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Reviewer participation may vary by guide. Checked against Cambridge IGCSE Biology 0610 (2026–2028); last reviewed August 2026.
Key concept
Phagocytes engulf and digest pathogens (phagocytosis). Lymphocytes recognise specific antigens on pathogens and produce complementary antibodies that bind to those antigens, marking the pathogen for destruction. Memory cells provide long-term immunity.
The immune response is one of the highest-mark topics in IGCSE Biology. Key terms that must be used precisely: antigen (molecule on pathogen surface), antibody (protein produced by lymphocyte), and phagocyte (white blood cell that engulfs pathogens). Confusing any of these costs marks.
Phagocytosis
Phagocytes are white blood cells that provide a non-specific defence. When they encounter a pathogen, they change shape to engulf it, enclosing it in a vacuole inside the cell. Digestive enzymes are then released into the vacuole to break down and destroy the pathogen.
Phagocytes respond to any pathogen: they do not distinguish between different types. This makes them a rapid first response, active within minutes of infection.
Lymphocytes and antibody production
Lymphocytes provide a specific response. Each pathogen has unique molecules on its surface called antigens. A lymphocyte that recognises a particular antigen divides rapidly and produces antibodies: Y-shaped proteins that are complementary to that specific antigen.
Antibodies bind to antigens on the pathogen surface. This can neutralise toxins, cause pathogens to clump together (agglutination) making them easier for phagocytes to engulf, or mark pathogens for destruction. After the infection is cleared, some lymphocytes remain as memory cells. If the same pathogen enters again, memory cells respond faster and in greater numbers, often preventing symptoms: this is immunological memory.
Active and passive immunity
Feature
Active immunity
Passive immunity
How acquired
Body makes its own antibodies after infection or vaccination
Antibodies received from another source (e.g. Mother's milk, injection)
Memory cells
Yes: long-lasting protection
No: protection is temporary
Speed of protection
Slower (takes days to weeks)
Immediate
Duration
Long-term or permanent
Short-term (antibodies break down)
Breast milk provides passive immunity to a newborn through antibodies. Active immunity from vaccination stimulates the body to produce its own memory cells.
Try a focused question (4 marks)
Explain how lymphocytes protect the body against a specific pathogen.
Show the answer and marking guidance
Lymphocytes recognise specific antigens on the surface of the pathogen.
They divide rapidly and produce antibodies complementary to those antigens.
Antibodies bind to the antigens, neutralising the pathogen or marking it for destruction.
Some lymphocytes become memory cells that respond rapidly if the same pathogen enters again.
Original Cambridge-style practice written for this site. It is not an official Cambridge past-paper question.
Common mix-ups to avoid
Check these points against your own answer before moving on.
Confusing antigens (on pathogen surface) with antibodies (produced by lymphocytes).
Saying phagocytes produce antibodies: antibodies come from lymphocytes.
Describing passive immunity as long-lasting: it is temporary because no memory cells are formed.
Forgetting to mention that antibodies are specific to a particular antigen.