Innate and adaptive immunity, and phagocytes

Organisms (Interaction and interdependence) · Defence against disease · note 3 of 8

Spec C3.2.4, C3.2.5Download all Defence against disease notes (PDF)Download all notes (PDF)11 pages

Innate and adaptive immunity, and phagocytesSpec C3.2.4, C3.2.5

In short

The innate immune system responds to broad categories of pathogen and does not change during life, whereas the adaptive immune system responds specifically to particular pathogens and builds up a memory of them. Phagocytes, part of the innate system, reach infection sites by amoeboid movement, engulf pathogens by endocytosis and digest them with lysosome enzymes.

Innate and adaptive immune systems
Feature	Innate immune system	Adaptive immune system
What it responds to	Broad categories of pathogen	Particular pathogens, in a specific way
Change during life	Does not change during an organism's life	Builds up a memory of pathogens encountered
Effect of a second infection	Same response as the first time	Response becomes faster and more effective
Cells	Phagocytes	Lymphocytes (B-cells and T-cells)

Both systems work together: the innate system responds immediately to any infection, while the adaptive system takes longer the first time but produces a targeted response with long-term memory.

Infection control by phagocytes

Phagocytes circulate in the blood. At a site of infection they squeeze out of capillaries through gaps between the wall cells by amoeboid movement.
They move through the tissue to the pathogens and recognize them as foreign.
The phagocyte engulfs the pathogen by endocytosis, enclosing it in a vesicle.
Lysosomes fuse with the vesicle and release enzymes that digest the pathogen.

Four stages of phagocytosis: a phagocyte squeezing between cells of the capillary wall by amoeboid movement; the phagocyte extending pseudopodia around a bacterium; the bacterium enclosed in a vesicle by endocytosis; lysosomes fusing with the vesicle and releasing digestive enzymes that break the bacterium down. — Phagocytosis: amoeboid movement, engulfing by endocytosis, then digestion by enzymes from lysosomes.

Exam tip:

Phagocytes are the only components of the innate immune system you need to know. Use the words amoeboid movement, endocytosis and lysosomes in answers.

Written and checked against the IB Biology HL specification · Updated October 2026

Frequently asked questions

How do vaccines work?

Vaccines contain antigens, or DNA or RNA coding for antigens, from a specific pathogen. They trigger a primary immune response in which B-cells are activated and memory cells are produced, without causing the disease. If the real pathogen infects later, memory cells respond quickly, producing antibodies before the person becomes ill.

Why can't antibiotics kill viruses?

Antibiotics work by blocking processes found in bacteria but not in eukaryotic cells, such as cell wall synthesis or protein synthesis by 70S ribosomes. Viruses have no metabolism of their own, so there is nothing for antibiotics to block. They replicate using the host cell's processes, which antibiotics do not target.

What is the difference between innate and adaptive immunity?

The innate immune system responds to broad categories of pathogen and does not change during an organism's life; phagocytes are part of it. The adaptive immune system, based on lymphocytes, responds specifically to particular pathogens and builds up a memory of pathogens encountered, so later responses are faster and more effective.

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