Immunity in Humans
The human body defends itself against disease-causing pathogens through three layers of defence that work together. This chapter opens with the physical and chemical barriers that stop most pathogens from entering, then explains how phagocytes and lymphocytes deal with anything that gets through, how antibodies recognise and disable specific antigens, and how active and passive immunity differ in how long their protection lasts.
You will also study how vaccination trains the immune system to respond quickly to a pathogen it has not truly met before, by introducing a weakened or dead antigen that triggers antibody and memory cell production without causing the disease itself. This links directly to why some illnesses can be prevented while others, once caught, still leave the body with lasting protection afterwards.
The final content standard covers health issues that arise when the immune system malfunctions in some way: allergy, where it overreacts to a harmless substance; autoimmune disease, where it attacks the body's own tissue; immunodeficiency, where a defence line such as the T-lymphocyte count is weakened, as in HIV infection; and the rejection of transplanted organs, where donor tissue is recognised as foreign.
Exam questions often ask you to compare terms that sound similar, such as antigen against antibody, or active against passive immunity, and to apply the three lines of defence to an unfamiliar scenario rather than simply listing them. Building a clear mental sequence, barrier, then phagocyte, then lymphocyte and antibody, makes both structured and essay-style questions easier to answer accurately.
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Content standards in this chapter
- 11.1 Body Defence Against Pathogens
- 11.2 Actions of Antibodies
- 11.3 Types of Immunity: Active and Passive
- 11.4 Health Issues Related to Immunity
Key concepts
- Body defence
- The first line of defence, skin, mucus in the airways, and stomach acid, forms a physical and chemical barrier that stops most pathogens from entering the body. The second line is non-specific: phagocytes engulf and digest any pathogen that slips past the barrier. The third line is specific: lymphocytes recognise particular antigens and produce matching antibodies.
- Antibodies
- Antibodies are Y-shaped proteins made by lymphocytes. Each type has a binding site shaped to match one specific antigen, much like a key fits one lock, so the body builds up a different antibody for every pathogen antigen it has encountered before. This specificity is why an antibody made against one pathogen normally gives no protection against a different, unrelated pathogen.
- Actions of antibodies
- Antibodies act in three ways: they agglutinate pathogens by clumping them together so phagocytes can engulf many at once, they neutralise toxins that pathogens release, and they coat pathogens so phagocytes recognise and destroy them faster. This combined action clears an infection while limiting the damage a pathogen can do first.
- Active immunity
- Active immunity develops when the body's own lymphocytes produce antibodies, either after fighting a real infection or after vaccination with a weakened antigen. Because memory cells are formed in the process, this protection takes time to build up but then lasts for years, sometimes for life.
- Passive immunity
- Passive immunity happens when ready-made antibodies from another source enter the body, such as those a baby receives from its mother through the placenta or breast milk, or antibodies given by injection after exposure to a dangerous pathogen. Because no memory cells are made, protection is immediate but fades within weeks or months.
- Vaccination
- Vaccination introduces a weakened, dead or partial form of a pathogen, the antigen, into the body without causing the disease itself. Lymphocytes respond by producing antibodies and, importantly, memory cells, so a real infection later triggers a much faster and stronger antibody response than the first exposure did.
- Allergy
- An allergy is an exaggerated immune response to a normally harmless substance called an allergen, such as pollen, dust mites or certain foods. The immune system treats the allergen as dangerous and releases chemicals that cause symptoms such as sneezing, rashes or swelling; a severe reaction, anaphylaxis, needs urgent treatment.
- Autoimmune disease
- An autoimmune disease occurs when the immune system fails to distinguish the body's own cells from foreign antigens and attacks healthy tissue as if it were a pathogen. The tissue that is targeted determines the symptoms, for example joint damage when the attack is directed at the lining of the joints. Common examples studied at this level include the immune system attacking the pancreas or the joints, which is why such conditions usually need long-term medical management rather than a single cure.
- Immunodeficiency
- Immunodeficiency is a weakened or missing part of the immune system, leaving a person less able to fight infections that a healthy body would usually clear quickly. HIV is a well-known cause: the virus infects and destroys T-lymphocytes, so the immune system gradually loses its ability to coordinate a defence. Immunodeficiency can also be present from birth as a genetic condition, not only acquired later through infection.
- Organ transplant rejection
- A transplanted organ carries antigens that differ from the recipient's own, so the recipient's immune system may recognise the graft as foreign and attack it. The risk of this rejection is reduced, though not removed, by matching donor and recipient tissue types closely and by giving immunosuppressant drugs. This is also why an organ from a close relative, who is more likely to share similar tissue antigens, is often a better match than one from an unrelated donor.
How this chapter is examined
SPM Biology is assessed over three papers. Paper 1 has 40 objective questions (40 marks) in 1 hour 15 minutes; Paper 2 carries 100 marks across Sections A, B and C in 2 hours 30 minutes; Paper 3 is the practical, testing science process skills. Content from this chapter can appear in any of them, so lessons drill recall for Paper 1 and structured answers for Paper 2.
Common exam angles
- Comparing active and passive immunity in a table, including how quickly each acts and how long protection lasts.
- Explaining how a vaccine gives long-term protection by triggering memory cell formation.
- Describing the three lines of defence in the order a pathogen would meet them.
- Explaining how allergy, autoimmune disease and immunodeficiency each arise from a different fault in the immune response.
- Describing why a transplanted organ may be rejected and how that risk is lowered.
- Linking HIV infection to a falling T-lymphocyte count and greater vulnerability to other infections.
Common mistakes
What students write: Saying antibodies destroy pathogens directly by 'eating' them.
What earns the mark: Antibodies bind to antigens and either clump or neutralise the pathogen; it is phagocytes, not antibodies, that then engulf and digest it. Antibodies act as a labelling and disabling step that makes the phagocyte's job faster and more effective.
What students write: Calling passive immunity long-lasting.
What earns the mark: Passive immunity is immediate but short-lived because no memory cells are formed, so the protection fades once the borrowed antibodies break down. This is why passive immunity from a mother's antibodies gradually wears off as a baby grows, usually within the first year of life.
What students write: Writing that a vaccine contains antibodies.
What earns the mark: A vaccine contains a weakened or dead pathogen, the antigen, that causes the body to produce its own antibodies rather than supplying ready-made ones. This is also why a vaccinated person still needs their own immune system to respond.
What students write: Confusing antigen and antibody.
What earns the mark: An antigen is the foreign marker on a pathogen or transplanted tissue; an antibody is the specific protein the body produces to bind that marker. A simple check is that an antigen is found on the invader, while an antibody is made by the host to match it.
What students write: Believing a vaccine can cause the disease it protects against.
What earns the mark: A vaccine's antigen is weakened, dead or only a fragment of the pathogen, so it cannot cause the full disease; it can only trigger antibody and memory cell production. Any mild symptoms after vaccination, such as a sore arm or slight fever, come from this immune response, not from the disease itself.
What students write: Treating an allergy as an infection.
What earns the mark: An allergy is the immune system overreacting to a harmless substance such as pollen or dust, not the body fighting off a living pathogen.
What students write: Assuming every transplanted organ is automatically rejected.
What earns the mark: Rejection happens because the immune system may recognise donor antigens as foreign; close tissue matching and immunosuppressant drugs lower this risk considerably. Even with a good match, patients are usually monitored closely and may need immunosuppressant medication for as long as they keep the transplant.
What students write: Treating HIV and AIDS as the same term.
What earns the mark: HIV is the virus that infects and destroys T-lymphocytes; AIDS is the later stage, once the immune system is too weakened to resist infections it would normally clear.
Study this chapter
Processes in this chapter
Frequently asked questions
What is the difference between active and passive immunity?
How does a vaccine work?
What are the three lines of defence?
What happens in an autoimmune disease?
Why does HIV weaken the immune system so severely?
Source:SRC-DSKP-EN, SRC-FORMAT
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