Anatomical Barriers of Immune System: Skin and Mucus

Before a microorganism or parasite can initiate an infection within a host, it must first adhere to and breach the surface layers of epithelial tissues. These invaders gain access to the body through either active means, such as penetrating the skin, or passive means, like being ingested, inhaled, or entering through a wound.

Regardless of the entry point, they must navigate through the external physical barriers that shield the body’s interior from pathogens in the external environment. These barriers, known as anatomical barriers, encompass the skin and the mucous epithelial layers (mucous membranes) lining the respiratory, gastrointestinal, and urogenital tracts, as well as the ducts of exocrine glands.

Collectively, the skin and mucous membranes act like a protective layer, akin to living “plastic wrap,” safeguarding the inner regions of the body from potential infections. Additionally, they play roles in physical and mechanical processes that facilitate the removal of pathogens. Moreover, they actively contribute to the body’s defense mechanisms by producing and deploying molecules with antimicrobial properties or inducing such activity.

The Skin

The skin, the body’s largest organ, plays a vital role as a physical barrier against pathogens due to the following reasons:

1. The epidermal layer primarily consists of specialized keratinocytes, which produce a water-resistant protein called keratin. This property makes it challenging for many microorganisms to penetrate the skin.
2. The skin’s relatively dry nature, coupled with the high salt concentration in sweat as it dries, proves inhibitory or lethal to numerous microbes.
3. Epithelial cells also manufacture defensins and cathelicidins, which are substances that have the capacity to kill microbes.
4. Sebum, a secretion of the skin, hinders the growth of various microorganisms. It contains lactic acid and fatty acids that help maintain the skin’s pH level between 3 and 5, creating an environment inhibitory to many bacteria.
5. The outermost layer of the skin is comprised of dead cells that are consistently shed. This shedding process dislodges organisms and also hinders viruses that rely on living cells for their metabolic processes.
6. Keratinocytes additionally release several cytokines, which may induce a localized inflammatory response.
7. Within the matrix of epithelial cells in the epidermis, Langerhans cells are interspersed. These are dendritic cells resident in the skin that internalize antigens through phagocytosis. These cells undergo maturation and then migrate from the epidermis to nearby lymph nodes, where they act as potent activators of naïve T cells.

Besides these roles in non-specific defence mechanism, the skin also has following functions:

Within the epidermis, there are intraepidermal lymphocytes, primarily composed of T cells. These cells are thought to have a significant role in combating infections that make their entry through the skin.

Beneath the epidermis lies the dermal layer, housing a variety of immune cells like lymphocytes, dendritic cells, monocytes, macrophages, and potentially even hematopoietic stem cells.

A significant proportion of the lymphocytes in the skin are either previously activated or memory cells. Many of these cells engage in traffic to and from nearby lymph nodes, which serve as central hubs coordinating responses to pathogens that have breached the skin’s protective barrier.

The Mucous Membrane

Instead of the skin, the respiratory, gastrointestinal, and urogenital tracts, along with the ducts of salivary, lacrimal, and mammary glands, are lined by resilient layers of mucous epithelial cells.

Several non-specific defense mechanisms work in tandem to thwart the entry of microorganisms through these mucous membranes. Mucus, a thick fluid secreted by the epithelial cells of mucosal membranes, ensnares invading microorganisms. Additionally, tight junctions bind these epithelial cells together, creating a barrier that prevents pathogens from infiltrating the body.

Saliva, tears, and mucous secretions serve to cleanse potential invaders, effectively preventing their attachment. Moreover, these secretions contain antibacterial and antiviral components that eliminate pathogens. For instance, mucus houses lysozyme, which degrades bacterial peptidoglycan, as well as antibody IgA that hinders microbe attachment and traps them within the mucus. Additionally, lactoferrin sequesters iron, denying its use by microbes, and lactoperoxidase generates toxic superoxide radicals that exterminate microbes.

Much like the skin, the mucous membrane continuously sheds cells to eliminate microbes adhered to its surface. In the lower respiratory tract, cilia, hair-like projections of epithelial cells, propel mucus and trapped particles out of the body.

Beneath the mucosal membrane lies the mucosa-associated lymphoid tissue (MALT), harboring Langerhans’ cells that play a crucial role in activating naive T cells.

In addition to their roles in non-specific defense mechanisms, intraepithelial T-lymphocytes and B-1 lymphocytes are linked with both the epidermis and the mucosal epithelium. These cells identify microbes commonly encountered in these regions and promptly initiate adaptive immune responses against them.

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