2.5.1 Innate Immunity

Overview of Innate Immunity 🧬

Innate immunity is the body’s built in, immediate defense system against infection. It is present from birth, reacts within minutes to hours, and does not require prior exposure to a microbe. Unlike adaptive immunity, it does not become stronger or more specific with repeated encounters and it does not create immunologic memory.

The innate immune system focuses on recognizing general patterns that are common to many microbes and damaged cells. These patterns are stable and essential for the microbe’s survival, so they do not change easily. Because of this, the innate system can respond rapidly but in a relatively fixed way.

Innate immunity works as a coordinated network of physical and chemical barriers, soluble molecules in body fluids, and specialized cells. Together they prevent pathogens from entering, detect those that do enter, and attempt to destroy them or limit their spread.

Physical and Chemical Barriers 🛡️

The first level of innate defense is formed by barriers that block entry of pathogens into the body. These are not simply walls. They are active, living surfaces with constant turnover and secretions that make infection more difficult.

The skin provides a tough physical shield. Its outermost layer is made of keratinized cells that are tightly connected, making it difficult for microbes to pass through intact skin. The skin also has an acidic pH and produces antimicrobial substances, such as defensins, that damage microbial membranes. Normal skin flora compete with pathogens for space and nutrients, which further protects against invasion.

Mucosal surfaces in the respiratory, gastrointestinal, and genitourinary tracts are lined by epithelial cells that form tight junctions. In the respiratory tract, cilia and mucus trap and move particles upward to be swallowed or expelled. In the gastrointestinal tract, gastric acid creates a low pH environment that kills many ingested microbes. Digestive enzymes and bile salts also have antimicrobial effects. In the genitourinary tract, urine flow mechanically flushes microbes, and vaginal flora plus low pH limit pathogen growth.

Chemical barriers include lysozyme in tears, saliva, and mucus, which breaks down bacterial cell walls, particularly in gram positive bacteria. Sebum and fatty acids on the skin surface inhibit bacterial growth. Defensins, short cationic peptides produced by epithelial cells and neutrophils, insert into microbial membranes and create pores that lead to lysis.

Innate barriers are not perfect, but they significantly reduce the number of microbes that reach underlying tissues and delay infection. When these barriers are disrupted, through burns, skin cuts, indwelling catheters, or changes in gastric pH, susceptibility to infection rises sharply.

Pattern Recognition and PAMPs 🧩

Innate immunity relies on a system of pattern recognition. Host cells express pattern recognition receptors, or PRRs, that detect conserved molecular structures on microbes called pathogen associated molecular patterns, or PAMPs. These patterns are not found on normal host cells, which allows discrimination between self and non self at a basic level.

PAMPs include structures such as bacterial lipopolysaccharide on gram negative bacteria, lipoteichoic acid on gram positive bacteria, flagellin in bacterial flagella, unmethylated CpG DNA motifs in bacterial and viral genomes, and double stranded RNA or single stranded RNA with specific features in viruses. Because these molecules are vital for microbial survival, they do not mutate easily, which makes them reliable targets.

In addition to PAMPs, innate receptors also detect damage associated molecular patterns, or DAMPs. These are molecules released or exposed by stressed, injured, or necrotic host cells, for example extracellular ATP, uric acid crystals, high mobility group box 1 protein, and nuclear or mitochondrial components that appear where they should not. Recognition of DAMPs helps the innate system respond not only to infection but also to sterile tissue injury.

Pattern recognition receptors are located in different parts of the cell. Some are on the cell surface, some in endosomes, and others in the cytoplasm. This arrangement allows them to sense microbes in the extracellular environment, in phagosomes, or inside the cytosol. When PRRs bind their ligands, they trigger intracellular signaling cascades that lead to inflammatory cytokine production, expression of costimulatory molecules, and other responses that will be important for both innate and later adaptive immunity.

Toll Like Receptors and Other PRRs 🔔

Among the best studied pattern recognition receptors are the toll like receptors, or TLRs. These are transmembrane proteins that are expressed on many innate immune cells such as macrophages, dendritic cells, neutrophils, as well as on some epithelial and endothelial cells. Each TLR type recognizes a different category of PAMP.

Surface TLRs usually detect extracellular or cell wall associated microbial components. For instance, one TLR recognizes bacterial lipopolysaccharide, another recognizes lipoteichoic acid from gram positive cell walls, and others can recognize flagellin or components of fungal cell walls. Endosomal TLRs recognize microbial nucleic acids that are encountered when microbes are internalized. Examples include double stranded RNA, single stranded viral RNA, and CpG rich DNA.

Engagement of a TLR leads to activation of adaptor proteins inside the cell that stimulate transcription factors such as NF κB and IRFs. These factors move to the nucleus and promote transcription of inflammatory cytokines like TNF α, IL 1, and IL 6, as well as type I interferons, mainly IFN α and IFN β, which are key antiviral mediators. TLR signaling also increases expression of molecules needed to activate T lymphocytes later.

Toll like receptors are not the only PRRs. NOD like receptors, or NLRs, are cytosolic receptors that detect bacterial cell wall fragments or other danger signals inside the cell. Some NLRs participate in forming an inflammasome, a multiprotein complex. When activated, the inflammasome leads to activation of caspase 1, which then processes pro IL 1β into its active form. IL 1β is a major inflammatory cytokine that contributes to fever and leukocyte recruitment.

RIG I like receptors, or RLRs, are another group of cytosolic sensors that detect viral RNA in the cytoplasm. Their activation results in production of type I interferons and other antiviral molecules. C type lectin receptors on cell surfaces recognize specific carbohydrate patterns such as mannose or β glucans on fungi and some bacteria. These receptors help coordinate antifungal responses and phagocytosis.

Although the molecular details can seem complex, for USMLE purposes it is important to keep in mind that different PRR families occupy different locations, sense distinct microbial structures, and converge on a limited number of transcriptional pathways that promote inflammation and antiviral defense.

Cells of the Innate Immune System 🧫

Cells that participate in innate immunity act rapidly and often at the site of infection. These cells do not require prior antigen sensitization, and they respond in a similar way each time they encounter a given stimulus.

Neutrophils are abundant circulating granulocytes that are often the first leukocytes to arrive at a site of acute bacterial infection. They are short lived cells that move from blood to tissues in response to chemotactic signals, such as IL 8 and complement fragments. Once in tissues, they phagocytose microbes and kill them using reactive oxygen species and granule enzymes.

Macrophages arise from circulating monocytes that migrate into tissues and differentiate. There are also resident macrophages in many organs, such as Kupffer cells in the liver and microglia in the brain. Macrophages ingest and destroy microbes and dead cells, release cytokines that coordinate inflammation, and produce growth factors that participate in tissue repair. They also bridge innate and adaptive immunity through antigen presentation, which will be covered elsewhere.

Dendritic cells are present in peripheral tissues and act as sentinels. They capture antigens and sense pathogens through PRRs. While they do have innate effector functions, their primary importance lies in processing antigen and migrating to lymphoid organs, where they present antigen to naive T cells and initiate adaptive responses.

Natural killer, or NK, cells are innate lymphoid cells that recognize and kill virus infected cells and some tumor cells. They do not express T cell receptors or antibodies, and they do not require antigen presentation. Instead, they integrate signals from activating and inhibitory receptors. The most important inhibitory signal comes from recognition of self MHC class I molecules on target cells. When MHC class I is reduced or lost, as often occurs in virus infected or malignant cells, this inhibitory input is removed and NK cells are more likely to kill the target.

A key functional concept is that NK cells respond to a "missing self" pattern when MHC I expression is low or absent. They also respond to stress induced ligands on target cells. NK cells kill targets through release of perforin and granzymes that induce apoptosis, and they secrete IFN γ, which activates macrophages and shapes adaptive immune responses.

Other innate cells include mast cells and basophils, which are especially active in responses to parasites and in allergic conditions, and eosinophils, which attack helminths and participate in certain hypersensitivity reactions. While these cell types are prominent for their roles in allergy and parasitic infection, they also contribute to innate defense by releasing mediators that influence blood vessels, smooth muscle, and surrounding immune cells.

Complement System Basics 💧

The complement system is a group of plasma proteins that circulate mostly as inactive precursors and become activated in a cascade. Complement is part of innate immunity, although it can be engaged by antibodies from the adaptive system. Its main functions are to mark microbes for phagocytosis, directly damage microbial membranes, and promote inflammation.

Complement can be activated by three main pathways. The classical pathway is triggered when certain antibodies bound to antigens interact with the first complement component. This connects adaptive antibody responses with innate complement activity. The lectin pathway begins when mannose binding lectin, an innate pattern recognition molecule, binds to mannose residues on microbial surfaces. The alternative pathway is activated spontaneously at a low level when complement proteins undergo hydrolysis and bind to microbial surfaces that lack regulatory proteins. For the purpose of understanding innate immunity, the lectin and alternative pathways are especially relevant because they do not require antibodies.

All three pathways converge at the generation of C3 convertase, an enzyme complex that cleaves complement component C3 into C3a and C3b. C3b binds to microbial surfaces and acts as an opsonin. Opsonization improves phagocytosis, because phagocytic cells express complement receptors that recognize bound C3b. C3a and the related fragment C5a function as anaphylatoxins. They increase vascular permeability, cause smooth muscle contraction, and act as chemoattractants that recruit neutrophils and other leukocytes.

As the cascade proceeds, complement components form the membrane attack complex, or MAC, composed mainly of C5b, C6, C7, C8, and multiple C9 molecules. The MAC inserts into the lipid bilayer of certain microbes, particularly gram negative bacteria, and forms pores that lead to osmotic lysis.

Because complement is powerful, it is tightly regulated by host proteins that prevent excessive activation on self cells. Defects in these regulatory factors or in certain complement components can lead to recurrent infections or autoimmune hemolysis, topics that are addressed in pathology and hematology.

Cytokines and Interferons in Innate Responses 📡

Innate immune cells communicate through soluble mediators known as cytokines. These small proteins are produced rapidly in response to PRR signaling and other stimuli. They act locally and systemically to coordinate inflammation, recruit cells, and induce antiviral states.

Proinflammatory cytokines such as TNF α, IL 1, and IL 6 are produced mainly by activated macrophages and other innate cells. They promote expression of adhesion molecules on endothelial cells, increase vascular permeability, and help recruit neutrophils to sites of infection. Systemically, they act on the hypothalamus to induce fever, on the liver to stimulate production of acute phase proteins, and on the bone marrow to increase leukocyte production.

Chemokines are a subgroup of cytokines that direct cell movement. For instance, IL 8 attracts neutrophils to sites of tissue damage or infection. These gradients are essential for efficient migration of leukocytes from the bloodstream into tissues.

Type I interferons, primarily IFN α and IFN β, are central to innate antiviral defense. They are produced by virus infected cells and by plasmacytoid dendritic cells upon sensing viral nucleic acids via PRRs such as TLRs and RIG I like receptors. Type I interferons bind to receptors on neighboring cells and induce expression of antiviral proteins that inhibit viral replication, degrade viral RNA, and enhance antigen presentation.

Innate cytokines also influence the development and direction of adaptive immune responses. For example, IL 12 from macrophages and dendritic cells promotes differentiation of certain T helper cell subsets and stimulates NK cells to produce IFN γ. In this way, innate signals at the time of infection shape the quality of the later adaptive response.

Phagocytosis and Microbial Killing 🍽️

Phagocytosis is a central effector mechanism of innate immunity. It involves recognition, ingestion, and destruction of microbes by phagocytes, primarily neutrophils and macrophages. Although antigen presentation is important later, for innate purposes the focus is the immediate elimination of pathogens.

The process begins with recognition of targets. Phagocytes express receptors for PAMPs, such as mannose receptors and scavenger receptors, as well as receptors for opsonins, including complement receptors for C3b and Fc receptors for the constant region of certain antibodies. Binding through multiple receptors triggers actin rearrangement and engulfment of the particle into a membrane bound vesicle called a phagosome.

The phagosome then fuses with lysosomes to form a phagolysosome, where acidic pH, hydrolytic enzymes, and antimicrobial peptides attack the microbe. In addition, phagocytes activate an oxidative burst, which involves the NADPH oxidase complex. This enzyme converts oxygen to superoxide, which is then converted to hydrogen peroxide and other reactive oxygen species. These reactive molecules damage microbial proteins, lipids, and nucleic acids.

In some phagocytes, especially macrophages, inducible nitric oxide synthase generates nitric oxide from arginine. Nitric oxide and its derivatives further contribute to microbial killing. The combined effects of low pH, degradative enzymes, reactive oxygen species, and reactive nitrogen species make the phagolysosome highly hostile to most pathogens.

Certain microbes have evolved mechanisms to resist or evade phagocytic killing, for example by inhibiting phagosome lysosome fusion or surviving inside the hostile environment. These specialized adaptations are important in the pathogenesis of specific infections and are considered in microbiology and pathology.

Inflammation as an Innate Response 🔥

Inflammation is a coordinated local tissue response that arises when innate immune sensors detect infection or damage. It is characterized clinically by redness, heat, swelling, pain, and sometimes loss of function, which correspond to increased blood flow, vascular permeability, and accumulation of leukocytes and fluid.

Innate cells such as macrophages and mast cells release mediators when they recognize PAMPs or DAMPs. Histamine from mast cells causes rapid vasodilation and increased vascular permeability. Prostaglandins and leukotrienes derived from arachidonic acid contribute to vasodilation, vascular leak, and chemotaxis. Cytokines like TNF α and IL 1 upregulate adhesion molecules on nearby endothelium so that leukocytes can slow, adhere, and migrate out of the bloodstream into tissue.

Complement fragments such as C3a and C5a act as anaphylatoxins and chemoattractants. Together, these mediators cause endothelial cells to retract, which allows plasma proteins, including antibodies and complement, to enter the tissue space. Chemokine gradients guide neutrophils first, then monocytes and other cells, to the site of injury or infection.

Inflammation aims to contain and remove the initial cause of tissue damage, clear out dead cells and debris, and initiate repair. When successful and self limited, it is highly beneficial. However, uncontrolled or chronic inflammation can cause tissue damage and contribute to disease.

Innate immunity controls the magnitude and duration of inflammation mainly through feedback mechanisms. As pathogens are cleared and damage is reduced, the stimuli that triggered PRRs decrease, and anti inflammatory mediators such as IL 10 and TGF β gain influence. Understanding how innate signaling leads to inflammation and how that inflammation is terminated is important both for infection control and for preventing immunopathology.

Interaction with Adaptive Immunity 🔗

Although innate and adaptive immunity are conceptually distinct, they function as a single integrated defense system. Innate immunity responds first and creates the conditions that determine if and how adaptive responses will be activated.

Innate cells provide the initial detection of pathogens and generate cytokines and costimulatory signals that are essential for activation of T and B lymphocytes. Dendritic cells and macrophages that have recognized pathogens through PRRs process and present antigens to T cells. The pattern of PRRs engaged and the cytokines produced help determine which type of adaptive response will dominate, for example responses directed against intracellular pathogens or extracellular parasites.

Complement activation by innate pathways increases the efficiency of B cell responses by enhancing antigen uptake and presentation. At the same time, antibodies produced by activated B cells feed back into innate mechanisms by triggering the classical complement pathway and by acting as opsonins that improve phagocytosis.

Thus, while innate immunity can limit or clear many infections by itself, it is also indispensable for proper initiation and shaping of the adaptive response. Defects in innate recognition or signaling can impair both early defense and the later generation of specific, long term immunity.