Illustration of the immune response initiated by the presence of foreign pathogen (bacteria N. Meningitidis, foreground, bottom middle) which activates the host immune response. Macrophages (blue, upper right), the major cellular component of the immune system, release cytokines, which stimulate the blood vessels to release Albumin, a straw colored liquid containing red blood cells and platelets, into the surrounding tissue, causing edema, the accumulation of fluid in intercellular spaces of tissues.