The adaptive cellular immune response
- Francisco A Bonilla, MD, PhD
Francisco A Bonilla, MD, PhD
- Section Editor — Immunology and Immunodeficiency
- Associate Professor of Pediatrics
- Harvard Medical School
T cells regulate the activities of B cells, T cells, and other cells participating in immune responses. They provide help for antibody production by B cells, and they are also the effectors of antigen-specific cell-mediated immunity (CMI). CMI is important in the elimination of cells infected with pathogens that replicate intracellularly (eg, viruses, mycobacteria, and some bacteria) and cells exhibiting aberrant differentiation (eg, neoplasms). CMI also destroys allogeneic cells (graft rejection). In addition, it is involved in cellular autoimmune responses, as well as type IV allergic reactions to drugs and contact dermatitis. (See "Transplantation immunobiology" and "Overview of autoimmunity" and "Drug allergy: Classification and clinical features", section on 'Type IV Reactions' and "Overview of dermatitis", section on 'Allergic contact dermatitis'.)
T cell receptors (TCRs), in contrast to immunoglobulins, exist only as multimeric membrane-bound complexes and are not secreted intact in soluble form. Also unlike immunoglobulins, TCRs recognize fragments (peptides) of protein or glycoprotein antigens in complexes with major histocompatibility (MHC) molecules on the surfaces of antigen-presenting cells (APCs, also called accessory cells), or on targets of cytotoxicity. Note the distinction between the processed antigen (peptide) eliciting a response and the major histocompatibility complex (MHC) antigen with which it becomes associated to stimulate peptide-specific T cells. (See "T cell receptor genetics" and "Normal B and T lymphocyte development".)
The cellular interactions that form the basis of CMI are discussed in this topic review. Related topics and T cell help for antibody production are discussed separately. (See "T cell receptor genetics" and "Normal B and T lymphocyte development" and "The humoral immune response".)
T cell responses are initiated by antigen-presenting cells (APCs), which are reviewed briefly here and discussed in more detail elsewhere. (See "Antigen-presenting cells".)
Dendritic cells (DCs) are the predominant class of "professional" APCs . The main subtypes include plasmacytoid DCs (pDCs) and conventional DCs (cDCs). These are further divided into additional subtypes that have distinct surface marker expression, morphology, tissue distribution, and cytokine production, and that lead to distinct pathways of T cell stimulation. Monocytes/macrophages may also exert some APC activity, and monocytes may differentiate into DCs.
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- ANTIGEN-PRESENTING CELLS
- Mechanism of presentation
- T CELL ACTIVATION
- Structure of the T cell receptor CD3 complex
- T cell accessory molecules
- T cell interactions and movements
- - Overview of CD4+ T cell activation
- T cell activation via the two signal model
- - Intracellular events
- Cytokine secretion profiles of activated T cells
- - Th1
- - Th2
- - Th3
- - Th9
- - Th17
- - Th22
- - Treg
- - Tfh
- Memory T cells
- CELLULAR REGULATION OF IMMUNE RESPONSES
- Positive regulation
- - Cognate T helper cell-B cell interaction
- - Activation of APCs
- Negative regulation
- - Autoregulation via CTLA-4
- - Inhibition via PD-1
- - Activation-induced cell death
- - Suppression
- T cell cytotoxicity
- - Activation of cytotoxic T cells
- Gamma-delta T cells