Pathogens and the immune system: WACE Year 12 Human Biology Unit 3
“Describe the lines of defence against pathogens, including non-specific defences and the specific immune response with B and T lymphocytes”
The body defends against pathogens with three lines of defence: non-specific physical and chemical barriers, non-specific internal responses such as phagocytosis and inflammation, and the specific immune response in which B lymphocytes make antibodies (humoral immunity) and T lymphocytes destroy infected cells (cell-mediated immunity), leaving memory cells for faster future responses.
What this dot point is asking
WACE wants you to organise the immune system into a layered defence and to explain how the specific response recognises and remembers particular pathogens. Pathogens are disease-causing agents and you should know the main types: bacteria, viruses, fungi, protozoa and other parasites, and prions. Each carries antigens, molecules on its surface that the immune system can recognise as non-self.
The first line of defence (non-specific, external)
The first line stops pathogens entering and is non-specific, meaning it acts against anything regardless of type. It includes physical barriers such as the skin and the cilia and mucus lining the airways, and chemical barriers such as stomach acid, lysozyme in tears and saliva, and the slightly acidic skin surface. Useful microorganisms (the normal flora) also compete with pathogens. If these barriers are intact, most pathogens never get inside.
The second line of defence (non-specific, internal)
If a pathogen breaches the barriers, the second line responds, still without targeting a specific pathogen. Its key features are:
- Phagocytosis: white blood cells called phagocytes (for example macrophages and neutrophils) engulf and digest pathogens.
- Inflammation: damaged cells release histamine, blood vessels dilate and become more permeable, and the area becomes red, warm, swollen and painful as more white cells and plasma arrive.
- Fever: the hypothalamus raises the temperature set point, which slows pathogen reproduction and speeds immune activity.
- Other defences: complement proteins and interferons (which interfere with viral replication) support the response.
An antigen is a molecule, usually a protein on the surface of a pathogen or cell, that the immune system recognises as self or non-self. Non-self antigens trigger the specific immune response. An antibody is a protein made by B cells that binds to a specific antigen.
The third line of defence (specific immune response)
The third line is specific: it targets a particular pathogen by recognising its antigens, and it produces memory. It has two arms, both involving lymphocytes (a type of white blood cell) made in the bone marrow.
Humoral (antibody-mediated) immunity is driven by B lymphocytes. When a B cell with the matching receptor meets its antigen (often presented by a helper T cell), it is activated and divides into many plasma cells that secrete antibodies. Antibodies bind to the antigen and neutralise the pathogen, clump pathogens together (agglutination), or mark them for phagocytosis.
Cell-mediated immunity is driven by T lymphocytes, which mature in the thymus. Helper T cells release chemicals (cytokines) that activate B cells and other T cells. Cytotoxic (killer) T cells destroy the body's own cells that are infected by viruses or that have become cancerous.
A new bacterial antigen enters the blood
A macrophage engulfs the bacterium and displays its antigen (acting as an antigen-presenting cell). A helper T cell with the matching receptor binds the displayed antigen and is activated, releasing cytokines. These cytokines activate the B cell that carries the matching receptor. The activated B cell divides into plasma cells, which secrete large numbers of antibodies specific to that antigen, and into memory B cells. The antibodies bind the bacteria, causing agglutination and marking them for phagocytosis. After the infection clears, the plasma cells die but the memory cells remain, so a second exposure to the same antigen produces a faster, larger antibody response.
Immunological memory and immunity
The first (primary) response is slow because the matching lymphocytes are rare and must be selected and multiplied. Memory B and T cells made during this response survive long term. On a second exposure to the same antigen, these memory cells trigger a secondary response that is faster and produces far more antibody, often stopping the disease before symptoms appear. This is the basis of long-lasting immunity.
You also need active versus passive immunity:
- Active immunity is when the person's own immune system makes antibodies and memory cells, either by catching the disease (natural) or by vaccination (artificial). It is slow to develop but long-lasting.
- Passive immunity is when ready-made antibodies are received from another source, such as across the placenta or in breast milk (natural) or by injection of antibodies (artificial). It is immediate but temporary, because no memory cells form.
Active immunity means you make your own antibodies and memory cells, so it lasts a long time but takes time to develop. Passive immunity means you receive antibodies made elsewhere, so it protects immediately but fades because no memory cells are produced.
How this maps to the exam
Expect questions that ask you to place a defence in the correct line, distinguish non-specific from specific defences, explain why the secondary response is faster, or compare active and passive immunity in the context of vaccination. Use precise terms: phagocyte, lymphocyte, antigen, antibody, plasma cell and memory cell.
Students swap antigen and antibody: the antigen is the foreign marker on the pathogen, the antibody is the protein the body makes to bind it. Another common error is mixing helper and cytotoxic T cells; helper T cells activate other immune cells, while cytotoxic T cells kill infected body cells. Also avoid saying vaccines "give you the disease"; a vaccine presents antigens so memory cells form without serious illness.
Exam-style questions
Questions in the style of SCSA exam questions on this dot point, each with a worked answer. They are written by ExamExplained unless tagged "Past paper"; the year shows the paper a question is modelled on.
WACE 2022-style7 marksDescribe the three lines of defence against pathogens, and explain the roles of B lymphocytes and T lymphocytes in the specific immune response.Show worked answer →
A 7 mark response needs the three lines plus the specific roles of B and T cells.
- First line (non-specific, external)
- Barriers that stop entry, such as the skin, mucous membranes, cilia and stomach acid.
- Second line (non-specific, internal)
- General responses if a pathogen enters, including phagocytes that engulf pathogens, inflammation and fever. These act against any pathogen.
- Third line (specific)
- The immune response targeted to a particular antigen.
- B lymphocytes (humoral)
- Recognise a specific antigen, divide and differentiate into plasma cells that secrete antibodies, which bind the antigen and mark pathogens for destruction. Memory B cells remain for a faster future response.
- T lymphocytes (cell-mediated)
- Helper T cells activate B cells and other immune cells, while cytotoxic T cells destroy the body's own infected cells. Memory T cells also persist.
Markers reward all three lines named in order and the distinct humoral and cell-mediated roles.
WACE 2024-style4 marksDistinguish between active and passive immunity, and give one natural example of each.Show worked answer →
A 4 mark distinguish answer needs the contrast plus a natural example of each.
Active immunity. The body's own immune system makes antibodies and memory cells in response to an antigen, so protection is long-lasting. Natural example: recovering from a measles infection.
Passive immunity. Ready-made antibodies are received from another source, so protection is immediate but short-lived because no memory cells are made. Natural example: antibodies passing from mother to baby across the placenta or in breast milk.
Markers reward the makes-own versus receives-ready-made distinction and a correct natural example for each.