What to make the most of your existing natural defenses against colds, flus, and other respiratory illnesses? Look no further than your own reflection; the state of your skin and mucous membranes. If your hands, nose, or throat are feeling dry, replenish the lost moisture stat before an invading pathogen makes it across the breached barrier! Together the skin and mucous membranes make up the epithelium, the first line of physical defense against pathogenic bacteria, viruses, fungi (yeast), and other foreign cells and proteins.
Healthy skin and mucous membranes protect your body
If dewy skin is like a shiny coat of steel armour that keeps potentially harmful substances from penetrating deeper (where they can cause more serious damage and disease), then healthy mucous membranes is like chain mail. Mail also forms part of the “body armour” but is lighter and more flexible; it can vary in thickness (just like skin). A wonderful feature is that mail is self-maintaining: the links grind together as you move and remove rust! Likewise, the epithelium has a large capacity for self-repair under normal circumstances, which is a blessing consider how fragile it can be. While traditional armouring (and beauty) may only be skin deep, immune health involves structures below the surface too, namely the digestive system! 70 – 80% of immune cells reside within the gastrointestinal tract (GIT) or gut. Your gut mucosa (lining) consists of many specialized features, including gut associated lymphatic tissue (GALT) and Peyer’s patches, which reinforce its immune function.
Mucous membranes offer physical and chemical protection
Mucous membranes are soft, pliable, and susceptible to injury as a result of their function and locations. None more than the mucosa of the small intestine, which is only one cell layer thick! Mucous membranes line body cavities and are continuations of your skin, which is exposed to the outside world. They are found throughout the body across five systems: digestive (mouth, stomach, and intestines); respiratory (nose, trachea (windpipe), lungs); urogenital (ureters, urethra, kidney, bladder, uterus and vagina / glans penis); eyes; and ears. Thankfully, all mucous membranes (with the exception of the urinary tract—because urine is inherently sterile) are able to secrete moist, vicious secretions, i.e. mucous, which not only physically blocks out pathogens but also contains antimicrobial peptides (AMPs) such as defensins and cathelicidin (natural antibiotics). AMPs are an example of one of your body’s many chemical lines of defense. Your chances of staying healthy are high when pathogens and other foreign substances must cross a moat of mucous filled with chemical weapons! However, injury to the epithelial layer (or a chink in your armour) from abrasion, inflammation, or repeated immunological challenges renders it less effective.
Reinforcements: Division of labour between the innate and adaptive immune system
Thus far, we have only discussed the innate (non-specific) immune system because it is the 1st-line, acting within hours of antigen (toxin or foreign substance) exposure. Alongside physical and chemical defenses lie cell-mediated responses: phagocytes (Pac-Man like cells that chomp up bacteria and other small particles, including neutrophils, macrophages, mast cells, and others), many of which also release cytokines (signalling molecules) and inflammatory mediators—a specialty of natural-killer cells against virally infected cells and tumours. The first diagram shows the three lines / layers of immune protection, which have a great deal of crossover: (1) physical and chemical barriers; (2) innate inflammatory responses, including phagocytosis; and (3) innate and adaptive immune responses.
DIAGRAM 1: Physical and Chemical Barriers
Adaptive immunity (acquired immunity) is more specific and based on past exposure to antigens and immune cell memory. Once antigens are processed and recognized, a trained immune response is mounted to attack them. Adaptive immunity is the driver of anaphylactic IgE antibody-mediated hypersensitivity reactions. The next diagram clarifies how the adaptive immune system detects an antigen and its subsequent response (production of histamine, free oxygen radicals, prostaglandins, leukotrienes, etc.), which results in allergic conditions such as hayfever, asthma, dermatitis, etc. It chronicles the steps involved from first contact when the allergen enters through the skin, lungs, or gut mucosa. Both the innate and adaptive immune system contribute to gut barrier function.
DIAGRAM 2: An Allergic Immune Response
Other unsung heros of the immune system: Peyer’s patches, GALT, and secretory IgA
Peyer’s patches are found primarily in the mucosa of the ilium (distal small intestine). They help determine what antigens can pass into the gut lumen (interior) where they can be introduced to antigen presenting cells (APCs), i.e. the yellow blob in the diagram above. In turn, APCs introduce them to T cells (which mature in the Thymus gland). Gut associated lymphatic tissues (GALT) contains an abundance of antibody-secreting B cells, which secrete the vast majority of IgA antibodies. Sufficient amounts of IgA (and IgG) in the body can help neutralize IgE-mediated reactions. Uniquely, IgA is found in external secretions (breastmilk, tears, and saliva) along with blood. This allows it to interact with antigens before they enter your body. IgA is the lance or long sword in the weapon and armour analogy, it keeps your opponents at bay. When you are in a state of acute stress and acting out fight-or-flight responses, your glandular secretions are reduced (due to contraction of blood vessels). One of the many ways that stress negatively impacts your immune system and digestion is by inhibiting mucosal secretions. In fact, secretory IgA can be used as a biomarker of stress-induced immune suppression.
Maintaining the quality of both your outer (skin) and inner (mucousal) epithelial linings is crucial for a well-functioning immune system. To learn about ways you can support healthy epithelial function and optimize the layers of your immune defense, please refer to my next article in this two-part series on Strengthening Your 1st Line of Immune Defense.