Inflamed heart sac. Light micrograph of a section through a human heart showing the heart sac (pericardium, purple), a fibrinous fluid (exudate, yellow) and cardiac muscle (red) in a case of pericarditis (heart sac inflammation). A large number of granulocytes, a type of white blood cell (leucocyte) that have an irregular nucleus and a grainy appearance, can be seen at upper right (dark purple). Pericarditis can be caused by fungal, viral or bacterial infection, an injury, cancer, a heart attack, or an autoimmune disorder. Granulocytes are involved in the immune system's defence mechanisms, protecting the body against the invasion of foreign pathogens in the blood and lymph.