Introduction to the Immune System

Comprehensive vocabulary flashcards covering the introductory concepts of immunology, cellular and molecular mediators, innate versus adaptive immunity, antigen recognition, complement, cytokine signaling, and immunopathology.

Intracellular Pathogens

Microorganisms that must infect and replicate inside individual host cells, such as viruses and certain parasites like Plasmodium.

Extracellular Pathogens

Disease-causing organisms that divide and replicate outside host cells in extracellular fluids, blood, tissues, or body cavities, such as streptococci.

Phagocytes

Key mediator cells of the immune system that internalize, degrade, and destroy microorganisms and cellular debris.

B Lymphocytes (B Cells)

Adaptive immune cells that develop in the bone marrow and produce antibodies directed against extracellular pathogens.

Cytotoxic T Lymphocytes (CTLs)

Effector T cells ($\text{CD8}^+$) that recognize and kill virally infected host cells and certain tumor cells.

Helper T Cells ($\text{T}_{\text{H}}$ Cells)

T lymphocytes ($\text{CD4}^+$) that coordinate immune responses through direct cell-cell contacts and the release of cytokines.

Specificity and Memory

The two essential hallmark features of the adaptive immune system that allow it to mount faster, stronger, and targeted responses upon re-exposure to a given antigen.

Antigen

Any molecule recognized by antigen receptors on B cells (in intact form) or T cells (as peptide fragments displayed on MHC molecules).

Epitope

The specific restricted molecular shape or antigenic determinant on the surface of an antigen that is directly recognized and bound by an antibody.

Fab Region

The antigen-binding fragment of an antibody molecule comprising the variable regions that contain the specific antigen-binding sites.

Fc Region

The constant stem region of an antibody that binds to Fc receptors on phagocytic cells, functioning as an adapter linking pathogens to phagocytes.

Clonal Selection

The process by which an encounter with an antigen selectively stimulates only those pre-existing lymphocytes carrying matching receptors, driving their proliferation and differentiation into effector and memory cells.

Major Histocompatibility Complex (MHC)

Host cell-surface glycoproteins that bind peptide fragments of pathogen-derived antigens and display them for recognition by T cell receptors (TCRs).

Complement System

A group of soluble serum proteins that activate in an enzymatic cascade to mediate cytolysis via the membrane attack complex, recruitment of phagocytes, and opsonization of bacteria.

Membrane Attack Complex (MAC)

A multi-protein pore assembled on microbial surfaces following complement activation that causes direct cytolysis and osmotic destruction.

Opsonization

The immune mechanism of coating a microorganism with molecules such as complement component C3b or antibodies to dramatically enhance phagocyte binding and engulfment.

C-Reactive Protein (CRP)

An acute-phase reactant whose serum concentration rises rapidly during infection; it binds pneumococcal C-protein and promotes phagocytosis by acting as an opsonin.

Cytokines

A broad category of soluble protein and glycoprotein signaling molecules released by leukocytes and other cells to mediate cell-to-cell communication and direct immune responses.

Interferons (IFNs)

Cytokines that limit the spread of viral infections by signaling to uninfected neighboring cells to induce an antiviral state and inhibit viral replication.

$\text{T}_{\text{H}}1$ Cells

A subset of helper T lymphocytes that secretes IFN-$\gamma$ to activate mononuclear phagocytes and eliminate intracellular pathogens.

$\text{T}_{\text{H}}2$ Cells

A subset of helper T lymphocytes that signals to B cells, promoting their division, differentiation into plasma cells, and antibody secretion.

Regulatory T Cells (Tregs)

Specialized T cells that control and downregulate active immune responses to prevent tissue damage and suppress autoimmunity against self tissues.

Polymorphonuclear Granulocytes (PMNs)

A class of leukocytes characterized by multi-lobed nuclei and granules in their cytoplasm, encompassing neutrophils, eosinophils, and basophils.

Neutrophils

Short-lived phagocytic PMNs that comprise the majority of circulating blood leukocytes and rapidly mobilize to inflammatory sites.

Kupffer Cells

Resident tissue macrophages belonging to the mononuclear phagocyte lineage strategically stationed in the liver.

Microglial Cells

Specialized tissue macrophages of the mononuclear phagocyte lineage permanently resident within the brain.

Phagocytosis

The cellular ingestion of a microbe by pseudopodia to form an intracellular phagosome, followed by lysosomal fusion and enzymatic degradation.

Diapedesis

The multi-step process wherein an activated leukocyte adheres to venular endothelium, extends pseudopodia between endothelial cells, and crosses into tissue, after which the vessel wall reseals.

Chemotaxis

The directed movement of phagocytes along a concentration gradient of chemotactic mediators (such as chemokines and C5a) toward an inflammatory locus.

Toxoid

A chemically modified microbial toxin that has lost toxicity but retains its original epitopes, stimulating a protective primary antibody response and establishing immune memory.

Immunopathology

Damage or disease states caused by immune system malfunction, categorized into immunodeficiency, hypersensitivity, and autoimmunity.

Autoimmunity

An inappropriate immune failure where the body mounts an immune response targeting its own self-antigens and tissues.

Immunodeficiency

An ineffective or defective immune response leaving the host vulnerable to infections, which can be hereditary (primary) or acquired (secondary, such as HIV/AIDS).

Hypersensitivity

An overactive immune response disproportionate to the threat or directed against harmless environmental antigens, such as pollen in allergic rhinitis and asthma.

Monoclonal Antibodies (mAbs)

Immune-derived identical antibodies designed to identify and bind specific cancer antigens with high selectivity to guide other immune cells to destroy tumors.