' or 'ID badge' that every authorized staff member wears. This ID badge is the 'self antigen'. As long as the security team sees these specific badges, they let everyone pass. However, if someone walks in wearing a different outfit or a mask—something that doesn't match the hotel's official ID—that is a 'non-self antigen'. The security team recognizes this mismatch and immediately triggers an alarm to start a defense protocol.
📖 Definitions
🔍 Worked example — Easy
A student is asked to identify the nature of an antigen. Which of the following best describes an antigen? (a) A protein produced by B-lymphocytes to neutralize pathogens. (b) A glycoprotein on the surface of a pathogen that triggers an immune response. (c) A white blood cell that engulfs bacteria. (d) A molecule produced by T-helper cells to signal other cells.
🔍 Worked example — Standard
Compare the role of self antigens and non-self antigens in maintaining homeostasis.
🔍 Worked example — Exam-style
Explain why a person might experience an immune response after receiving a blood transfusion of an incompatible blood group.
⚠️ Common errors
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🎯 Exam question
(a) Define the term 'antigen'. [1] (b) Explain why the recognition of non-self antigens leads to the rejection of the transplanted organ. [2] [3 marks]
Key ideas — The primary immune response, step by step
The primary response takes several days, because there are very few lymphocytes matching any one antigen at first. During this time the pathogen multiplies, so the person feels ill.
🧑🏫 Teacher's explanation
Think of your immune system as a highly specialized security force for a high-security building. When a stranger (a pathogen) enters, the first responders (phagocytes) don't just stop them; they take a 'mugshot' of the intruder and show it to the central command center.
📖 Definitions
🔍 Worked example — Easy
A patient is infected with a new strain of influenza. Describe the role of a macrophage in the first stage of the primary immune response.
🔍 Worked example — Standard
Compare the roles of plasma cells and memory B-cells following clonal expansion of B-lymphocytes.
🔍 Worked example — Exam-style
A pathogen enters the body. Calculate the percentage increase in the number of specific B-lymphocytes if the initial count is 5 and after clonal expansion it is 500.
⚠️ Common errors
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🎯 Exam question
(a) Describe the sequence of events involving macrophages, T-helper cells, and B-lymphocytes that leads to the production of antibodies. [4] (b) Explain why the person may experience symptoms of illness before antibodies are present in the blood. [2] [6 marks]
Key ideas — Memory cells and the secondary response
Memory cells (memory B- and T-cells) are long-lived and stay in the blood for years. If the same antigen enters the body again, the memory cells recognise it at once and divide rapidly, producing far more plasma cells and T-killer cells than in the primary response. This secondary response is faster, makes much more antibody, and the antibody level stays high for longer. The pathogen is usually destroyed before symptoms appear: the person is immune. This is the basis of long-term immunity, after both infection and vaccination (11.2).
🧑🏫 Teacher's explanation
Think of your immune system like a security team for a high-end building. The first time a thief (a pathogen) tries to break in, the security guards (phagocytes) have to identify them, call the police (T-helper cells), and then wait for the specialized SWAT team (B and T-lymphocytes) to be trained and deployed. This takes time, and the thief might cause damage while the team is getting ready. This is the 'primary response'.
📖 Definitions
🔍 Worked example — Easy
A patient is exposed to a virus for the first time. Describe the role of T-helper cells in initiating the primary immune response.
🔍 Worked example — Standard
Compare the primary and secondary immune responses in terms of speed and antibody concentration.
🔍 Worked example — Exam-style
A graph shows antibody concentration over time. In the primary response, the peak concentration is 10^4 units. In the secondary response, the peak concentration is 10^7 units. Calculate the factor by which the antibody concentration increases in the secondary response compared to the primary response.
⚠️ Common errors
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🎯 Exam question
(a) Describe the role of memory cells in the secondary immune response. [2] (b) Explain why the person is often immune to the pathogen during the second exposure. [2] [4 marks]
Worked example
Question. Explain why a macrophage is described as an antigen-presenting cell, and why this is important in coordinating an effective immune response. [4]
🔍 Worked example — Easy
Define the term 'antigen' and distinguish between self and non-self antigens.
🔍 Worked example — Standard
Describe the role of a macrophage in the primary immune response.
🔍 Worked example — Exam-style
Compare the primary and secondary immune responses in terms of memory cells.
⚠️ Common errors
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