Psoriasis. Artwork of a section through the skin in a case of pustular psoriasis. Psoriasis is the presence of scaly lesions on the skin, caused by an increase in the rate at which new skin cells are produced. The scaly overgrowth is seen across top (grey) over living skin cells (pink). Pustular psoriasis is when pus (yellow) forms in the skin as a complication of psoriasis. Excessive amounts of granulocytes (cells in pus with lobed nuclei), a type of white blood cell, also accumulate in the blood (vessels across bottom) and tissues. This is thought to be an excessive response of the immune system to the psoriasis. Treatment depends on the cause, which can include stress and infections.