bacteria (singular: bacterium) Single-celled organisms. These dwell nearly everywhere on Earth, from the bottom of the sea to inside other living organisms (such as plants and animals). Bacteria are one of the three domains of life on Earth.
cell The smallest structural and functional unit of an organism. Typically too small to see with the unaided eye, it consists of a watery fluid surrounded by a membrane or wall. Depending on their size, animals are made of anywhere from thousands to trillions of cells. Most organisms, such as yeasts, molds, bacteria and some algae, are composed of only one cell.
immune (adj.) Having to do with the immunity. (v.) Able to ward off a particular infection. Alternatively, this term can be used to mean an organism shows no impacts from exposure to a particular poison or process. More generally, the term may signal that something cannot be hurt by a particular drug, disease or chemical.
immune system The collection of cells and their responses that help the body fight off infections and deal with foreign substances that may provoke allergies.
infection A disease that can spread from one organism to another. It’s usually caused by some type of germ.
liquid A material that flows freely but keeps a constant volume, like water or oil.
lymph A colorless fluid that bathes tissues in the body. It contains a type of white blood cells (lymphocytes) that help the body fight disease. This liquid drains from intercellular spaces to lymph vessels and eventually moves on into the bloodstream.
lymph nodes Also known as lymph glands, these are mall nodules located in the armpits, groin and stomach. They are part of the lymph system. They produce white blood cells that the body uses to fight infection. They also serve as a storage place for some other cells in the immune system.
lymphocyte A type of white blood cell that is used by the immune system. These cells are made by the bone marrow. There are two main types. B lymphocytes make antibodies.T lymphocytes help kill cancer cells and help control how the immune system responds to threats by invaders (bacteria and viruses, for instance) or certain pollutants, drugs and other substances considered "foreign" (as in not made by the body).
measles A highly contagious disease, typically striking children. Symptoms include a characteristic rash across the body, headaches, runny nose, and coughing. Some people also develop pinkeye, a swelling of the brain (which can cause brain damage) and pneumonia. Both of the latter two complications can lead to death. Fortunately, since the middle 1960s there has been a vaccine to dramatically cut the risk of infection.
microscopic An adjective for things too small to be seen by the unaided eye. It takes a microscope to view objects this small, such as bacteria or other one-celled organisms.
organ (in biology) Various parts of an organism that perform one or more particular functions. For instance, an ovary is an organ that makes eggs, the brain is an organ that makes sense of nerve signals and a plant’s roots are organs that take in nutrients and moisture.
plasma (in chemistry and physics) A gaseous state of matter in which electrons separate from the atom. A plasma includes both positively and negatively charged particles. (in medicine) The colorless fluid part of blood.
spleen An organ in the upper left abdomen, not far from the stomach, and protected by the rib cage. Among its many roles, it can filter foreign substances from the blood and can take blood cells out of vessels when the cells become too old. It also can store certain blood constituents, such as platelets and white blood cells.
tissue Made of cells, it is any of the distinct types of materials that make up animals, plants or fungi. Cells within a tissue work as a unit to perform a particular function in living organisms. Different organs of the human body, for instance, often are made from many different types of tissues.
tonsils Short for palatine tonsils. This is a pair of soft tissues at the back of the throat. They are part of the immune system, which can help to fight infections. Many people get infected tonsils removed, however, and seem no more vulnerable to infection afterward. The tonsils’ structure somewhat resembles that of lymph nodes. The tonsils’ outer pink cover is similar to that of the mouth’s lining.
virus Tiny infectious particles consisting of RNA or DNA surrounded by protein. Viruses can reproduce only by injecting their genetic material into the cells of living creatures. Although scientists frequently refer to viruses as live or dead, in fact no virus is truly alive. It doesn’t eat like animals do, or make its own food the way plants do. It must hijack the cellular machinery of a living cell in order to survive.
waste Any materials that are left over from biological or other systems that have no value, so they can be disposed of as trash or recycled for some new use.
white blood cells Blood cells that help the body fight off infection.
