Comprehensive Study Guide on Non-Specific (Innate) Immunity

Fundamentals of Host Defense and the Two-Pronged Immune Response

Immunity is defined as the complex set of mechanisms used to build an individual's immunity to recognize foreign substances while neutralizing, destroying, and metabolizing those substances without bringing harm to the human host. The immune system is characterized by a two-pronged response: innate immunity and adaptive immunity. Innate immunity, also known as non-specific, natural, or first-line defense, is present at birth and provides a rapid response within several hours of pathogen exposure. It does not involve specific recognition of a microbe and lacks a memory concept, acting as an early warning system that protects against any type of invading agent. Its primary effector cells are phagocytes, including neutrophils, macrophages, and dendritic cells, which provide a general response to a broad range of pathogens. In contrast, adaptive immunity, also called specific or acquired immunity, involves focused attacks on specific pathogens through lymphocytes, specifically B and T cells. This system is slower to respond, taking several days, but it possesses immune memory that allows for a rapid and common postexposure immunity reaction. Responses in adaptive immunity include the production of antibodies by plasma cells and the action of cytotoxic T cells to clear specific infections.

First Line of Defense: Physical and Chemical Barriers to Infection

The first line of defense consists of physical and chemical barriers that prevent pathogen invasion and deny access to internal tissues. The skin, a cutaneous membrane composed of the dermis and epidermis, serves as a primary physical barrier. The top layer of the epidermis consists of tightly linked dead cells filled with keratin, creating a dry, unfavorable environment that is constantly shed and remains impermeable unless moist or damaged. Mucous membranes provide a less protective but essential thick, moist epithelium containing goblet cells for mucus production. In the respiratory tract, the ciliary escalator utilizes ciliated epithelial cells that beat in a wave motion to clear microbes trapped in mucus away from the lungs. Other physical mechanisms of defense include the lacrimal apparatus, where the lacrimal gland and canals produce constant tears that wash microbes off the eyes. Salivary glands produce amylase, and the gastrointestinal tract utilizes rapid contractions resulting in vomiting and diarrhea to flush out microbes and toxins.

Chemical factors further enhance these physical barriers. Mucus in the nose and nasopharyngeal regions is composed of glycoproteins and water, creating a thick substance that inhibits colonization. Sebum, an oily substance from sebaceous glands, contains fatty acids and lactic acids that maintain the skin pH between $3$ and $5$, inhibiting microbial growth. Perspiration consists of water and salts, where the water flushes microbes and the high salt concentration inhibits growth. Lysozyme is an enzyme found in tears, saliva, nasal secretions, and perspiration that breaks down peptidoglycan cell walls of bacteria. Gastric juice in the stomach, composed of $HCl$, enzymes, and mucus, maintains an acidic pH between $1$ and $3$, which destroys most bacteria and toxins. Additionally, the normal microbiota provides microbial antagonism by competing for nutrients and space, altering conditions such as pH and oxygen levels, and producing toxins like bacteriocins. Specific examples include Staphylococcus epidermidis on the skin and Escherichia coli in the intestinal tract.

Cells and Organs of the Immune System

The immune system structures include cells in the blood, specialized organs, the reticulum-endothelium system, and the lymphatic system. Blood is composed of approximately $60\%$ liquid plasma and $40\%$ formed elements, which include erythrocytes (red blood cells), platelets, and leukocytes (white blood cells). Platelets are short-lived fragments of large cells called megakaryocytes and are essential for blood clotting. All blood cells are derived from pluripotent stem cells in the bone marrow. These stem cells differentiate into two main lineages: myeloid stem cells and lymphoid stem cells. The myeloid lineage produces granulocytes (neutrophils, eosinophils, basophils, and mast cells) and monocytes. The lymphoid lineage produces lymphocytes, including B cells, T cells, and Natural Killer (NK) cells. Leukocytes are the primary cellular agents of immunity, further categorized into granulocytes (possessing granular cytoplasm and lobed nuclei) and agranulocytes (possessing round nuclei and no granular cytoplasm).

Human leukocytes are categorized by their percentages in the blood and their specific functions. Neutrophils, also known as polymorphonuclear neutrophilic leukocytes or PMNs, account for $55\%$ to $65\%$ of circulating leukocytes and function to phagocytize and digest engulfed materials. Eosinophils make up $2\%$ to $4\%$ and participate in inflammatory reactions and immunity to parasites. Basophils ($0\%$ to $1\%$) and mast cells release histamine and other inflammation-causing chemicals; while basophils circulate, mast cells are present in most tissues. Monocytes ($3\%$ to $8\%$) differentiate into macrophages or dendritic cells when they migrate into tissues. Macrophages are present in virtually all tissues and are given various names based on their location. Dendritic cells gather antigens from tissues and present them to lymphocytes. Lymphocytes ($25\%$ to $35\%$) include several types that participate in the adaptive immune response and congregate in secondary lymphoid organs like lymph nodes, the spleen, the thymus, the appendix, and tonsils.

The Phagocytosis Process and Phagocyte Response

Phagocytes are specialized cells that engulf and destroy pathogens, dead cells, and cellular debris. The process of phagocytosis involves four distinct steps: find, adhere, ingest, and digest. The initial step, chemotaxis, involves the movement of phagocytes toward chemical stimulants. Phagocytes in the tissues recognize invading microorganisms using pattern recognition receptors (PRRs) to identify pathogen-associated molecular patterns (PAMPs). Macrophages and dendritic cells can distinguish between gram-negative and gram-positive bacteria. Damaged tissues and infectious agents release chemical substances, while basophils and mast cells release histamine and phagocytes release cytokines to activate the inflammatory response. Chemokines are small signaling proteins that induce chemotaxis, such as CXCR1 and CXCR2 inducing the migration and degranulation of neutrophils.

Adherence and ingestion follow chemotaxis. Infectious agents attach to the plasma membrane of phagocytic cells, though some pathogens have defense mechanisms. For example, Streptococcus pneumoniae and Haemophilus influenzae possess antiphagocytic capsules that make adherence difficult, while Streptococcus pyogenes contains M protein to interfere with the process. Host-produced antibodies and complement proteins act as opsonins to help phagocytes bind. Once bound, pseudopodia surround the microbe and fuse to form a vacuole called a phagosome. During digestion, lysosomes containing digestive enzymes and small proteins called defensins fuse with the phagosome membrane to create a phagolysosome. Pathogens are broken down into small molecules like amino acids, sugars, and fatty acids. Phagocytes also use oxygen to form reactive species such as $H_2O_2$, $NO$, superoxide ions, and hypochlorite ions to damage the microbial plasma membrane. Indigestible material remains in a residual body and is eventually excreted via the plasma membrane. Some microbes, however, can multiply within phagocytes or prevent destruction within the phagolysosome.

Inflammation and Clinical Signs of Tissue Injury

Inflammation is a non-specific response to tissue injury, infection, mechanical injury, heat, UV exposure, chemicals, or allergies. It is characterized by five cardinal signs: redness (rubor/erythema), increased heat (calor), swelling (tumor/edema), pain (dolor), and loss of function. The process begins when cell damage triggers basophils and mast cells to release histamine and leukotrienes. Histamine causes vasodilation, which increases blood flow and leads to redness and heat. Leukotrienes increase the permeability of blood vessel walls, making them leaky. Phagocytes, including neutrophils and macrophages, migrate to the site by squeezing between endothelial cells in a process called diapedesis. This accumulation of fluid and cells causes edema or swelling. Blood also delivers clotting factors, such as fibrin, to wall off the injured area.

Pus is a white or yellow fluid that accumulates at the site of infection, consisting of dead phagocytes, damaged cells, ingested microorganisms, and tissue debris. While Streptococcus pyogenes produces leukocidins that destroy phagocytes and promote pus formation, viruses typically do not cause pus. An abscess is an accumulation of pus in a cavity hollowed out by tissue damage, such as boils or pimples. Pain during inflammation is associated with the release of bradykinin, a small peptide at the injury site. Macrophages also release cytokines like tumor necrosis factor alpha (TNF-$\alpha$), which further promotes vasodilation and edema. The systemic response to inflammation often includes leukocytosis, where the number of leukocytes in the blood increases to coordinate the defense.

Fever and Endogenous and Exogenous Pyrogens

Fever is a systemic increase in body temperature, usually triggered by microbial substances or cytokines from activated phagocytes that reset the thermostat in the hypothalamus. The normal body temperature is set at $37^\circ\text{C}$ ($98.6^\circ\text{F}$). A fever is generally defined as an oral temperature of $37.8^\circ\text{C}$ ($100.5^\circ\text{F}$) or a rectal temperature of $38.4^\circ\text{C}$ ($101.5^\circ\text{F}$); temperatures exceeding $43^\circ\text{C}$ can result in death. Pyrogens are the substances that induce fever. Endogenous pyrogens include proinflammatory cytokines such as IL-1, IL-6, and TNF-$\alpha$, which stimulate the hypothalamus to produce prostaglandins. Exogenous pyrogens, such as gram-negative endotoxins (LPS), cause phagocytes to release IL-1. This circulates to the blood-brain barrier where it activates neurons to secrete prostaglandins, resetting the hypothalamic thermostat to a higher temperature within approximately $20$ minutes. This lead to physiological responses including shivering and an increased metabolic rate to reach the new temperature set point.

Molecular Defenses: Interferons and Natural Killer Cells

Molecular defenses against viruses include interferons and Natural Killer (NK) cells. Interferons are small, soluble proteins responsible for viral interference, preventing virion replication in other cells. There are three groups: Alpha ($\alpha$), Beta ($\beta$), and Gamma ($\gamma$). IFN-$\beta$ is produced by virus-infected cells and triggers neighboring cells to produce antiviral proteins (AVPs) that block RNA virus replication. IFN-$\alpha$ is produced by leukocytes, triggers AVP production in body cells, and activates NK cells. IFN-$\gamma$ is produced by lymphocytes and NK cells, activating macrophages and neutrophils for phagocytosis while also blocking viral replication through AVP synthesis. Natural Killer cells are cytotoxic lymphocytes that engage infected or diseased cells. They contain granules with perforin, which forms pores in the target cell membrane, and granzymes (proteases), which enter the target cell and induce death. These cells recognize targets based on the levels of major histocompatibility complex (MHC) proteins on the cell surface.

The Complement System and Pathways of Activation

The complement system consists of a group of sequentially interacting proteins produced in the liver that boost the efficiency of both innate and adaptive immune responses. The three main outcomes of complement activation are opsonization (enhanced phagocytosis), inflammation, and the lysis of invading cells through the Membrane Attack Complex (MAC). There are three pathways of activation: the Classical, Alternative, and Lectin pathways. The Classical pathway is initiated when antibodies (IgG) bind to surface antigens on microbes, which then fix complement protein C1, followed by C4 and C2. The Alternative pathway is a non-specific mechanism activated by contact between complement proteins (Factor B, Factor D, Factor P) and lipid-polysaccharides on the pathogen surface. This pathway is activated earlier than the classical pathway, with C3 being a key protein. The Lectin pathway is triggered when macrophages ingest pathogens, stimulating the liver to produce lectins like mannose-binding lectin (MBL), which binds to mannose-containing polysaccharides on bacterial cell walls.

Complement fixation results in several antimicrobial effects. Opsonization occurs when C3b and antibodies act as opsonins to facilitate recognition by phagocytes. The Membrane Attack Complex (MAC) is formed when the C5b-7 complex recruits C8 and C9, creating a large C5b-9 unit that perforates the pathogenic cell membrane with pores. This allows extracellular fluids to rush in, leading to cell lysis. Dozens to hundreds of MACs may perforate a single cell. Finally, inflammation is enhanced as C3a and C5a trigger histamine release from mast cells, increasing vascular permeability and promoting chemotaxis of neutrophils toward the site of infection.