biology 2

• Pathogens


  Disease-causing agents that can enter our bodies through eating, drinking, breathing, or interacting with people and other animals
• We are not constantly sick because the immune system enables the body to recognize its own cells and defend against infections, cancer, and foreign substances
• Immune system


  • Consists of a network of cells, defensive chemicals, and fluids that permeate the body
  • Unlike other systems, it does not have prominent organs
• White blood cells


  Infection-fighting cells suspended in blood plasma and occupying the interstitial fluid between cells

  
• Where white blood cells are located

  Bloodstream and travel through vessel walls and tissues to locate site of infection

  
• White blood cells


  • Colorless to very light purple to pink color under a microscope
  • Extremely tiny cells with a round shape and a nucleus

  
• Stem cells


  Give rise to white blood cells in red bone marrow

  
• An average individual's immune system consists of approximately 1.8 trillion immune cells, weighing around 1.2 kg
• Roles of white blood cells

  • Phagocytes (75% of white blood cells)
  • Lymphocytes (remaining 25%)

  
• Phagocytes


  Scavenger cells that engulf and destroy bacteria and debris through phagocytosis
• Macrophages


  Large phagocytic cells that play important roles in initiating the body's defenses
• Types of lymphocytes


  • B cells
  • T cells
  • Natural killer cells
• Basophils


  Least common white blood cells, involved in allergic responses
• Mast cells


  Close relatives of basophils, carry histamine
• Primary lymphoid organs


  Bone marrow and thymus, where lymphocytes are produced, matured, and selected
• Lymphocytes learn to distinguish between self and non-self in the primary lymphoid organs
• Changes in the thymus gland as the body ages

  1. At birth, it is fairly large but decreases in size as we age, eventually being replaced with adipose tissue

  2. Reduced role of the thymus in adulthood, as the immune system produces most T cells during childhood
• The number of T cells in the body remains fairly stable due to their constant replication in secondary lymphatic tissues/organs
• Decreased immunity as we age


  Latent pathogens can be reactivated, and increased susceptibility to various types of cancers
• Secondary lymphoid organs


  Lymph nodes, spleen, and lymphoid nodules, where mature lymphocytes encounter pathogens and become activated

  
• Lymphatic system


  A subsystem of the immune system that collects fluid that leaks from the blood vessels, removes bacteria, debris, cancer cells, and returns the liquid
• Formation of lymph


  1. Blood plasma is filtered by blood capillaries, passing into the interstitial spaces between tissue cells as interstitial fluid

  2. Interstitial fluid enters lymphatic capillaries and becomes lymph

  
• Lymph


  Colorless fluid of the lymphatic system, primarily water but also containing plasma solutes, substances from tissue cells, and fats from the digestive tract
• Lacteals


  Special lymphatic vessels in the villi of the small intestine that absorb fats and transport them to the blood
• Lymphatic capillaries


  • Much larger and more permeable than blood capillaries, with large gaps to allow entry of bacteria, immune cells, macrophages, and other large particles
• Lymph circulation


  1. Lymph enters lymphatic capillaries, then larger lymphatic vessels, eventually reaching lymph nodes

  2. At lymph nodes, antigenic microorganisms, foreign substances, or cancer cells stimulate lymphocytes to divide

  
• Swollen lymph nodes occur when there is an infection, as extra white blood cells accumulate in the lymph nodes in the neck, armpit, or groin

  
• Phagocytosis


  Microorganisms and foreign substances are engulfed
• What causes swollen lymph nodes?
• When you have infection, lymph nodes in the neck, the armpit, or groin become swollen and tender because of extra WBCs accumulation. They call this 'swollen glands.'
• Lymph circulation


  1. Plasma of blood is filtered by blood capillaries and passes into interstitial spaces as interstitial fluid

  2. Interstitial fluid passes from interstitial spaces into lymphatic capillaries, becoming lymph

  3. Lymph is drained by lymphatic capillaries into lymphatic vessels with one-way valves

  4. Lymph passes through lymph nodes where antigenic microorganisms, foreign substances, or cancer cells stimulate lymphocytes

  5. Macrophages phagocytize the attacked foreign substances

  6. Efferent vessels leave nodes and pass on to other lymph nodes or lymphatic trunks

  7. Lymph flow is enabled by skeletal muscle contractions, respiratory movements, and smooth muscle contraction in lymphatic vessels

  
• Principal lymphatic trunks


  • Lumbar trunk
  • Intestinal trunk
  • Bronchomediastinal trunk
  • Intercostal trunk
  • Subclavian trunk
  • Jugular trunk
• Lymph drainage


  1. Lymphatic trunks pass lymph into two main channels

  2. Thoracic duct empties lymph into left subclavian vein

  3. Right lymphatic duct empties lymph into right subclavian vein

  
• Immunity


  The ability of the body to resist infection from disease-causing microorganisms
• Adaptive immunity


  • Acts against individual targets
  • Involves B cells and T cells
• Antigen


  Any molecule that stimulates an immune reaction by B and T cells
• Pathogens are bacteria, viruses, parasites or fungi that can cause disease, and their subparts that cause antibody formation are called antigens
• Antibodies


  • Y-shaped defensive proteins that recognize specific antigens
  • Can inactivate or neutralize microbes
• Immune cells can respond to antigens from pathogens in body fluids but cannot see inside cells, only molecules on cell surfaces
• B and T cells


  • Each is genetically programmed to produce receptors that recognize and bind to only one target antigen