21.2 Barrier Defenses and the Innate Immune Response

Barrier Defenses and Innate Immunity

Your body's first line of defense against invaders is a multi-layered system of barriers and innate immunity. Physical barriers like skin and mucous membranes keep pathogens out, while chemical and microbiological barriers create hostile environments for microbes that do get in.

If pathogens breach these barriers, innate immunity kicks in. Cellular components like phagocytes engulf invaders, while humoral components like complement proteins enhance the immune response. This rapid, broad-spectrum defense buys time for the adaptive immune system to develop a targeted response.

Primary Barrier Defenses

The body uses three types of barriers working together to prevent pathogens from ever gaining a foothold.

Physical barriers prevent pathogens from entering the body:

• The skin provides a tough, largely impermeable barrier. Its outer layer of dead, keratinized cells is difficult for microbes to penetrate, and its slightly acidic surface pH inhibits bacterial growth.
• Mucous membranes line the respiratory, digestive, and urogenital tracts. They secrete sticky mucus that traps pathogens. In the respiratory tract, cilia then sweep that mucus (and the trapped microbes) upward toward the throat to be swallowed or expelled.

Chemical barriers create an inhospitable environment for pathogens:

• Lysozyme, an enzyme found in saliva, tears, and nasal secretions, breaks down peptidoglycan in bacterial cell walls.
• Acidic pH in the stomach (around pH 1.5–3.5) and on the skin surface (around pH 4–5.5) directly kills or inhibits many microbes.
• Defensins are antimicrobial peptides that disrupt bacterial membranes, found in skin and mucosal epithelial cells.

Microbiological barriers use the body's own resident microbes as defense:

• Normal flora (commensal bacteria) colonize surfaces like the skin and gut. They compete with pathogens for nutrients and attachment sites, making it harder for invaders to establish infection.
• Some normal flora also produce antimicrobial substances like bacteriocins and lactic acid that directly inhibit pathogen growth.

Components of Innate Immunity

Once a pathogen breaches the barriers, the innate immune system responds. It has both cellular and humoral (soluble) components, plus a recognition system that detects foreign invaders.

Cellular components directly interact with and destroy pathogens:

• Phagocytic cells engulf and digest pathogens. Neutrophils are the most abundant and arrive first at an infection site. Macrophages are larger, longer-lived phagocytes that also help activate the adaptive immune system.
• Natural killer (NK) cells don't phagocytize. Instead, they release cytotoxic granules (containing perforin and granzymes) that kill virus-infected cells and tumor cells. NK cells recognize the absence of normal "self" markers (MHC class I) on target cells.

Humoral components are soluble factors circulating in blood and tissue fluid:

• The complement system is a cascade of over 30 plasma proteins that enhance phagocytosis, promote inflammation, and can directly lyse pathogens. More detail on this below.
• Cytokines are signaling molecules that regulate immune cell activity and coordinate the overall response. Key families include interleukins and interferons.

Pattern recognition receptors (PRRs) are how innate immune cells detect invaders:

• Toll-like receptors (TLRs) sit on cell surfaces and endosomal membranes, while NOD-like receptors (NLRs) are found inside the cytoplasm.
• Both recognize pathogen-associated molecular patterns (PAMPs), which are conserved structures found on microbes but not on human cells. Examples include lipopolysaccharide (LPS) on gram-negative bacteria and peptidoglycan on gram-positive bacteria.
• When PRRs bind PAMPs, they trigger release of pro-inflammatory cytokines like IL-1 and TNF, launching the innate immune response. This recognition step is what starts the whole cascade.

Soluble Factors in Innate Immunity

Three major categories of soluble factors amplify and coordinate the innate response.

Complement proteins enhance immunity through three distinct mechanisms:

• Opsonization: C3b coats pathogen surfaces, making them easier for phagocytes to recognize and engulf.
• Chemotaxis: C5a acts as a chemoattractant, recruiting immune cells to the infection site.
• Lysis: C5b through C9 assemble into the membrane attack complex (MAC), which punches holes in pathogen membranes, killing them directly.

Cytokines are signaling molecules with specific roles:

• Interleukins (IL-1, IL-6) promote inflammation and activate immune cells.
• Interferons (IFN-α, IFN-β) induce an antiviral state in neighboring uninfected cells and activate NK cells. The name comes from their ability to "interfere" with viral replication.
• Tumor necrosis factor (TNF) enhances phagocytosis and can induce apoptosis in infected cells.

Acute phase proteins are produced by the liver in response to inflammatory cytokines (especially IL-6):

• C-reactive protein (CRP) opsonizes pathogens and activates the complement system. Clinically, elevated CRP levels in blood are used as a marker of inflammation.
• Hepcidin binds and sequesters iron, limiting the iron available for bacterial growth (many bacteria need iron to reproduce).

Inflammation and Pathogen Elimination

Inflammation is the innate immune system's coordinated tissue-level response. Here's how it unfolds step by step:

Step 1: Vasodilation and increased vascular permeability Mast cells and resident macrophages at the injury site release histamine and prostaglandins. These cause local blood vessels to dilate and become more permeable. This produces the classic signs of inflammation: redness, heat, swelling, and pain. The increased permeability allows immune cells and plasma proteins (including complement) to move from the blood into the infected tissue.

Step 2: Leukocyte extravasation Leukocytes (white blood cells) must leave the bloodstream to reach the infection. This happens in stages:

• Leukocytes slow down and roll along the vessel wall, loosely binding to selectins on the endothelium.
• Chemokines at the site activate integrins on the leukocyte surface, causing firm adhesion.
• The leukocyte then squeezes between endothelial cells (diapedesis) and migrates toward the infection, following a chemokine gradient.

Step 3: Recruitment of immune cells Chemokines like IL-8 and MCP-1 attract neutrophils and monocytes. Neutrophils arrive first and are the primary phagocytic responders. Monocytes arrive later and differentiate into macrophages at the tissue site, continuing and expanding the phagocytic response.

Step 4: Phagocytosis and pathogen destruction Phagocytic cells engulf pathogens into a phagosome, which fuses with a lysosome. Inside, the pathogen is destroyed by reactive oxygen species (the "respiratory burst") and lysosomal enzymes. Macrophages also process pathogen fragments and present antigens on MHC class II molecules to T cells, forming the critical bridge between innate and adaptive immunity.

Step 5: Resolution and tissue repair Anti-inflammatory cytokines like IL-10 downregulate the inflammatory response once the threat is controlled. Macrophages clear cellular debris and dead neutrophils, and release growth factors that promote tissue repair.

Effectiveness of Early Immune Responses

Speed and breadth are the innate system's greatest strengths:

• Innate immune mechanisms activate within minutes to hours of pathogen detection, far faster than the adaptive response (which takes days).
• Because PRRs recognize conserved PAMPs shared across many pathogen types, no prior exposure to a specific pathogen is needed. The system works against threats the body has never encountered before.

Limitations are real, though:

• The lack of specificity means inflammation can cause collateral damage to healthy tissues. In extreme cases, a runaway inflammatory response leads to sepsis, a life-threatening systemic inflammation.
• There's no immunological memory. Each new infection is treated as if it's the first time, with no faster or stronger response on re-exposure.
• Some pathogens have evolved evasion strategies. Staphylococcus aureus can resist phagocytosis, and HIV directly infects immune cells.

Priming the adaptive immune response is arguably the innate system's most important long-term function:

• Dendritic cells and macrophages act as antigen-presenting cells (APCs), displaying pathogen fragments to T cells.
• Cytokines produced during the innate response (IL-12, IFN-γ) help shape what type of adaptive response develops.
• By containing the pathogen early, innate immunity buys the 5–7 days the adaptive immune system needs to produce antigen-specific antibodies and memory cells for lasting protection.