Priya gets a splinter of wood stuck under the skin of her finger while gardening. She removes the splinter, but does not clean the small wound. Bacteria on the splinter had already entered the wound. Two days later, the area around the wound is red, swollen and slightly painful, and a small amount of thick, yellowish-white fluid (pus) has collected at the site.
(a) State the name of the process by which white blood cells engulf and destroy pathogens such as the bacteria in Priya's wound. [1]
(b) Describe how Priya's white blood cells respond to the bacteria that entered through the wound in her finger. [3]
(c) Suggest what the pus that has collected at the wound site is made of. [2]
The worked solution
Part (a): Naming the process
White blood cells destroy pathogens such as bacteria by engulfing them; this process is called phagocytosis.
Part (b): How Priya's white blood cells respond
Once the bacteria from the splinter have entered the tissue of Priya's finger, her white blood cells travel to the site of the wound. There, they surround and engulf the bacteria, taking them inside the white blood cell, and then destroy them. The redness, swelling and slight pain around the wound are signs of this response taking place at the site of the infection.
Part (c): What the pus is made of
As white blood cells engulf and destroy large numbers of bacteria at the wound site, many of the white blood cells themselves die in the process. The pus that collects at the wound is a mixture of these dead white blood cells, dead bacteria, and fluid that has leaked out of the surrounding tissue.
Final answers
- (a) Phagocytosis.
- (b) White blood cells travel to the wound site, then engulf and destroy the bacteria there.
- (c) Pus is a mixture of dead white blood cells, dead bacteria and tissue fluid.
What loses the marks
- In (a), naming the defence as simply "white blood cells" rather than naming the specific process, phagocytosis, by which they engulf and destroy pathogens.
- In (b), stating only that "white blood cells fight the bacteria" without describing that they travel to the site of the wound and engulf and destroy the bacteria there.
- In (b), confusing this response with an antibody-based response, describing antibodies binding to the bacteria's antigens rather than white blood cells directly engulfing the bacteria.
- In (c), suggesting that pus is made only of bacteria, without recognising that it also contains white blood cells that have died while destroying the bacteria, as well as tissue fluid.
- In (c), suggesting pus is simply "infected blood", rather than describing it as a mixture of dead white blood cells, dead bacteria and fluid from the tissue.