Chemical Defense Systems

The digestive tract has tight junctions and goblet cells that secrete mucus but it generally does not have cilia like the respiratory tract. Instead, there is peristalsis or strong muscular contractions that propel the mucus out of the body along with feces. The goblet cells are always the cells in the body responsible for making mucus through secretory vesicles near the cell’s surface. The cells that line the urogenital tract, lymph vessels, and blood vessels are called endothelial cells. These are also the cells that make the blood-brain barrier, which prevents disease in the central nervous system. These cell junctions are the tightest in the body because they prevent serious infections from getting into the delicate tissues of the brain and spinal cord.

There are other mechanical defenses besides skin sloughing and cilia movement. The urine provides a mechanical defense by flushing bacteria out of the urinary tract. Eyelashes, eyelids, and tears also prevent the invasion of bacteria in the eyes. Peristalsis is a mechanical defense system in the GI tract.

The normal microbiome of the body will be a good line of defense against invading microorganisms. They compete for nutrients and occupy the area so pathogens can’t get into the body. Places where the microbiome is important include the vagina, the GI tract, the respiratory tract, and the skin. There are chemical contributions to the microbiome as well. The lack of a good microbiome plays a role in getting an opportunistic infection after taking an antibiotic.

CHEMICAL DEFENSE SYSTEMS

The innate immune system has chemical defense mechanisms that also block pathogens. Some are produced in the human body itself, while others are produced by friendly microbes. The sebaceous glands secrete sebum that seals off the hair follicle the gland is attached to. There are bacteria in the microbiome that feed off of sebum to make a waste product called oleic acid, which is acidic and prevents bacterial contamination. The oleic acid is not endogenous to humans but is made by skin bacteria and fungi.

The digestive tract also produces chemicals that protect the body against infection. There are lactoperoxidase enzymes in saliva and lysozyme in the esophagus. Gastric acid in the stomach is also protective against pathogens. There are antibacterial peptides and bile in the small intestine; lysozyme is produced in the small intestine as well.

Acidity is important in the female reproductive system and the urinary tract. The urine contains acids that protect the urinary tract. There is lactate in the vagina that lowers the pH. This is made by lactobacilli that ferment glycogen as part of their metabolism, which results in a lowered pH.

The eyes have tears that are made with lysozyme and lactoferrin, which block microbial activity. Lysozyme affects the peptidoglycan layer of the bacteria and lactoferrin sequesters iron so it cannot be used by microbes. Ears make cerumen or earwax that decreases the pH of the ears and makes fatty acids that inhibit bacteria. The respiratory tract makes all of the major enzymes, such as lactoferrin, lysozyme, and lactoperoxidase, which block bacterial growth. Surfactant in the lungs is also antibacterial.

Antimicrobial peptides have diffuse antimicrobial properties. Some are naturally produced by the body, while others are made only when there is a bacterial invasion. They can damage the cell membrane, damage RNA or DNA, or block cell wall synthesis. Some act only on bacteria, while others will act against a variety of pathogens. Most of these peptides are found in the skin. Others are called defensins, made by epithelial and other cells.

There are actually several types of antimicrobial peptides. The genes that make these are often transferred from cell to cell through plasmids. Bacteriocins are made in the GI tract. Defensins are made by macrophages, epithelial cells, and neutrophils. Dermcidin is made by sweat glands. Cathelicidin is made by cells in the skin. Histatins are made by the salivary glands.

Plasma protein mediators are found in blood plasma. These include certain acute-phase proteins, cytokines, and complement proteins that exert a nonspecific response in the innate immune system. There are numerous acute-phase proteins made by the liver and sent to the blood. These include ferritin, transferrin, fibrinogen, serum amyloid A, and C-reactive protein—each of which is antimicrobial.

The complement system is important to the immune response. There are more than thirty proteins in this system; they get activated when stimulated by microbial invasion. There are several pathways involved, including the classic pathway, the alternative pathway, and the lectin pathway. These are actually cascades activated under specific circumstances.

The function of the complement cascade is to cause opsonization. This is when bacteria get coated with a substance that is recognized by phagocytic cells. Opsonin is what the coating is called. There are opsonins made by proteins in the complement cascade, antibodies, and mannose-binding proteins. Certain complement proteins called C3a and C5a strongly affect the inflammatory response, triggering mast cells to release their chemical mediators, including heparin and histamine. C5a will attract neutrophils and other white blood cells.

There are certain complement proteins that aggregate to form a membrane attack complex, which participates in making pores in the cell membrane of gram-negative bacteria so that there is bursting of the cell and death of the pathogen. It is only effective against gram-negative bacteria.

Cytokines are also important proteins in the innate immune system. These are stimulators of the innate immune system. Cytokines can do several things which can be described as autocrine, paracrine, or endocrine. Autocrine responses affect the same cell that makes it. Paracrine responses affect nearby cells. Endocrine responses affect distant cells.

Cytokines can be interleukins, chemokines, or interferons. Interleukins do many things to moderate the immune system and are mainly made by white blood cells. Chemokines attract other white blood cells to the site of an infection. Interferons are signaling molecules in the immune system. They do many things but mainly defend against viruses.

Cytokines will stimulate basophils and mast cells, which release histamine and heparin as part of the immune response. Histamine can also cause bronchoconstriction. Mast cells will also release leukotrienes that are potent, proinflammatory molecules. These