Anatomical Barriers of Immune System- Skin and Mucus


The skin is the largest organ in the body and a critical anatomic barrier against pathogens. It covers the entire surface of the body and protects the underlying tissues and organs from physical damage, dehydration, and microbial invasion. The skin consists of two main layers: the epidermis and the dermis. The epidermis is the outermost layer that is in direct contact with the external environment. The dermis is the inner layer that supports and nourishes the epidermis and contains blood vessels, nerves, hair follicles, sweat glands, and sebaceous glands.

The epidermis is composed largely of specialized epithelial cells called keratinocytes that produce a waterproofing protein called keratin. Keratin is not easily degraded by many microorganisms, which makes skin penetration difficult. The epidermis also contains other types of cells that contribute to skin defense, such as melanocytes that produce melanin to protect against ultraviolet radiation, Langerhans cells that act as antigen-presenting cells to activate adaptive immunity, and intraepidermal lymphocytes that are mostly T cells that recognize and eliminate infected or abnormal cells.

The skin has several mechanisms to prevent or limit microbial growth on its surface. These include:

  • The relatively dry skin with high salt concentration in drying sweat is inhibitory or lethal to many microbes.
  • Skin secretes sebum, which prevents growth of many microorganisms. Sebum consists of lactic acid and fatty acids that maintain the pH of the skin between 3 and 5, which is inhibitory to many bacteria.
  • Epithelial cells also produce antimicrobial peptides such as defensins and cathelicidins that kill microbes by disrupting their membranes or interfering with their metabolism.
  • The outer surface of the skin consists of dead cells that are continuously shed, causing the organisms to dislodge and also preventing viruses that require living cells for their replication.
  • Keratinocytes also secrete a number of cytokines that may function to induce a local inflammatory reaction and recruit immune cells to the site of infection.

The skin is not only a passive barrier but also an active participant in immune responses. It contains various types of immune cells that can detect and respond to pathogens that have breached the skin barrier. These include:

  • Langerhans cells, which are skin-resident dendritic cells that internalize antigen by phagocytosis. These Langerhans cells undergo maturation and migrate from the epidermis to regional lymph nodes, where they function as potent activators of naive T cells.
  • Dermal dendritic cells, which are similar to Langerhans cells but reside in the dermis and can also present antigen to T cells in lymph nodes.
  • Macrophages, which are phagocytic cells that can engulf and destroy microbes and also secrete cytokines and chemokines to modulate inflammation and immunity.
  • Mast cells, which are granular cells that release histamine and other mediators that cause vasodilation, increased vascular permeability, smooth muscle contraction, and itching. These effects facilitate the entry of immune cells and fluids into the infected tissue and also act as an alarm signal to alert the host of a potential threat.
  • Natural killer (NK) cells, which are cytotoxic lymphocytes that can recognize and kill infected or abnormal cells without prior sensitization. They also secrete cytokines such as interferon-gamma (IFN-gamma) that enhance the activity of macrophages and other immune cells.
  • B cells and plasma cells, which are antibody-producing lymphocytes that can bind to specific antigens on pathogens or their products and neutralize them or mark them for destruction by other immune mechanisms.
  • T cells, which are divided into helper T (Th) cells and cytotoxic T (Tc) cells based on their functions. Th cells secrete cytokines that regulate the activation and differentiation of other immune cells such as B cells, macrophages, NK cells, and Tc cells. Tc cells kill infected or abnormal cells by releasing perforin and granzymes that induce apoptosis.

The skin is a complex organ that provides both physical and immunological barriers against pathogens. It has multiple layers of defense mechanisms that work together to prevent or limit microbial invasion and infection. The skin also interacts with other components of the immune system to coordinate effective responses against pathogens that have breached the skin barrier.