The immune system protects skin by regulating bacteria and responding to injuries. For example, neutrophils increase 50-fold in wounds within 24 hours, aiding repair.
Protección contra patógenos
The immune system acts as the first layer of protection to the body, the skin, against harmful pathogens. A few studies estimated that the skin hosts approximately 1.5 trillion microorganisms—namely, bacteria, viruses, and fungi. A strong immune system keeps the balance with these microorganisms, which it does not allow to overgrow into harmful species. It is estimated, for instance, that Staphylococcus aureus colonizes in 20-30% of the general population; a competent immune response, however, ensures that the bacteria do not infect the individual. In the event such defenses fail, these types of bacteria will readily penetrate the skin to cause infection, as evidenced by the facility with which infectious disease takes hold without a properly functioning immune system.
Regarding comparative age-related changes, skin defenses in younger adults tend to be more resilient compared to older adults. Indeed, one publication in Immunology Today estimated that skin of individuals above 65 years had a 30% increased pathogen colonization compared to their younger counterparts. He termed this age-related decline as immunosenescence, which impairs the capacity of skin to fend off common cutaneous infections, such as fungal or bacterial ones. Skin infections occur in as many as 40% of nursing home residents over age 70, a finding that nicely illustrates how age-related immune changes can make older adults more susceptible to skin problems. This difference, in healthy maintenance of the skin across the lifespan, directly reflects the impact of immune efficiency on the skin’s ability to effectively fight off pathogens.
Hygiene and skin care habits may also affect the immunological health of the skin. For example, the overuse of antimicrobial soaps can result in an imbalance of the skin microbiome. It is important to state that it has been estimated that individuals who apply antibacterial soaps daily have a 25% increased risk of developing dry skin and cracks on the skin, conditions that give way to new infections since bacteria can cross the skin barrier much more easily. Conversely, subjects using milder cleansers with a consideration for microbiome preservation show better balance and as such have up to 15% reductions in pathogen colonization. These data underpin the importance of skin care routines in a mindful way to support the protective role of the immune system.
Inflamación y respuesta a lesiones
The skin heals from injury with the important assistance of the inflammatory response from the immune system. After an injury to the skin, neutrophils and macrophages, for example, are rapidly mobilized at the wound site to clean away debris and fight infection. Neutrophils have been reported to increase in concentration as much as 50 times above normal levels within the first 24 hours after injury. This initial surge is crucial to fight off bacterial infections, such as a study that neutropenia individuals are 40% more likely to have infections in the wound compared to individuals who do not suffer from any kind of immune deficiency.
On the other hand, chronic inflammation hinders the process of healing while facilitating the process of long-term dermatological conditions. Diseases such as psoriasis and eczema are just two examples where there is an overactive response eliminating healthy skin tissue. It has also been proven by research in dermatological clinics that psoriasis patients do have raised inflammatory markers, such as IL-17, as high as 300% compared to patients without the disease. On one hand, chronic inflammation prevents ulcer healing; on the other hand, it promotes complicating infections that occur in about 25% of cases with severe psoriasis.
Age and underlying diseases are important in skin inflammatory responses. The immune reaction, in the case of skin lesions, is slower and not so effective compared to the healthy ones. On average, diabetic patients showed 30-50% delay in wound healing as compared to nondiabetic subjects. Such retardation is related to an impairment in blood flow and recruitment of immune cells at the injured site. This becomes dramatically evident in studies where, for instance, 45% of wounds failed to heal within an eight-week period in patients with poorly controlled diabetes, compared with a similar delay in only 10% of non-diabetic patients. These data emphasize the critical role of both systemic health and immune efficiency in the inflammation and healing process.
Regulación de la microbiota cutánea
The immune system is of utmost importance in controlling the skin microbiota, composed of trillions of microorganisms that colonize the skin and contribute to the good health of this organ. Scientific research shows that a healthy skin microbiota would entail over 1,000 species of bacteria up to a species of 80 fungi. These same microbes communicate back to the immune system to maintain the delicate balance that suppresses pathogenic overgrowth. For example, Staphylococcus epidermidis, a commensal bacterium, produces antimicrobial peptides that inhibit the colonization of the skin by Staphylococcus aureus, one of the common pathogens. It has been seen in individuals with compromised skin microbiota that there is a 40% increase in the colonization of pathogenic bacteria, leading to diseases such as eczema and acne.
Aging and environmental exposure modulate not only skin microbiota but also its immune regulation. The skin microbiota is less complex and primarily dominated by species like Streptococcus and Staphylococcus, showing lower microbial diversity in newborns. Whereas aging leads to an increased diversity of microbiota, in later years of adulthood, the microbiota might alter its composition once again with the changing immune function. One such finding came from a study that compared less than 30-year-old subjects to those older than 60 years and found that microbial diversity was reduced by 25% in the elderly. This is associated with an increased incidence of skin infections and inflammatory diseases. Such a transition underlines the interplay between the immune system and the maintenance of a healthy microbiota over the course of a lifetime.
The immunological impact on the skin microbiota is further reflected in the scope of diverse cutaneous disorders. In atopic dermatitis patients, the skin microbiota balance is generally disrupted, and overgrowth by Staphylococcus aureus can be seen in 70% of cases. Indeed, this overgrowth may trigger the inflammatory response, thus aggravating the symptoms, such as itching and erythema. Indeed, clinical trials showed that treatments which attempt to restore microbiota balance—probiotics or antimicrobial peptides—reduce symptoms by up to 40%. These findings stress the interconnection of immune regulation with microbial balance in the preservation of skin health.
Sensibilidad y reacciones alérgicas
The immune system, generally by overreacting against harmless substances such as pollen, food proteins, or chemicals in skin-care products, directly manages the sensitivity of the skin and allergic reactions. One kind of allergic contact dermatitis is suffered by about 10-20% of the population in the world, normally caused by fragrances and preservatives. For example, it is estimated that up to 11% of the general population is allergic to a common cosmetic preservative called methylisothiazolinone, which causes erythema, itching, and edema at application sites.
Data also show that allergic reactions are more prevalent in individuals with a history of other immune diseases. For instance, patients with asthma or hay fever have a 30-50% chance of developing skin allergies, also known as atopic march. In the research of 5,000 participants, it was found that 38% of the people suffering from eczema also had allergic rhinitis, whereas in the whole population, it was only 10%. This fact illustrates that immune hypersensitivity can be correlated within different systems and, hence, a hyperactive response in one region gives a good possibility for the other areas reacting similarly.
Age and environmental exposure are two significant factors that influence skin sensitivity and allergic reactions. It predominantly affects young children, with the rates of prevalence as high as 20% in children below five years. A majority of them, however, grow out of eczema as their immune system matures and rates decline to about 3% in adults. On the other hand, sensitivity brought about by allergens is acquired in adults through cumulative exposure. A follow-up of 1,000 workers continuously exposed to industrial chemicals revealed that, after five years, 22% had developed contact dermatitis, while only 5% in a control group with no comparable exposure had developed the condition. These findings support that exposing an individual to an allergen for a lesser period of time will reduce the chances of sensitization.
Envejecimiento e inmunosenescencia
Immunosenescence is the age-associated gradual decline in immune function. The skin immune defenses are severely affected by aging. By age 65, there is up to a 50% reduction in the number of Langerhans cells, critical immune cells resident in the epidermis that play an important role in the detection of pathogens and the presentation of antigens. This directly translates to a heightened vulnerability to skin infections and defective wound healing in the elderly. Indeed, it has been speculated that there are 30-40% more chronic wounds, including pressure ulcers, likely to happen in older individuals than in younger populations, further emphasizing the lost effectiveness of immune responses within aged skin.
Immunosenescence also diminishes the skin’s capability related to the repair of environmentally induced damage. For example, the DNA repair processes in skin cells are 35% less efficient in people over the age of 60 than in those under age 30. This inefficiency is the reason for the trend where non-melanoma skin cancers occur more frequently in older adults and almost double after the age of 65 years. It has been shown in the studies that 80% of skin cancers develop in people aged 55 years or older, a fact linked to the interplay between weakened immune surveillance and increased vulnerability to the mutagenic action of environmental stressors at the cellular level.
Another hallmark of immunosenescence is chronic inflammation, a process recently termed “inflammaging.” It is characterized by the persistent and systemic elevation in pro-inflammatory markers such as IL-6 and TNF-α with age in the absence of overt infection or injury. A study involving 1,200 participants showed that levels of IL-6 were 30% higher among people over 70 years than among people between 40 and 50 years of age. The result of this chronic, low-grade inflammation is the speeding up not only of the skin-aging process but also the impairment of the skin’s barrier function, with increased vulnerability to dryness and irritation. Indeed, older adults who have higher levels of these markers are twice as likely to develop conditions such as rosacea and chronic dermatitis.