Excess hormone can prompt these glands to produce too much sebum. And if the hair follicle gets blocked by dead skin cells, oil builds up with nowhere to go. This build up of oil creates an ideal environment in which acne bacteria can multiply, triggering a chemical reaction and the formation of spots. We have lots of sebaceous glands in our face, chest, neck and back – which is why spots are most common in these areas.
Blackheads, also called open comedones, look like benign black dots on your skin. They usually develop in larger pores that allow air to get in and react with the oil and dead skin cells inside, causing the plug to darken and appear black.
Whiteheads, or closed comedones, are filled with the same matter as blackheads, but the pore isn't as large. Air can't get in and react with the sebum, so it stays white.
Both are milder types of spot, and for many people spots don't progress beyond this stage. If pores remain blocked, the build up of oil encourages normally harmless acne bacteria to multiply. This high level of bacteria puts the body's immune system into attack mode, causing inflammation. Red inflamed bumps are a sign of infected pores caused by acne bacteria, and usually require stronger treatment.