The sebaceous glands produce oil called sebum, sometimes this mixes with dead skin cells and bacteria to form a hard plug. Exposure to the air changes the plug to a black colour, resulting in a blackhead. Its colour is not caused by dirt but by the chemicals in the mixture. If the sebum remains under the skin, then it does not become exposed to the air. It remains white and is called a whitehead.
Whiteheads are the major problem.
Sebum build-up may become infected as the bacteria feed on the oil and cells. White blood cells move into the area to try to destroy the bacteria. Dead white cells and bacteria form the yellowy mixture known as pus. Cut out the diagrams below and stick them into your book. Blackheads Whiteheads Colour in the diagrams and explain how blackheads and whitheads form.