Immunology Terms Starting With C

CD4 T Cell is a T lymphocyte expressing the CD4 co-receptor that recognizes peptide antigens presented on MHC class II molecules and coordinates adaptive immune responses through cytokine secretion and direct cell contact.

CD4 T cells are activated when their T cell receptor, together with CD4, binds peptide-MHC class II complexes on antigen-presenting cells along with co-stimulatory signals such as CD28-B7 engagement. Activated CD4 T cells differentiate into functionally distinct subsets: Th1 cells promote cellular immunity through interferon-gamma secretion, Th2 cells drive antibody and allergic responses via IL-4 and IL-13, Th17 cells direct neutrophil-mediated inflammation through IL-17, and regulatory T cells suppress immune activation to prevent tissue damage. In HIV-infected individuals, a CD4 T cell count below 200 cells per microliter of blood defines clinical AIDS and marks the threshold at which opportunistic infections become likely.

CD4-Positive is a designation for immune cells that express the CD4 surface glycoprotein, including helper T cells and some macrophages and dendritic cells, which use CD4 to co-recognize MHC class II molecules on antigen-presenting cells.

CD4 binds alongside the T cell receptor, engaging the same MHC class II molecule and amplifying the intracellular activation signal by recruiting the tyrosine kinase Lck. The CD4 molecule is also the primary entry receptor for HIV, making CD4-positive cells the target population depleted in AIDS. CD4 cell counts and CD4:CD8 ratios are standard clinical measurements used to monitor HIV disease progression and assess immune reconstitution during antiretroviral therapy, with a healthy adult typically maintaining 500 to 1,500 CD4-positive T cells per microliter of blood.

CD8 T Cell is a T lymphocyte bearing the CD8 co-receptor that recognizes peptide antigens on MHC class I molecules displayed by nucleated cells and kills infected or cancerous targets through cytotoxic mechanisms.

CD8 T cells, also called cytotoxic T lymphocytes, are activated by peptide-MHC class I complexes combined with co-stimulatory signals provided by dendritic cells, and full activation typically requires CD4 T cell help during the priming phase. Once activated, CD8 T cells can expand by more than 10,000-fold within a week, migrate to sites of infection or tumors, and kill target cells by releasing perforin and granzymes or through Fas-FasL interactions that trigger apoptosis. After the acute response resolves, roughly 5 to 10 percent of the expanded population survives as long-lived memory CD8 T cells that respond faster and more vigorously upon re-exposure to the same pathogen.

CD8-Positive is a designation for immune cells that display the CD8 protein on their surface, marking them primarily as cytotoxic T lymphocytes capable of killing virus-infected or cancerous cells through MHC class I-restricted antigen recognition.

CD8 consists of alpha and beta chains that bind the non-variable portion of MHC class I, stabilizing the interaction between the T cell receptor and its target and amplifying activation signals by recruiting the kinase Lck. T cells bearing CD8 clear intracellular infections caused by viruses and certain bacteria, and they recognize tumor cells that present abnormal peptides on MHC class I. The CD8:CD4 T cell ratio in peripheral blood shifts toward CD8 predominance during chronic viral infections such as HIV and cytomegalovirus, where CD4 cells are depleted or CD8 populations expand in response to persistent antigen.

Cell-Mediated Immunity is the branch of adaptive immunity in which T lymphocytes, rather than antibodies, directly attack infected cells, activate phagocytes, and coordinate defense against intracellular pathogens and tumors.

Cytotoxic T lymphocytes recognize and kill cells displaying viral peptides or tumor antigens on their surface, using perforin and granzyme proteins to trigger target cell apoptosis. Helper T cells secrete cytokines like interferon-gamma that activate macrophages to destroy intracellular pathogens such as Mycobacterium tuberculosis, forming granulomas that wall off persistent infection. Memory T cells persist for years after infection or vaccination and rapidly expand upon re-exposure, providing long-term protection against viruses like measles and varicella-zoster.

Regulatory T cells suppress excessive immune responses to prevent autoimmunity and collateral tissue damage.

Chemokine is a small secreted signaling protein that directs the migration of immune cells toward sites of infection or inflammation by establishing concentration gradients that cells follow through chemotaxis.

Over 50 human chemokines have been identified, classified by the spacing of conserved cysteine residues into CC, CXC, CX3C, and XC families. Each chemokine binds to G-protein-coupled receptors on target cells, triggering cytoskeletal rearrangements that drive directed migration up the concentration gradient toward the source. Tissue injury and infection stimulate chemokine production by macrophages, endothelial cells, and stromal cells, orchestrating a sequential recruitment pattern in which neutrophils arrive first, followed by monocytes and lymphocytes hours to days later.

Complement System is a group of over 30 plasma proteins that form a biochemical cascade amplifying innate immune responses by opsonizing pathogens, recruiting inflammatory cells, and directly lysing microbial targets.

Complement activation proceeds through three pathways: the classical pathway, triggered by antibody-antigen complexes; the lectin pathway, triggered by mannose-binding lectin recognizing microbial surface sugars; and the alternative pathway, triggered by spontaneous C3 hydrolysis on foreign surfaces. All three pathways converge on the formation of C3 convertase, which cleaves C3 into C3a, a pro-inflammatory anaphylatoxin, and C3b, an opsonin that coats pathogens for phagocytosis. Downstream cleavage events ultimately assemble the membrane attack complex, a pore-forming structure that inserts into microbial membranes and causes osmotic lysis.

Host cells are protected from accidental complement attack by surface-expressed regulatory proteins such as CD55 and CD59, which disrupt convertase assembly and block membrane attack complex formation.

Cytokine is a small signaling protein secreted by immune and non-immune cells that regulates the growth, differentiation, activation, and recruitment of immune cells, coordinating both innate and adaptive immune responses.

Cytokines include interleukins, interferons, tumor necrosis factors, colony-stimulating factors, and chemokines, each acting through specific receptors on target cells to alter gene expression and cellular behavior. Most act in a paracrine or autocrine manner at picomolar to nanomolar concentrations and form regulatory networks with synergistic, antagonistic, or pleiotropic effects. A single cytokine can trigger different outcomes depending on the target cell type, a property called pleiotropy that distinguishes cytokines from classical hormones.

Cytokine storms, in which uncontrolled cytokine release causes systemic inflammation and multi-organ damage, are a life-threatening complication of severe infections, sepsis, and immunotherapy.

Cytotoxic T Cell is an activated CD8 T lymphocyte that kills virus-infected, cancerous, or allogeneic cells by recognizing specific peptide-MHC class I complexes on target cells and releasing perforin and granzymes that trigger apoptosis.

Upon recognizing its target, a cytotoxic T cell forms a tight immunological synapse and directionally releases perforin, which polymerizes into pores in the target cell membrane, and granzymes, which enter through those pores and activate caspase-dependent apoptosis. This directional secretion spares adjacent healthy cells from collateral damage. A single cytotoxic T cell can kill multiple targets sequentially, a property called serial killing that is critical for controlling widespread viral infections.

CD8 T cells can also trigger apoptosis through the Fas-FasL pathway without granule release, providing a second killing mechanism particularly relevant in immune-privileged tissues.