The main function of the immune system is defense against microbial infection. The other major functions of the immune system depend on a number of major actions that defend us against threats to our health. These actions help our immune system sort out a system of complicated reactions that our body uses every day to defend us from infection and disease. When these properties are not functioning, they often spell disaster if not watched carefully.
The first set of functions is usually called "natural" or "innate" immunity since it does not have to be turned on and is always (under normal circumstances) present. This set of functions is non-specific, which means that it does not single out a specific microbe when defending the body and does not require the body to identify the potential pathogen before it acts.
Natural immunity includes things like our skin or other organs that create a barrier between our insides and the infectious outside world. Another type of barrier is a secretion like a tear or mucous that is sloughed off and takes the infectious agent with it.
Natural immunity includes proteins called compliments that make it more attractive for a microbe to undergo phagocytosis. These compliments attach themselves to the target microbe to mark it for death by labeling it an unauthorized microbe.
Natural immunity includes phagocytes that "eat" microbes appearing to be a threat within the body. Phagocytes are any cell that eats another. The most common of these is the white blood cell. If the phagocyte cannot identify the microbe, it eats it by surrounding it and dumping digestive enzymes to break it down into digestible pieces.
Natural immunity includes inflammation. Inflammation is also referred to as flare, flame and wheal in medical schools. The "flare" is the redness. The "flame" is the inflammation and itch. "Wheal" is the swelling of the area caused by fluid getting under the area and phagocytes migrating into the area which results in puss formation.
Acquired immunity includes all of the reactions in the body that need lymph node involvement. Acquired immunity is also known as adaptive immunity or specific immunity. This process is broken into two parts that include what happens in the fluids (humoral immunity) and what happens in the rest of the body (including cellular immunity).
Humoral vs. Cellular Immunity
Humoral immunity involves making antibodies when B lymphocytes are educated by antibodies about the threat. The lymphocytes then make antibodies that bind to either a microbe or a toxin to mark it so that it is seen as a good candidate for elimination and speeds up the process. Cellular immunity involves T cells which do two things: They change so that they activate B cells to "raise the alarm" and start inflammation. T cells also undergo changes to allow them to become a cell that kills other cells by bursting them.