Immunogenetics: Immune System Components

Flashcards covering the components of the immune system, including organs, cell lineages, cytokines, and histological features of lymphoid tissues.

CCR5 mutation

A genetic alteration that can block $HIV$ entry into cells, used as an example of how altering immunity can prevent or treat diseases.

Primary Lymphoid Organs

Sites where lymphocytes differentiate; includes the bone marrow (where B cells develop and T cell precursors develop) and the thymus (where T cell lymphocytes differentiate).

Secondary Lymphoid Organs

Sites where mature lymphocytes reside and immune responses are generated, including lymph nodes, spleen, mucosal immune system, skin, and adipose tissue.

Hematopoiesis Timeline (Embryogenesis)

Stem cells appear in the yolk sac in the $1^{st}$ month; blood cell production starts in the liver at the $6^{th}$ week; T-cell precursors populate the thymus at $7-8$ weeks; the liver is the major source until the $6^{th}$ month, when it shifts to bone marrow.

Endosteal niche

A location in the bone marrow close to the bone containing osteoblasts and quiescent hematopoietic stem cells ($HSCs$) near arterioles.

Vascular niche

A bone marrow location associated with sinusoidal endothelium where actively dividing $HSCs$ are located near sinusoids close to central veins.

$G-CSF$ (Granulocyte-Colony Stimulating Factor)

A peptide that induces the mobilization of stem cells from the bone marrow into the peripheral blood.

Cytokines

Peptides secreted into extracellular fluid that function as autocrines, paracrines, or endocrine hormones to mediate intercellular communication and promote or inhibit cell growth.

Myeloid lineage

One of the two main immune lineages which gives rise to granulocytes, monocytes, red blood cells ($RBCs$), and megakaryocytes.

Lymphoid lineage

One of the two main immune lineages which includes B and T lymphocytes, Natural Killer ($NK$) cells, and Innate Lymphoid Cells ($ILCs$).

Cluster of Differentiation ($CD$) markers

Specific cell surface molecules whose programmed appearance is associated with the development and regulation of immune system cells.

Interleukin-1 ($IL-1$)

A cytokine that promotes stem cell growth by inducing bone marrow stromal cells to release additional cytokines and synergistically stimulating cells.

Transforming growth factor-$\beta$ ($TGF-\beta$)

A factor that mediates the maintenance of pluripotent capacity by keeping $HSCs$ quiescent and can also promote terminal differentiation.

Thymus

An organ derived from the $3^{rd}$ pharyngeal pouch (endoderm) where T cells progress from the cortex to the medulla to differentiate.

Hassall corpuscles

Specialized cells in the thymus that produce thymic growth factors.

Thymic Involution

The process starting at puberty where an increase in steroids reduces immature thymocyte numbers, drastically reducing the ability to make new T cells by age $75$ years.

Neutrophils

Polymorphonuclear granulocytes whose granules fuse with ingested organisms to form phagolysosomes to kill invading pathogens, mainly bacteria.

Eosinophils

Granulocytes containing Major Basic Protein ($MBP$), which neutralizes heparin and is toxic against parasites.

Basophils and Mast Cells

Cells that release heparin, histamine, and other substances to mediate immediate allergic reactions or anaphylaxis.

Tissue-resident Macrophages

Mononuclear phagocytes found in specific tissues, including Kupffer cells (liver), alveolar macrophages (lungs), osteoclasts (bone), and microglia (brain).

Antigen-presenting cells ($APCs$)

Cells that uptake and degrade protein antigens, expressing $MHC$ class $II$ molecules and accessory molecules to interact with and program T helper ($Th$) cells.

Dendritic Cells ($DCs$)

Sentinels that detect danger and initiate immune responses; names include Langerhans cells (skin), Follicular $DCs$ (B-cell areas), and Interdigitating $DCs$ (T-cell areas).

Natural Killer ($NK$) cells

Lymphoid cells belonging to the innate immune response that control virally infected and malignant cells and kill specifically via the $Fc\gamma$ receptor ($CD16$).

$$CD3$$

A surface marker expressed by T cells along with their specific antigen receptors ($TCRs$).

$CD4^+$ T cells

Commonly known as T helper ($Th$) cells, they mediate immune responses predominantly through the secretion of cytokines.

$Th1$ cells

A subset of $CD4$ T cells that secrete inflammatory cytokines like $IL-2$ and $IFN-\gamma$ for cell-mediated immunity.

$Th2$ cells

A subset of $CD4$ T cells that synthesize cytokines like $IL-4$ and $IL-13$ to regulate antibody responses and defend against parasites.

$Tregs$ (Regulatory T cells)

Lymphocytes identified by $CD4$, $CD25$, and $Foxp3$ that suppress the functions of other lymphocytes and produce anti-inflammatory cytokines like $IL-10$.

$CD8^+$ T cells

Cytotoxic T lymphocytes ($CTLs$) that kill target cells by inserting perforins to create pores for granzymes or inducing apoptosis.

B cells

Lymphocytes expressing membrane immunoglobulin ($mIg$) and markers like $CD19$, $CD20$, and $CD21$; they differentiate into plasma and memory cells in germinal centers.

Lymphatic Fluid (Lymph)

Fluid connective tissue similar to interstitial fluid, with protein concentrations ranging from $3$ to $5\,g/dL$ ($6\,g/dL$ in liver) and fat up to $2\%$ in the thoracic duct.

Lymph Nodes

Encapsulated structures where B cells are found in follicles (cortex), T cells in the paracortex, and plasma reticular cells in the medulla.

Spleen - Red Pulp

The largest compartment of the spleen where blood is filtered and cleansed of senescent erythrocytes and bacteria by macrophages.

Spleen - White Pulp

The splenic area functioning similarly to lymph nodes, with T cells in the periarteriolar lymphoid sheath ($PALS$) and B cells in follicles.

$MALT$ (Mucosa-associated lymphoreticular tissue)

Organized secondary lymphoid structures in the mucosa where inductive immune responses occur; includes $GALT$, $NALT$, and $BALT$.

Peyer patches

Aggregates of lymphoid follicles located in the intestinal lamina propria, predominantly in the ileum.

Intraepithelial lymphocytes ($IELs$)

Lymphocytes located within the mucosal epithelium and lamina propria responsible for effector functions.

Langerhans cells

Specialized antigen-presenting dendritic cells ($APCs$) residing in the suprabasal layer of the skin epidermis.

$M1$ vs. $M2$ Macrophage Switch

In obesity, macrophages in adipose tissue switch from the anti-inflammatory $M2$ type to the pro-inflammatory, microbicidal $M1$ type.