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• 1 structure and function

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•  Maintenance of fluid balance

   Fluid continually seeps out of capillaries into surrounding tissues. The capillaries reabsorb about 85% of the fluid, leaving about 15% behind. This amount may seem minimal, but, over the course of a day, the remaining fluid would total as much as 4 liters, enough to cause massive swelling and even death. One of the roles of the lymphatic system is to absorb this fluid and return it to the bloodstream

• efferent lymphatic vessel.

  After slowly filtering through the node, lymph leaves through a single efferent lymphatic vessel.

• • Destruction of old red blood cells

  : Macrophages in the sinuses digest worn-out RBCs and imperfect platelets. Macrophages also recycle hemoglobin from the destroyed RBCs, salvaging the iron and globin and returning it to the bone marrow and liver for later use.

• Second line of defense

  If a pathogen penetrates the first line of defense, the body launches several mechanisms geared at repelling a wide variety of threats, including the production of phagocytic white blood cells and triggering inflammation and fever. Because these responses are aimed at a broad range of attackers, rather than one specific pathogen, the response is called nonspecific immunity. Another term is innate immunity, because the mechanisms are present from birth, allowing the body to repel pathogens to which it has never been exposed.

• membrane attack complex

  The final five proteins (called the membrane attack complex) embed themselves into the bacterium’s plasma membrane in ring-like circles, effectively punching a hole in the bacterium.

• cellular (cell-mediated) immunity

  For this third line of defense, the body employs two mechanisms. One aims to destroy foreign cells or host cells that have become infected with a pathogen. This is called cellular (cell-mediated) immunity.

•  humoral (antibody-mediated) immunity

   The other mechanism, which focuses on pathogens outside the host cells, sends out antibodies to “mark” a pathogen for later destruction. This is called humoral (antibody-mediated) immunity

• Natural active immunity

  This type of immunity occurs when the body produces antibodies or T cells after being exposed to a particular antigen. (For example, if you become ill with the measles, your body will produce antibodies to this particular virus, making you immune to infection in the future.)

• Artificial active immunity

  This results when the body makes T cells and antibodies against a disease as a result of a vaccination (such as for tetanus or influenza). By injecting a vaccine containing dead or weakened (attenuated) pathogens, the recipient’s body produces an immune response without actually developing the illness.

• Natural passive immunity

  This form of immunity results when a fetus acquires antibodies from the mother through the placenta, or when a baby acquires them through breastfeeding.