Immunological Memory and T-Helper Subsets

Comprehensive vocabulary flashcards covering immunological memory, memory T-cell and B-cell subsets, T-helper polarizations (Type 1, Type 2, Type 3 immunity), receptor-cytokine signaling pathways, and innate lymphoid cells from BIOS 355 lecture notes.

Immunological Memory

A key characteristic of the adaptive immune system in which a fraction of antigen-specific B-cells and T-cells are retained long term following pathogen elimination, providing protection that can last decades (such as CD4 and CD8 T-cells detected $75$ years after initial exposure) and serving as the basis for vaccination.

Linear Model of Memory T-cell Origin

A developmental model proposing that memory T-cells arise directly from differentiated effector T-cells.

Branching (Asymmetric Division) Model

A developmental model proposing that upon naive T-cell activation, asymmetric cell division generates daughter cells that diverge to become either memory T-cells or effector T-cells.

Central Memory T-cells ($T_{CM}$)

A memory T-cell subset that expresses L-selectin (CD62L) and CCR7, allowing it to recirculate between the blood, secondary lymphoid organs (T-cell zones), and lymph, similar to naive T-cells.

Effector Memory T-cells ($T_{EM}$)

A memory T-cell subset expressing $\beta_1$ and $\beta_2$ integrins that either remains in the circulation or recirculates between tissues, lymph, lymph nodes, and blood to access inflamed peripheral sites.

Tissue-Resident Memory T-cells ($T_{RM}$)

A memory T-cell subset permanently confined to a single peripheral tissue frequently exposed to pathogens (such as the skin or GI tract); its migration is directed by chemokine receptors (CCR, CXCR) and retention is maintained by adhesion proteins induced by epithelial cytokines.

Interleukin-7 (IL-7)

The principal cytokine survival signal required for naive and memory T-cells to persist for months or years; synthesized by epithelial cells, endothelial cells, fibroblastic reticular cells in the T-cell zone, keratinocytes, and bone marrow stromal cells.

Clonal Contraction

The apoptotic death of effector T-cells following pathogen clearance caused by the downregulation of IL-7 receptor (IL-7R) expression, which deprives them of the essential IL-7 survival signal.

Memory T-cell Metabolism

A metabolic state characterized by lower energy demand than effector T-cells, utilizing fatty acid oxidation while maintaining high spare respiratory capacity for oxidative phosphorylation.

Memory B-cells (MBCs)

Long-lived, activated B-cells that have already undergone class switching to express IgG, IgA, or IgE; they circulate through the blood until establishing residence in the spleen and lymph nodes to detect secondary antigen exposure.

Follicular Helper T-cells ($T_{FH}$)

CD4 T-cells that express CXCR5 to migrate toward CXCL13 gradients produced by follicular dendritic cells, residing in the follicular zone to guide B-cell activation, class switching, and memory formation.

Homeostatic Chemokines

Chemokines that are constitutively synthesized within lymphoid tissues (such as CXCL13 produced by follicular zone dendritic cells) to orchestrate baseline immune cell migration.

Inflammatory Chemokines

Chemokines synthesized inducibly by immune and non-immune cells in direct response to infection or tissue inflammation to recruit immune effector cells to the affected site.

Type 1 Immunity

The branch of adaptive immunity specialized in eliminating intracellular pathogens (such as Salmonella, Leishmania, Mycobacterium, and Candida); mediated primarily by $T_H1$ cells and M1 macrophages.

M1 Macrophages

Classically activated macrophages stimulated by $T_H1$ cells via two required signals—contact co-stimulation through CD40-CD40L and the soluble mediator IFN$\gamma$—to enhance intracellular phagolysosomal killing.

Granuloma

An organized immunological structure consisting of persistent $T_H1$ cells and M1 macrophages surrounding a central core of bacteria and apoptotic cells; formed during infections like Mycobacterium tuberculosis to prevent systemic pathogen dissemination, resulting in chronic low-level inflammation.

Type 2 Immunity

The branch of adaptive immunity directed against large extracellular parasites that cannot be phagocytosed (such as helminths ranging from $1\,\text{mm}$ to $> 1\,\text{m}$); mediated by $T_H2$ cells, M2 macrophages, eosinophils, and mast cells.

M2 Macrophages

Alternatively activated macrophages stimulated by $T_H2$ responses that function in helminth defense and promote tissue repair, remodeling, and homeostasis.

Type 3 Immunity

The branch of adaptive immunity targeting extracellular bacteria and fungi; driven primarily by $T_H17$ cells and neutrophils.

$T_H17$ Cells

The unique CD4 T-cell subset defined by the exclusive ability to synthesize IL-17; their effector actions include recruiting neutrophils, inducing antimicrobial peptide synthesis in epithelial cells, and increasing epithelial turnover.

IL-12p40 Subunit

A shared cytokine protein subunit common to both heterodimeric inflammatory cytokines IL-12 (paired with p35) and IL-23 (paired with p19).

IL-12 Signaling Axis

The pathway wherein IL-12 ($\text{p35}+\text{p40}$) engages the IL-12R$\beta$1 and IL-12R$\beta$2 receptor heterodimer to phosphorylate and activate STAT4, driving and maintaining $T_H1$ differentiation.

IL-23 Signaling Axis

The pathway wherein IL-23 ($\text{p19}+\text{p40}$) engages the IL-12R$\beta$1 and IL-23R receptor heterodimer to phosphorylate and activate STAT3, maintaining $T_H17$ cell effector responses.

Innate Lymphoid Cells (ILCs)

Lymphoid effector cells lacking T-cell receptors (TCRs) and surface immunoglobulins (Igs); grouped into ILC1, ILC2, and ILC3 in parallel to $T_H1$, $T_H2$, and $T_H17$ subsets, and activated primarily by cytokines synthesized by innate immune cells or pathogen recognition receptors (PRRs).