Functions of Leukocytes

• Leukocytes, also known as white blood cells (WBCs), are the mobile units of the body’s immune defense system.
• Immunity is the body’s ability to resist or eliminate potentially harmful foreign materials or abnormal cells.
• Leukocytes and their derivatives, along with a variety of plasma proteins, make up the immune system.
• The immune system defends against invading disease-producing microorganisms, functions as a “cleanup crew” that removes worn-out cells and tissue debris, and identifies and destroys cancer cells.
• The largest group of leukocytes is the granulocytes, which contain cytoplasmic granules that carry substances involved in allergic or inflammatory responses.
• Granulocytes include neutrophils, eosinophils, and basophils.
• Leukocytes, erythrocytes, and blood platelets are all ultimately derived from unspecialized cells in the bone marrow.
• These stem cells replace themselves by cell division so that the stem cell population is not exhausted.
• Lymphocytes produced in this manner seed the thymus, spleen, and lymph nodes, producing self-replacing lymphocyte colonies in these organs.
• There are normally only 4,000 to 11,000 white cells per μL of human blood, compared with 5,000,000 red cells per μL.
• Granulocytes, especially neutrophils, contain mechanisms by which they can progress rapidly from a harmless circulating intravascular cell to a specific phagocytic cell and killer of foreign particles, including bacteria.
• The granulocytes and monocytes have a special ability to “seek and destroy” function.
• Aulus Celsus first described four of the five cardinal signs of inflammation in De Medicina: rubor (redness), calor (heat), tumor (swelling), and dolor (pain).
• The redness and warmth in inflammation arises from the increased local blood flow.
• Within several hours after tissue damage begins, the area becomes well supplied with neutrophils.
• Cytotoxic T cells and helper T cells bind to antigens on the surfaces of target cells and kill them by secreting toxic chemicals.
• The swelling in inflammation stretches free nerve endings that can cause pain.
• B lymphocytes are born and mature in bone marrow.
• Tissue injury can be caused by bacteria, trauma, toxic chemicals, and heat.
• Memory T cells help the immune system respond more quickly to future invasions by the same agent.
• Galen added a fifth sign of inflammation: impaired function.
• Before blood enters the general circulation, it passes through the sinusoids of the liver, which are lined with tissue macrophages called Kupffer cells.
• B lymphocytes produce antibodies against specific antigens and are responsible for humoral immunity (antibody-mediated immunity).
• The processes of inflammation include vasodilation of the local blood vessels to increased blood flow, increased permeability of the capillaries to leakage of fluid into the interstitial space, clotting of the fluid in the interstitial space due to leakage of clotting proteins from the plasma into the interstitial fluid, and migration of large numbers of granulocytes and monocytes into the tissue.
• Helper T cells do not kill cells on their own but help activate both B cells and cytotoxic T cells by secreting cytokines.
• Microglia, the resident immune cells of the central nervous system (CNS), are involved in inflammation.
• Neutrophils are involved in the early stages of acute inflammation, making up 50 - 70 % of circulating WBCs.
• T lymphocytes do not produce antibodies but directly destroy target cells by releasing chemicals (cell mediated immunity).
• Monocytes are the largest of WBCs with a single large nucleus, kidney bean or horseshoe shaped, and enter the tissues by diapedesis to become tissue macrophages.
• The clotting of the fluid in the interstitial space effectively walls off the area so that pathogens cannot easily spread to the rest of the body.
• Supressor T cells block the actions of some other types of lymphocytes, to keep the immune system from becoming over-active; modulates the immune system functions.
• The edema or swelling in inflammation arises from the increased capillary permeability and subsequent exudation of fluid into the tissues.
• Secondary lymphoid organs capture and concentrate pathogens, present them to macrophages and other cells, and serve as sites where circulating lymphocytes can come into contact with the foreign antigens.
• Injured tissue releases a variety of substances that evoke a complex series of changes within the tissue, which is called inflammation.
• Neutrophils and tissue macrophages defend against infections, with the neutrophils being mature cells that can attack and destroy bacteria even in the circulating blood, and the tissue macrophages beginning life as blood monocytes, which are immature cells while still in the blood and have little ability to fight infectious agents at that time.
• Monocytes mature and enlarge in resident tissue, become professional phagocytes, and secrete a large number of lysosomal, chemotactic, complement-activating, and pyrogenic factors.
• Monocytes and macrophages are involved in the invasion of the inflamed area.
• A single macrophage can attract and activate 10 to 100 neutrophils before the macrophage itself becomes inactivated and dies.
• Within a few hours after the severe inflammation, the number of neutrophils sometimes increases fourfold to fivefold, a condition called neutrophilia, caused by products of inflammation that enter the blood stream transported to the bone marrow act on the stored neutrophils mobilize stored neutrophils into the circulating blood.
• T lymphocytes are born in bone marrow and mature in the thymus.