The Immune System: An Overview

Comprehensive vocabulary flashcards covering the components, cell lineages, lymphoid organs, kinetics, and differences between innate and adaptive immunity from introductory immunology lectures.

Immune system

A network of cells, molecules, tissues, and organs that enable responsiveness against pathogens and maintain homeostasis.

Anatomic barriers

Physical structures, such as skin and mucosal epithelia, providing the first level of pathogen containment to prevent infection.

Self-limiting response

A key regulatory feature of an effective immune system that downregulates activity to restore homeostasis after an infectious agent is cleared.

Immunological homeostasis

The physiological steady state that the immune system maintains under healthy conditions and reestablishes following the clearance of a pathogen.

Self versus non-self discrimination

The fundamental capacity of the immune system to distinguish healthy host cells ("self") from foreign pathogens or diseased host tissues ("non-self").

Preformed effectors

Nonspecific and broadly specific innate molecules and cells already present in tissues that mediate immediate pathogen recognition and removal.

Pathogen-associated molecular patterns (PAMPs)

Conserved molecular structures shared by groups of related microbes that are recognized by preformed receptors of the innate immune system.

Early induced innate response

The intermediate phase of host defense occurring early between $4-96\text{ hours}$ after infection, characterized by effector cell recruitment, PAMP recognition, and inflammation.

Adaptive immune response initiation threshold

The late phase of host defense initiated at $>96\text{ hours}$ after infection when early innate defenses fail and antigen is transported to lymphoid organs.

Clonal expansion and differentiation

The proliferation of antigen-specific naive B and T cells and their maturation into effector cells following antigen recognition in secondary lymphoid organs.

Pluripotent hematopoietic stem cell

The undifferentiated precursor cell residing in the bone marrow that gives rise to all leukocytes of the myeloid and lymphoid lineages.

Common lymphoid progenitor (CLP)

A bone marrow progenitor cell derived from the pluripotent hematopoietic stem cell that gives rise to B cells, T cells, NK cells, and ILCs.

Common myeloid progenitor (CMP)

A bone marrow progenitor cell derived from the pluripotent hematopoietic stem cell that gives rise to granulocyte/macrophage progenitors and megakaryocyte/erythrocyte progenitors.

Granulocyte/macrophage progenitor

A myeloid lineage precursor cell in the bone marrow that differentiates into granulocytes, monocytes, and dendritic cell precursors.

Megakaryocyte/erythrocyte progenitor

A myeloid lineage precursor cell in the bone marrow that differentiates into megakaryocytes and erythroblasts.

Megakaryocyte

A large bone marrow cell derived from the megakaryocyte/erythrocyte progenitor that fragments to produce blood platelets.

Platelets

Anucleate cellular fragments circulating in the blood derived from bone marrow megakaryocytes that mediate blood clotting and wound repair.

Erythroblast

An immature nucleated red blood cell precursor in the bone marrow that develops into a mature erythrocyte.

Erythrocyte

A mature red blood cell in the circulation derived from bone marrow erythroblasts.

Granulocytes (Polymorphonuclear leukocytes)

A category of white blood cells containing multi-lobed nuclei and cytoplasmic granules, comprising neutrophils, eosinophils, and basophils.

Neutrophils

Abundant polymorphonuclear granulocytes in the blood that are rapidly recruited to sites of infection to phagocytose and eliminate pathogens.

Eosinophils

Polymorphonuclear granulocytes containing cytoplasmic granules specialized in defense against parasites and involvement in allergic inflammation.

Basophils

Circulating polymorphonuclear granulocytes whose granules contain histamine and other mediators released during inflammatory and allergic responses.

Monocytes

Circulating mononuclear myeloid leukocytes that enter peripheral tissues to differentiate into macrophages.

Macrophages

Tissue-resident phagocytic cells derived from circulating monocytes that engulf pathogens, remove cell debris, and trigger inflammation.

Mast cells

Tissue-resident cells developing from an unknown precursor in the blood that contain granules packed with mediators of acute inflammation and allergy.

Immature dendritic cells

Endocytic myeloid sentinels residing in the blood and peripheral nonlymphoid tissues that actively capture and process microbial antigens.

Mature dendritic cells

Antigen-presenting cells residing in lymph nodes that express high levels of costimulatory molecules to prime and activate naive T cells.

Natural killer (NK) cells

Effector lymphoid cells of the innate immune system that kill virus-infected or abnormal host cells and produce early cytokines.

Innate lymphoid cells (ILCs)

Non-antigen-specific lymphocytes residing primarily in tissues that provide rapid cytokine responses during early stages of infection.

NKT cells

A specialized subset of T cells expressing surface markers and functional properties of both natural killer cells and conventional T lymphocytes.

B lymphocytes (B cells)

Adaptive lymphocytes that develop in the bone marrow, express antigen receptors (BCRs), and differentiate into antibody-producing plasma cells.

T lymphocytes (T cells)

Adaptive lymphocytes that originate in the bone marrow and migrate to the thymus for maturation, mediating cell-mediated immune responses.

Plasma cells

Terminally differentiated effector B lymphocytes that synthesize and secrete large quantities of antibodies.

Primary lymphoid tissues

Anatomical sites of leukocyte development and maturation, specifically comprising the bone marrow and thymus.

Secondary lymphoid tissues

Anatomical sites of lymphocyte activation by antigens, comprising lymph nodes, the spleen, and mucosal-associated lymphoid tissues (MALT).

Bone marrow

A primary lymphoid tissue where hematopoietic stem cells undergo hematopoiesis and where B cells develop.

Thymus

A primary lymphoid organ located in the upper thorax to which immature T lymphocytes migrate from bone marrow for further maturation.

Lymph nodes

Encapsulated secondary lymphoid organs positioned along lymphatic vessels that filter lymph and provide microenvironments for lymphocyte activation.

Spleen

A secondary lymphoid organ located in the abdomen that monitors blood-borne antigens, removes old red blood cells, and activates lymphocytes.

Mucosal-associated lymphoid tissues (MALT)

Secondary lymphoid tissues distributed along mucosal linings, including Peyer's patches, the appendix, adenoids, and tonsils.

Peyer's patches

Organized aggregates of mucosal-associated lymphoid tissue situated in the wall of the small intestine.

Appendix

A secondary lymphoid organ attached to the cecum of the large intestine that forms part of the mucosal immune system.

Tonsils

Aggregated secondary lymphoid tissues located at the pharynx that sample ingested or inhaled antigens entering through the oral cavity.

Adenoids

Aggregated secondary lymphoid tissues located in the nasopharynx that sample airborne antigens entering through the nasal passages.

Thoracic duct

The largest lymphatic vessel in the body, which drains lymph from the lower body and left side of the torso into the left subclavian vein.

Lymphatics

An extensive anatomical network of thin vessels that collect extracellular lymph fluid and transport antigens and immune cells to lymph nodes.

Pattern-recognition receptors (PRRs)

Germline-encoded innate receptors that detect pathogen-associated molecular patterns (PAMPs) and host molecules released by damaged cells.

Somatic recombination

The genetic mechanism of gene segment rearrangement that produces the vast diversity of antigen-specific receptors on B and T lymphocytes.

Innate immune specificity

Specificity oriented toward molecular patterns shared by groups of related microbes (PAMPs) and molecules produced by damaged host cells.

Adaptive immune specificity

Specificity tailored to distinct epitopes on individual microbial and nonmicrobial antigens.

Innate immune diversity

Limited diversity determined by a fixed repertoire of germline-encoded pattern-recognition receptors.

Adaptive immune diversity

Extremely large diversity achieved through the somatic recombination of variable gene segments in developing lymphocytes.

Innate immune induction time

Rapid induction time, deploying defenses within minutes, hours, or a few days following exposure to a pathogen.

Adaptive immune induction time

Slow induction time, requiring several days ($>96\text{ hours}$) upon initial exposure before full effector responses emerge.

Innate immune mediators

Host components mediating innate defenses, including the skin, mucosal epithelia, and antimicrobial molecules.

Adaptive immune mediators

Host components mediating adaptive defenses, including intraepithelial lymphocytes and antibodies secreted at epithelial surfaces.

Dendritic cells as immunological bridge

The cellular connection between innate and adaptive immunity, linking innate pathogen recognition with the priming and activation of naive B and T cells.

Type I interferons (Type I IFNs)

Innate antiviral cytokines whose titers peak early at days $1-2$ post-infection to restrict viral replication prior to peak viral load and adaptive responses.

Virus-specific cytotoxic T lymphocytes (CTLs)

Adaptive effector cells that proliferate after day $4$ of a viral infection, peak around days $8-9$, and clear virus-infected host cells.

Primary immune response

The initial adaptive immune reaction following first exposure to an antigen, characterized by a lag phase and a modest peak antibody titer around $10^0\,\text{mg}\cdot\text{ml}^{-1}$.

Secondary immune response

The rapid and robust adaptive response upon re-encountering an antigen, lacking an extensive lag phase and yielding antibody titers up to $10^4\,\text{mg}\cdot\text{ml}^{-1}$.

Lag phase

The initial delay of approximately $4-5\text{ days}$ following first antigen exposure during which naive lymphocytes are primed prior to measurable serum antibody production.

Vaccines

Biological preparations designed to induce a primary immune response and immunological memory without causing clinical disease.

Germinal centers

Specialized microstructures within secondary lymphoid tissues where activated B cells interact with T cells, proliferate, and differentiate into plasma cells and memory B cells.