Acne

A normal follicle looks like this:

For reasons no one completely understands, follicles, often called pores, sometimes get blocked. Sebum (oil) which normally drains to the surface gets blocked and bacteria begins to grow. Both whiteheads and blackheads start out as a microcomedone. The picture below is a microcomedone:

There are two types of acne

1. Non-inflammatory acne

Microcomedones become non-inflamed skin blemishes called comedones--either a whitehead or a blackhead:

1.a. Whitehead

When the trapped sebum and bacteria stay below the skin surface, a whitehead is formed. Whiteheads may show up as tiny white spots, or they may be so small that they are invisible to the naked eye.

1.b. Blackhead

A blackhead occurs when the pore opens to the surface, and the sebum, which contains the skin pigment melanin, oxidizes and turns a brown/black color. It is not dirt and can not be washed away. Blackheads can last for a long time because the contents very slowly drain to the surface.

A blackhead or whitehead can release its contents to the surface and heal. Or, the follicle wall can rupture and inflammatory acne can ensue (see below). This rupture can be caused by random occurrence or by picking or touching the skin. This is why it is important to leave acne prone skin relatively untouched.

2. Inflammatory acne

2.a. Papule

A papule occurs when there is a break in the follicular wall. White blood cells rush in and the pore becomes inflamed.

2.b. Pustule

A pustule forms several days later when white blood cells make their way to the surface of the skin. This is what people usually refer to as a "zit" or a "pimple".

An inflamed lesion can sometimes completely collapse or explode, severely inflaming the surrounding skin, and sometimes engulfing neighboring follicles. These lesions are called nodules or cysts:

2.c. Nodule

When a follicle breaks along the bottom, total collapse can occur, causing a large, inflamed bump that can be sore to the touch.

2.d. Cyst

Sometimes a severe inflammatory reaction can result in very large pus filled lesions.