Lymph node. Light micrograph of a section through a lymph node. The blue tissue at right is the fibrous outer capsule of the node. Beneath it is the subcapsular sinus (white). Tissue fluid (lymph) drains into the sinuses, where white blood cells called macrophages remove and digest cellular debris, bacteria and other pathogens from the fluid. Medullary cords (dark purple) contain B lymphocytes (a type of white blood cell) which are part of the body's immune system.