Innate immune system

The innate immune system is a primary defense strategy used by vertebrates to protect against infection. It serves as a nonspecific defense, meaning it responds to many different types of threats in a similar way. While vertebrates also use an adaptive immune system, the innate system is the dominant defense in plants, fungi, invertebrates, and prokaryotes.

Its main roles include recruiting immune cells to infection sites, identifying foreign substances in the blood or tissues, and acting as a physical and chemical barrier. It also plays a vital role in activating the adaptive immune system through a process called antigen presentation.

Protection begins with anatomical barriers that act as the first line of defense. Epithelial surfaces form a physical barrier that is largely impermeable to most infectious agents. In the skin, the process of desquamation, or shedding, helps remove bacteria that have adhered to the surface. The skin also uses chemical measures, such as sweat containing dermcidin and organic acids, to create an environment unsuitable for microbes. In the respiratory tract, cilia move to clear agents, while mucus traps them.

The gastrointestinal tract uses peristalsis, or muscular movement, and gastric acid to remove invaders. Even the eyes and mouth use the flushing action of tears and saliva to prevent infection.

When these barriers are breached, the body initiates an inflammatory response. This process is triggered by chemical factors released from injured cells. Resident cells, such as macrophages, dendritic cells, and mast cells, use pattern recognition receptors (PRRs) to identify pathogens. These receptors recognize pathogen-associated molecular patterns (PAMPs), which are molecules shared by many germs but not by the host. Once activated, these cells release inflammatory mediators like cytokines and chemokines. These chemicals cause vasodilation, which increases blood circulation and leads to redness, heat, and swelling.

This response also attracts phagocytes, such as neutrophils, to the site of the injury.

One essential part of this defense is the complement system. This system uses soluble plasma proteins, specifically proteins C1 through C9, to identify and eliminate pathogens. The system works through a cascade, which is a series of linked reactions. In the classical pathway, antibodies from the adaptive immune system bind to a pathogen and activate C1. This leads to the formation of C3 convertase, which splits into C3a and C3b. C3a increases inflammation, while C3b helps form the membrane attack complex. This complex inserts itself into the pathogen's membrane to cause cell lysis, which is the bursting of the cell.

An alternative pathway can also begin with the spontaneous activation of C3.

Specialized white blood cells, known as leukocytes, carry out many of these tasks. Most leukocytes are produced by hematopoietic stem cells in the bone marrow. Phagocytes, which literally means "eating cells," are a key group of these leukocytes. They work by engulfing pathogens through a process called phagocytosis, where they wrap their plasma membrane around the particle.

Other important cells include mast cells, which reside in connective tissues and mucous membranes. When activated, mast cells release granules rich in histamine and heparin. Histamine is particularly important because it dilates blood vessels to recruit more immune cells to the area. Natural killer cells and eosinophils are also part of this innate group.

In recent years, scientists have focused on how these barriers function through the epithelial barrier theory. Proposed by immunologist Cezmi Akdis in the early 2020s, this theory suggests that environmental toxins can impair epithelial junctions. Substances like air pollutants, microplastics, and detergents may increase permeability, allowing antigens to penetrate underlying tissues. This impairment can lead to microbial dysbiosis, which is an alteration in the balance of the body's microbiota. This theory helps explain the rising rates of chronic inflammatory diseases in industrialized societies. It builds upon earlier frameworks like the hygiene hypothesis and recent microbiome research.

Understanding barrier dysfunction is critical because it connects to many different health conditions. When epithelial barriers are damaged, it can contribute to allergic diseases such as asthma and atopic dermatitis. It has also been linked to autoimmune and metabolic disorders, including type 1 diabetes and multiple sclerosis. Even preliminary research suggests possible connections to neuropsychiatric conditions like Alzheimer's disease. While some critics argue that many of these links are currently only correlative, the study of how the innate immune system and its barriers interact remains a major field of medical research.