Module 1: Organization of the Immune System

Vocabulary practice flashcards covering the history of immunology, key definitions, barrier defenses, hematopoiesis, innate and adaptive immune cell types, and lymphoid anatomy.

Thucydides

Greek historian who recorded in 430 BC during the plague of Athens that individuals who had recovered from the disease could safely nurse the sick because they would not contract it a second time.

Abū Bakr Muhammad ibn Zakariyyā al-Rāzī (Rhazes)

Persian physician who differentiated smallpox from measles and observed that exposure granted disease-specific protection rather than cross-protection, recognizing that the body learns from exposure.

Variolation

An early method of immunization where dried smallpox pustule crusts were blown into the nose or introduced into skin cuts; it caused death in $1\text{–}3\%$ of recipients compared to $30\text{–}50\%$ mortality from natural smallpox outbreaks.

Emperor Kangxi

Chinese emperor who survived smallpox, ascended the throne in 1661, and mandated widespread variolation throughout his empire, protecting his military and popularizing the practice across Asia.

Lady Mary Wortley Montagu

Wife of the British ambassador in Constantinople who observed variolation among native Turks, had the procedure performed on her son Edward, and established it as standard practice in England.

Cotton Mather and Dr. Zabdiel Boylston

Advocates who introduced variolation to the United States during the 1721 Boston smallpox epidemic, inoculating 248 people with 6 deaths ($<3\%$ mortality rate) despite severe public backlash.

Edward Jenner

English physician who in 1798 introduced vaccination by inoculating an 8-year-old boy with cowpox fluid from a pustule, demonstrating cross-protection against smallpox.

Benjamin Waterhouse

American physician who vaccinated his own children against smallpox and convinced President Thomas Jefferson to initiate public vaccination programs in 1806.

Ludwig Panum

Danish epidemiologist who observed on the Faroe Islands that individuals previously infected during a measles outbreak years earlier remained protected, demonstrating long-lived immunity.

Vaccine (Pasteur's definition)

An attenuated, weakened strain of a microorganism capable of conferring immunity without causing fatal disease, named from the Latin 'vacca' (cow) in honor of Edward Jenner's work.

Joseph Meister

A young boy bitten repeatedly by a rabid dog who in 1885 became the first human successfully treated by Louis Pasteur using a series of attenuated rabies virus preparations.

Emil Adolf von Behring and Shibasaburo Kitasato

Researchers who showed that immunization with diphtheria toxin prevented disease and discovered that protective immunity could be passively transferred from host to host via serum.

Paul Ehrlich

Immunologist who showed toxin resistance could be transferred via blood or breast milk, hypothesized the presence of toxin-blocking blood proteins, and coined the term 'antikorper' (antibodies).

Ilya Mechnikov

Russian microbiologist who demonstrated in 1883 that cells contribute to immunity by observing white blood cells (phagocytes) ingesting microorganisms, proposing cellular immunity as the primary effector.

Gamma Globulin (Immunoglobulin)

A serum fraction identified in the 1930s by Elvin Kabat containing soluble, active antibody molecules responsible for humoral immunity.

Humoral Immunity

Immune protection mediated by soluble active molecules, primarily antibodies, found within body fluids (historically called humors).

Merrill Chase

Scientist who rekindled interest in cell-mediated immunity in the 1940s by successfully transferring immunity against tuberculosis between guinea pigs using white blood cells.

Bruce Glick

Researcher at Ohio State University whose experiments with chickens identified two separate lineages of lymphocytes: thymus-derived T cells and bursa of Fabricius-derived B cells.

Jules Bordet

Pasteur Institute scientist who demonstrated around 1900 that nonpathogenic foreign substances, such as red blood cells from different species, could induce specific immune responses.

Karl Landsteiner

Scientist who demonstrated that injecting an animal with almost any non-self organic chemical could elicit the production of antibodies that bind specifically to that substance.

Clonal Selection Theory

Theory proposed by Burnet, Jerne, and Talmage stating that each lymphocyte develops a single distinct antigen receptor prior to antigen encounter; binding to this receptor induces proliferation into identical clones.

Pathogen

An infectious microorganism—categorized into viruses, bacteria, fungi, and parasites—that induces illness or disease in a host.

Pathogenesis

The biological process and mechanism through which a pathogen induces illness or disease in a host.

Commensal Infection

An established colonization and growth of a microorganism within a host that causes no disease or harm to the host ('eating at the same table').

Symbiotic Infection

An established microbial colonization within a host where no disease occurs and both the microbe and the host derive mutual benefit.

Antigen

Any substance, whether a whole organism, protein, cell, or inert chemical, that is recognized as 'non-self' and can elicit an immune response.

Epitope

The specific structural portion or site of an antigen that is directly recognized and bound by receptors of the adaptive immune system.

Innate Immunity

An evolutionarily primitive, built-in defense system consisting of physical barriers, chemical secretions, and fixed-specificity receptors that responds to invaders within minutes to hours.

Adaptive Immunity

An antigen-specific defense mechanism mediated by B and T lymphocytes that emerges $5\text{–}6\text{ days}$ after exposure, adapting during the response and generating immunological memory.

Cardinal Features of an Immune Response

The four hallmark localized symptoms of inflammation: heat (calor), swelling (tumor), pain (dolor), and redness (rubor).

Keratin

A tough, waterproofing protein filling the tiers of dead epithelial cells within the epidermis of the skin, forming a dry barrier that prevents microbial replication.

Sebum

A waxy oil secreted by cutaneous sebaceous glands that contains antimicrobial fatty acids to inhibit microbial growth on the skin surface.

Paneth Cells

Specialized mucosal epithelial cells situated in intestinal crypts between villi that secrete $\alpha$-defensins and $\beta$-defensins to control invading microbes and maintain normal commensal gut flora.

Lactoferrin

An antimicrobial protein present in mucosal secretions that restricts microbial growth by sequestering and depleting free environmental iron.

Hematopoiesis

The highly regulated developmental process occurring primarily in the adult bone marrow through which pluripotent hematopoietic stem cells generate all red and white blood cells.

Hematopoietic Stem Cell (HSC)

The multipotent tissue-specific stem cell located in the bone marrow defined by the capacity to self-renew and give rise to all erythroid, myeloid, and lymphoid cell lineages.

Common Myeloid Progenitor (CMP)

A committed progenitor cell in the bone marrow that gives rise to erythrocytes, megakaryocytes/platelets, granulocytes, monocytes, and macrophages of the innate immune system.

Common Lymphoid Progenitor (CLP)

A committed progenitor cell in the bone marrow that gives rise to T lymphocytes, B lymphocytes, natural killer (NK) cells, and innate lymphoid cells (ILCs).

Granulocytes (Polymorphonuclear Leukocytes)

White blood cells characterized by multi-lobed nuclei and cytotoxic, membrane-bound cytoplasmic granules that are exocytosed in response to microbial infection.

Neutrophils

The most prevalent circulating leukocytes ($50\%$ to $70\%$ in adult humans) with purple-staining multi-lobed nuclei and pink cytoplasm that eliminate bacteria and fungi via phagocytosis, degranulation, and NETs.

Neutrophil Extracellular Traps (NETs)

Extracellular web-like structures composed of expelled decondensed chromatin decorated with granular antimicrobial enzymes that capture and kill invading microorganisms.

Eosinophils

Granulocytes with bi-lobed nuclei, bright red-staining granules, and IgG Fc receptors that combat multicellular parasites like helminths and contribute to allergy and asthma via histamine release.

Basophils

Rare ($<1\%$) nonphagocytic circulating granulocytes containing large blue-staining granules and IgE Fc receptors that respond to helminth parasites and participate in allergic reactions.

Mast Cells

Tissue-resident granulocytes that leave circulation as undifferentiated cells, mature upon tissue entry, and serve as the tissue equivalents of basophils.

Monocytes

Agranular, mononuclear leukocytes that comprise $2\text{–}12\%$ of circulating white blood cells, capable of migrating into tissues to differentiate into macrophages or dendritic cells.

Macrophages

Activated, tissue-resident phagocytic cells derived from monocytes that engulf microbes, clear dead host cells, and secrete signaling cytokines to assist the adaptive response.

Dendritic Cells (DCs)

Rare ($<1\%$) professional antigen-presenting cells (pAPCs) with dynamic membranous extensions that capture antigens in periphery and present them to naive T cells.

Natural Killer (NK) Cells

Large granular lymphoid cells ($5\%$ to $10\%$ of peripheral blood lymphocytes) that express innate-like rapid cytotoxicity toward tumor, infected, or stressed cells while retaining adaptive proliferative capabilities.

Missing Self Hypothesis

An NK cell activation mechanism in which the absence or down-regulation of MHC class I molecules on abnormal/infected host cells eliminates normal inhibitory signals, triggering NK cytotoxicity.

Perforin and Granzymes

Granule-derived cytotoxic effectors; perforin polymerizes to generate transmembrane pores in the target membrane, through which granzyme serine proteases enter to trigger apoptosis.

Fas/FasL Pathway

A contact-dependent cell death mechanism where FasL on a killer cell engages target cell Fas, recruiting FADD and procaspase-8 to form active caspase-8, inducing apoptosis.

Caspases

A family of cysteine proteases involved in executing apoptosis that specifically cleave target protein substrates after an aspartic acid residue.

Plasma Cells

Terminally differentiated effector B lymphocytes specialized in synthesizing and secreting high levels of antigen-specific, soluble antibodies into circulation.

CD8+ T Cells

Cytotoxic T lymphocytes that utilize CD8 coreceptors and TCRs to recognize digested intracellular microbial peptide antigens presented on host MHC class I molecules.

CD4+ T Cells

Helper T lymphocytes that utilize CD4 coreceptors and TCRs to recognize digested extracellular microbial peptide antigens presented on host MHC class II molecules.

Afferent Lymphatics

Lymphatic vessels that collect fluid, microbes, antigens, and traveling cells from peripheral tissues and transport them into the draining lymph node.

Efferent Lymphatics

Lymphatic vessels that transport filtered lymph fluid and departing lymphocytes away from the lymph node toward venous circulation.

Chemokines

A specialized family of secreted chemotactic cytokines that form molecular concentration gradients to guide the directional migration and trafficking of immune cells.

Primary Lymphoid Organs

Anatomical sites where hematopoiesis and lymphocyte maturation take place, specifically the bone marrow (all lineages, B cell maturation) and the thymus (T cell maturation).

Negative Selection

A central tolerance mechanism in the bone marrow (for B cells) and thymus (for T cells) that deletes developing lymphocytes expressing self-reactive antigen receptors.

Positive Selection

A central tolerance screening process in the thymus cortex that selects for survival only those thymocytes whose TCRs can appropriately recognize host self-MHC molecules.

High Endothelial Venules (HEVs)

Specialized post-capillary venules in the paracortex of lymph nodes lined with plump endothelial cells through which naive blood-borne lymphocytes extravasate into lymphoid tissue.

Germinal Center

A microenvironment within secondary lymphoid follicles filled with proliferating B cells undergoing somatic hypermutation, class switch recombination, and affinity maturation.

Spleen

An encapsulated secondary lymphoid organ specialized for filtering blood-borne pathogens, consisting of red pulp for erythrocyte disposal and white pulp for B- and T-cell activation.

Mucosa-Associated Lymphoid Tissue (MALT)

Organized unencapsulated lymphoid aggregates in the gastrointestinal, respiratory, and urogenital tracts that mount immune responses to pathogens and maintain tolerance to commensals.