A. Whiteheads, blackheads and acne are the result of blocked skin pores, in which dead skin cells, dirt and sebum clog together blocking the opening of the pores. Whiteheads usually appear at the early stage of the clogging. As the oxidization process takes place the exposed whiteheads turn into what we know as the blackheads.
The worst outcome of the clogging, pimples, appear when a bacteria known as the propionibacterium begins to manifests under the blocked skin pores. The bacteria multiplies in size and causes infection and inflammation. The multiplication can also cause under skin tissue damage, which is why the red swollen pimples feel sore on touch.
Also See Acne Skin Care