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 |