Comprehensive Study Notes on Innate and Non-Specific Immunity

Fundamentals of Host Defense and the Two-Pronged Immune Response

Immunity is defined as the collection of mechanisms used by an individual to recognize foreign substances and subsequently neutralize, destroy, or metabolize them without causing harm to the host's own body. The human immune system operates through two primary branches: innate immunity and adaptive immunity. Innate immunity, also known as non-specific or natural immunity, is present at birth and provides a rapid response within several hours of pathogen exposure. It acts as an early warning system and provides a general response to a broad range of pathogens without involving specific recognition or immunological memory. The primary effector cells of innate immunity include phagocytes, such as neutrophils, macrophages, and dendritic cells. Physical and chemical barriers serve as the first line of defense, while cellular and molecular responses constitute the second line.

Adaptive immunity, or specific acquired immunity, involves a focused attack on specific pathogens mediated by lymphocytes, specifically B and T cells. Unlike the innate system, the adaptive response takes several days to develop but results in post-exposure immunity through the creation of memory cells. This system recognizes specific antigens or immunogens and reacts by producing specific proteins called antibodies from plasma cells or by using cytotoxic T cells to clear infections. Adaptive immunity can be categorized into passive forms, such as antibodies transferred via the placenta or antitoxin injections, and active forms, such as recovery from a disease or vaccination.

First Line of Defense: Physical Barriers to Pathogen Invasion

The human body employs several physical barriers to prevent the entry of microorganisms. The skin consists of the cutaneous membrane, comprising the dermis and the epidermis. The top layer of the epidermis is composed of dead cells filled with keratin and tightly linked together. This layer is dry, unfavorable for microbial growth, and constantly shed. It remains impermeable to microbes unless it is damaged or excessively moist. Mucous membranes provide an alternative physical barrier; these consist of thick, moist epithelium that is less protective than keratinized skin but contains goblet cells for mucus production.

Specialized structures further protect various organ systems. The ciliary escalator in the respiratory tract consists of ciliated epithelial cells that beat in a wave-like motion to move microbes trapped in mucus out of the airways. The lacrimal apparatus, including the lacrimal glands and canals, constant produce tears that wash microbes off the surface of the eye. Salivary glands produce saliva containing amylase, and the mechanical actions of vomiting and diarrhea involve rapid contractions of the gastrointestinal tract to flush out microbes and toxins. Tight junctions between cells and various regional physical barriers also play a critical role in preventing systemic invasion.

First Line of Defense: Chemical Factors and Normal Microbiota

Chemical factors complement physical barriers to inhibit colonization. Mucus found in the nose and nasopharyngeal regions is composed of glycoproteins and water, creating a thick substance that inhibits microbial attachment. Sebum, an oily secretion from sebaceous glands, contains fatty acids and lactic acids that maintain the skin's pH between $3$ and $5$. This acidic environment effectively inhibits the growth of many microbes. Perspiration consists of water and salts; the water flushes microbes off the skin while the salt accumulates to inhibit growth through osmotic pressure. Lysozyme is a critical enzyme found in most body secretions, including tears, saliva, nasal secretions, and perspiration, which functions by breaking down the peptidoglycan cell walls of bacteria. Gastric juice in the stomach, composed of hydrochloric acid ($HCl$), enzymes, and mucus, maintains a highly acidic pH between $1$ and $3$, which destroys most bacteria and toxins.

Normal microbiota, or the resident microbial population of the body, contribute to host defense through microbial antagonism. These beneficial microbes compete with pathogens for nutrients and space, alter environmental conditions such as pH and oxygen availability, and produce toxins like bacteriocins to inhibit competitors. For example, $Staphylococcus\,epidermidis$ is a normal resident of the human skin, and $Escherichia\,coli$ is a primary resident of the intestinal tract. These populations also assist in the digestion and absorption of food.

The Blood and Lymphatic Systems in Host Defense

The immune system relies heavily on the blood and lymphatic systems for the production and transport of defense cells. Blood consists of approximately $60\%$ liquid plasma and $40\%$ formed elements, which include erythrocytes (red blood cells), platelets, and leukocytes (white blood cells). All these cells are derived from pluripotent stem cells in the bone marrow. Platelets are short-lived fragments of large cells called megakaryocytes and are essential for blood clotting. Leukocytes are the primary agents of immunity and are divided into granulocytes (neutrophils, eosinophils, basophils) and agranulocytes (monocytes, lymphocytes).

The lymphatic system includes the lymphatic capillaries, vessels, nodes, and organs like the thymus and spleen. The process of draining excess fluids from the spaces between cells begins with lymphatic capillaries, which collect fluid and plasma proteins that have leaked from the blood. This fluid, once inside the capillaries, is called lymph. Lymph travels through vessels and nodes before returning to the blood via the right and left subclavian veins through the right lymphatic duct and thoracic duct. Lymph flow is primarily dependent on the contractions of skeletal muscles. Lymphoid organs like the tonsils, appendix, and Peyer's patches in the small intestine serve as sites for lymphocyte congregation and antigen presentation.

Diversity and Function of Human Leukocytes

Leukocytes are categorized based on their morphology and lineage. Granulocytes, derived from myeloid stem cells, possess granular cytoplasm and lobed, irregularly shaped nuclei. Neutrophils, also known as polymorphonuclear leukocytes (PMNs), make up $55\%-65\%$ of circulating white blood cells. They are highly phagocytic, respond quickly to tissue injury, and guard skin and mucous membranes. Eosinophils comprise $2\%-4\%$ of leukocytes; they increase during allergic reactions and parasitic worm infections and help detoxify foreign substances to turn off inflammatory reactions. Basophils ($0\%-1\%$) and mast cells (found in tissues) release histamine to initiate inflammatory and allergic reactions.

Agranulocytes lack visible cytoplasmic granules and have round nuclei. Monocytes ($3\%-8\%$) are mononuclear phagocytes that circulate in the blood before migrating into tissues to differentiate into macrophages or dendritic cells. Macrophages, or "big eaters," are professional phagocytes that also release chemical messengers to alert other immune cells and present antigens to T cells. They can be fixed (stationary in tissues) or wandering (circulating in blood). Dendritic cells (DC) have long membrane extensions and are abundant in the epidermis, mucous membranes, and lymph nodes, where they gather antigens and initiate adaptive defenses. Lymphocytes ($25\%-35\%$), including B cells, T cells, and Natural Killer (NK) cells, are derived from lymphoid stem cells and are primarily responsible for adaptive immunity.

Second Line of Defense: The Mechanism of Phagocytosis

Phagocytosis is a multi-step process used by specialized cells to find, adhere to, ingest, and digest microorganisms. The first step is chemotaxis, where phagocytes move toward chemical stimulants. These stimulants include microbial products known as pathogen-associated molecular patterns (PAMPs), which are recognized by pattern recognition receptors (PRRs) on the phagocyte. Damaged tissues and cells like basophils and mast cells release histamine and chemokines to attract phagocytes to the infection site. The second step is adherence, where the phagocyte's plasma membrane binds to molecules on the microbe's surface. Some bacteria, like $Streptococcus\,pneumoniae$ and $Haemophilus\,influenzae$, use capsules to resist adherence, while $Streptococcus\,pyogenes$ uses M protein for the same purpose. Host-produced antibodies or complement proteins can act as opsonins to facilitate binding.

Following adherence, the phagocyte extends pseudopodia to surround the microbe, fusing them to create a vacuole called a phagosome. This leads to the third step: digestion. Lysosomes containing digestive enzymes and small proteins called defensins fuse with the phagocyte membrane to form a phagolysosome. Inside, pathogens are broken into small molecules like amino acids and sugars. Phagocytes also utilize oxygen to generate toxic reactive species such as $H_2O_2$, $NO$, superoxide ions, and hypochlorite ions to damage the microbial membrane. Any indigestible material remains in a residual body, which is eventually transported to the plasma membrane and excreted from the cell.

Inflammation as a Non-Specific Response to Injury

Inflammation is a localized, non-specific response to tissue injury, infection, or mechanical damage (cuts, burns, UV exposure) designed to limit pathogen spread and stimulate repair. It is characterized by five cardinal signs: redness (rubor or erythema), heat (calor), swelling (tumor or edema), pain (dolor), and sometimes loss of function. The process begins when cell damage triggers basophils and mast cells to release mediators like histamine and leukotrienes. Histamine causes vasodilation, increasing blood flow to cause redness and heat. Leukotrienes increase capillary permeability, allowing fluid and cells to leak into tissues.

During the inflammatory response, neutrophils and macrophages migrate from the blood into the tissue through a process called diapedesis, where they squeeze between the endothelial cells of the vessel wall. These phagocytes remove microbes and debris. The accumulation of dead phagocytes, damaged cells, and digested microbes forms a white or yellow fluid called pus. Infections caused by $Streptococcus\,pyogenes$ often produce pus because they release leukocidins that destroy phagocytes. An abscess refers to the accumulation of pus within a hollowed-out tissue cavity, such as a boil or pimple. Pain is often associated with the release of the small peptide bradykinin at the injury site. Finally, the blood delivers clotting factors like fibrin to wall off the area while macrophages release cytokines and tumor necrosis factor alpha ($TNF-\alpha$) to further stimulate the response.

Fever and Systemic Innate Defenses

Fever is a systemic increase in body temperature, usually triggered by the hypothalamus, which acts as the body's thermostat. While normal body temperature is roughly $37^\circ C$ ($98.6^\circ F$), a fever is generally defined as an oral temperature of $37.8^\circ C$ ($100.5^\circ F$) or a rectal temperature of $38.4^\circ C$ ($101.5^\circ F$). Temperatures exceeding $43^\circ C$ typically result in death. Fever acts to mobilize defenses, accelerate repairs, and inhibit pathogen growth. It is triggered by substances called pyrogens. Endogenous pyrogens include proinflammatory cytokines such as $IL-1$, $IL-6$, and $TNF-\alpha$, which stimulate the hypothalamus to produce prostaglandins that reset the thermostat to a higher level.

Exogenous pyrogens come from outside the body, such as the lipopolysaccharide ($LPS$) endotoxin of Gram-negative bacteria. When phagocytes ingest these bacteria, they release $IL-1$, which circulates to the hypothalamus. This activates neurons to secrete prostaglandins, raising the body's temperature set point within approximately $20$ minutes. The body then increases its metabolic rate and initiates shivering to reach the new, higher temperature. Localized heating at infection sites, caused by the same cytokines in smaller amounts, also promotes healing and increases blood flow.

Molecular Defenses: Interferons and Natural Killer Cells

Interferons are small, soluble proteins responsible for viral interference, stopping the replication of viruses in neighborhood cells. There are three groups in humans: alpha ($\alpha$), beta ($\beta$), and gamma ($\gamma$). Interferon beta ($IFN\beta$) is produced by virus-infected cells and triggers uninfected neighbors to produce antiviral proteins (AVPs) that specifically block RNA virus replication. Interferon alpha ($IFN\alpha$) is produced by leukocytes; it also triggers AVP production and activates Natural Killer (NK) cells. Interferon gamma ($IFN\gamma$) is produced by lymphocytes and NK cells to activate macrophages and neutrophils for phagocytosis while also inhibiting viral synthesis.

Natural Killer (NK) cells are cytotoxic lymphocytes distinct from B and T cells. They identify infected or diseased cells, including those that have lost their Major Histocompatibility Complex ($MHC$) surface proteins. When an NK cell engages a target, it releases granules containing perforin and granzymes. Perforin binds to the target cell's membrane to form a pore, allowing granzymes (proteases) to enter and induce cell death. This provides an essential mechanism for destroying abnormal cells, such as those infected by viruses or those that have become cancerous.

The Complement System Pathways and Outcomes

The complement system is a group of sequentially interacting proteins produced by the liver that circulate in the blood and tissue fluids. Its activation enhances phagocytosis, induces inflammation, and causes the lysis of invading cells. There are three primary pathways of activation. The Classical Pathway is initiated when antibodies (e.g., $IgG$) bind to surface antigens on a microbe, allowing complement protein $C1$ to attach and trigger a cascade involving $C4$ and $C2$. This pathway relies on the adaptive immune response. The Alternative Pathway is a non-specific mechanism activated by contact between complement proteins (Factors $B$, $D$, and $P$) and lipid-polysaccharides (LPS) on the pathogen surface. It activates earlier than the classical pathway, with protein $C3$ serving as a key components. The Lectin Pathway is triggered when macrophages stimulate the liver to produce lectins, such as mannose-binding lectin ($MBL$), which bind to mannose-containing polysaccharides on bacterial cell walls.

All pathways lead to three critical antimicrobial outcomes. First, opsonization occurs when the complement protein $C3b$ (and sometimes antibodies) coats a bacterium, making it easier for phagocytes to recognize and engulf. Second, the Membrane Attack Complex ($MAC$) is formed when $C5b$ recruits proteins $C6$, $C7$, $C8$, and $C9$ to create a large $C5b-9$ unit. This complex forms a pore in the cytoplasmic membrane, causing extracellular fluids to rush in and lyse the cell. Third, inflammation is stimulated as fragments $C3a$ and $C5a$ trigger mast cells to release histamine and act as chemoattractants for neutrophils, increasing vascular permeability and accelerating the immune response.