T-cell Lymphocyte and T-Cell Receptor Immunology

This set covers the fundamental concepts of T-cell maturation, various T-lymphocyte subtypes, the structure and function of the T-cell receptor (TCR), mechanisms of cytotoxic T-cell action, and the specific signaling required for helper T-cell activation and differentiation.

T-lymphocytes

Non-antibody producing cells that constitute the cell-mediated arm of immunity, protect the body from pathogens and cancer cells, and mature in the thymus.

Thymus

An organ located in the upper chest where immature lymphocytes from the bone marrow are educated to become mature T-lymphocytes.

B-lymphocytes

Lymphocytes that mature in the bone marrow and are responsible for overseeing humoral immunity.

Cytotoxic T cells ($CD8^+$)

Cells that actively destroy infected or cancerous cells through the use of granule sacs containing digestive enzymes or by inducing apoptosis.

Helper T cells

Cells that activate cytotoxic T cells and macrophages, and stimulate antibody production by B cell lymphocytes by interacting with antigen-presenting cells.

Regulatory T cells

Cells that suppress the actions of B and T cells to decrease the immune response when a high level of activity is no longer warranted.

Natural Killer T cells

Cells that distinguish infected or cancerous cells from normal body cells by attacking those that lack specific molecular markers identifying them as self-cells.

Memory T cells

Cells that protect against previously encountered antigens and may provide lifetime protection against some pathogens.

T-cell receptor (TCR)

A protein complex on the surface of T cells that recognizes antigen fragments presented as peptides bound to major histocompatibility complex (MHC) molecules.

Heterodimer

The structural form of a TCR, which is composed of two different protein chains; in humans, $95 \%$ of T cells consist of $\alpha$ and $\beta$ chains, while $5 \%$ consist of $\gamma$ and $\delta$ chains.

Variable (V) and Constant (C) regions

The two main regions of the $\alpha$ and $\beta$ chains of a TCR, where V regions contain hypervariable segments that determine antigen specificity.

Apoptosis

A programmed cell suicide mechanism that cytotoxic T cells can force upon diseased or pathogenic cells.

Perforin

A protein stored in cytotoxic granules that forms pores in the target-cell plasma membrane to damage the cell and allow effector proteins to enter the cytosol.

Granzymes

Cytotoxic effector proteins, such as Granzyme B, that trigger an enzymatic chain reaction once delivered into the target-cell cytoplasm, causing apoptosis.

Granulysin

A protein expressed in humans with antimicrobial activity that can induce apoptosis in target cells at high concentrations.

Fas ligand (FasL)

A protein on the surface of a CTL that binds to the Fas protein on a target cell to trigger a suicide program leading to apoptosis.

Antigen-presenting cells (APCs)

Cells such as macrophages or dendritic cells that ingest microbes, degrade them, and export antigen fragments to their surface in association with class II MHC molecules.

B7 and CD28

A costimulatory protein pair where B7 on the APC binds to CD28 on the helper T cell to provide the necessary second signal for activation.

Anergic

A state in which a T cell is rendered unable to respond to an antigen, typically caused by receiving the TCR signal without the required costimulatory signal.

Th1 Helper T Cells

A subset of T cells that produce $TNF$, $IFN-\gamma$, and $IL-2$ to activate macrophages and natural killer cells and influence B cells to produce $IgG3$ antibodies.

Th2 Helper T Cells

A subset of T cells that produce $IL-4$, $IL-5$, and $IL-13$ to stimulate mucus production and influence B cells to produce $IgA$ and $IgE$ antibodies.

Th17 Helper T Cells

A subset of T cells that secrete $IL-17$ and $IL-21$ to recruit neutrophils; defects in this profile can lead to devastating fungal infections such as Candida albicans.