1. Lysozyme (the antibacterial substance we covered at the start of the digestive system) in saliva, tears, earwax, body oils, etc.

2. Acidity in the stomach (around 1.5-2.0 ph) and in the female vagina (3.5-4.0 ph) both help make these entrances into the body harsh and inhospitable to microorganisms.

3. Antibiotics produced by the normal bacterial in the colon help to prevent “alien” bacteria from surviving there.

4. The Fever response—endothermic (birds & mammals) animals can turn up their body temperature making it less hospitable for the microbes. Fever seems especially adapted for combating bacteria. During fevers most of the body’s free iron and zinc is “sequestered” in the liver and is thus unavailable to bacteria elsewhere. These two elements are required for bacterial growth and reproduction. Fevers also speed up the healing of damaged tissues.

5. Interferons—these are protective proteins released by cells infected by viruses, and by some leucocytes during viral infections. When neighboring cells that have not been invaded by viruses receive these interferon messages, they start to manufacture protective proteins that make them much more resistant to viral takeover. I liken them to Paul Revere who rode out over the countryside warning of the British invasion. Those receiving this message took up arms and “resisted” the British. Interferons are obviously adapted mainly to resist virses.

6. Inflammation—Like fever, this is often though of as a bad thing caused by the infection, but inflammation too is a bodily response to aid in fighting infections. Inflamed tissues typically swell as capillaries become more leaky. Extra leaked plasma allows more clotting proteins, leucocytes, antibodies, etc. to wash over the cells in the area to aid in combating damage and microorganisms. Inflammation can result in pain—also good in some cases to protect damaged tissues, especially near joints. Some inflamed membranes increase mucus production (nasal membranes) which traps virus particles and some bacteria, allowing them to be sneezed out or swallowed (into the acid stomach). Inflammation is caused mainly by histamine & other chemicals released by special leucocytes called Mast Cells.

Complement—this is a series of 20 or more plasma proteins which can bind to bound antibodies (antibodies bound to foreign material) and sometimes directly to foreign material. If the problem is a cellular invader like bacteria or protist, complement can undergo a series of reactions that result in holes being formed in the membranes of these cells. These holes are large enough such that the cell looses its integrity and dies (“bleeds to death”). Complement also opsonizes the foreign cell & materials.

                                           Cell-Mediated Immunity

Another subclass of Lymphocytes, the Cytotoxic T Lymphocytes, are responsible for another type of immune response, which like the humoral immune response, is specific against one type of antigen.

“Naïve” Cytotoxic T Cells are activated by binding their receptors to antigens held in MHC I proteins presented on the surface of antigen-presenting cells and by interleukin stimulation. This activation usually involves Helper T Cells, but not always (as in humoral immunity).

Note that the antigens which trigger this response are presented on MHC I proteins, not on MHC II proteins, as in the Humoral immune response covered earlier. All body cells produce MHC I proteins to display or present their “wares” for inspection by patrolling leucocytes. If cells are infected or abnormal, their wares will be recognized as antigenic.

Once activated, Cytotoxic T Cells multiply to build up their numbers. This multiplication produces both Cytotoxic T Memory Cells, and “Armed” Cytotoxic T Cells.

Like in the Humoral response, Memory T Cells are held in reserve and can last for years. Armed Cytotoxic T Cells go into action and attack any cell expressing the same antigen on its MHC I Proteins. These “infected” cells are killed (these are usually your own body cells that have been infected with a virus, bacteria, etc. –or have become tumorous).

The killing is done through the release of perforin proteins, which form holes in the target cell (much like compliment), and through the release of granzymes that enter the target cell and activate nucleases there. These nucleases “chop up” the cell’s DNA causing apoptosis of the target cell. Armed Cytotoxic T Lymphocytes also produce other chemicals which aid in the attack/defense, such as interferon (if the antigen was viral).