thumb|upright=1.25|Immunofluorescence micrograph of three [[cytotoxic T cells (outer three) surrounding a cancer cell. Lytic granules (red) are secreted at the contact site, killing the target. Cytotoxic T cells are powerful agents of cellular immunity.]]

Cellular immunity, also known as cell-mediated immunity, is an immune response that does not rely on the production of antibodies. Rather, cell-mediated immunity is the activation of phagocytes, antigen-specific cytotoxic T cells ( cytotoxic T lymphocytes), and the release of various cytokines in response to an antigen.

History

In the late-19th-century Hippocratic tradition of medicine, the immune system was imagined having two branches: humoral immunity, for which the protective function of immunization could be found in the humor (cell-free bodily fluid or blood plasma) and cellular immunity, for which the protective function of immunization was associated with cells. CD4 cells or T helper cells (also known as helper T cells) provide protection against distinct pathogens. Naive T cells, which are immature T cells that have yet to encounter an antigen, are converted into activated effector T cells after encountering antigen-presenting cells (APCs). These APCs, such as macrophages, dendritic cells, and B cells, in some circumstances, load antigenic peptides onto the major histocompatibility complex (MHC) of the cell, in turn presenting the peptide to receptors on T cells. The most important of these APCs are highly specialized dendritic cells, which conceivably operate solely to ingest and present antigens. Activated effector T cells can be placed into three functioning classes, detecting peptide antigens originating from various types of pathogen:

  • Cytotoxic T cells, which kill infected target cells by apoptosis without using cytokines;
  • T<sub>h</sub>1 cells, which primarily function to activate macrophages;
  • T<sub>h</sub>2 cells, which primarily function to stimulate B cells into producing antibodies.

Role of dendritic cells in cell-mediated immunity

Acting as a bridge between the innate and adaptive immune systems, dendritic cells (DCs) initiate interactions that drive T-cell activation. Myeloid phagocytic DCs capture antigens in peripheral tissues and migrate through lymphatic vessels into the lymph node, where they present antigens to T-cells. Said migration relies on the expression of C-C chemokine receptor type 7 (CCR7), which guides DCs along CCL19 and CCL21 gradients toward the lymph node. Once DCs are inside the lymph node, they release chemokines such as CCL5 and CCR7 that help recruit and position naïve T-cells and other leukocytes within the lymph node for antigen recognition. and phagocytosis (for macrophages).

  • Stimulating cells to secrete a variety of cytokines that influence the function of other cells involved in adaptive immune responses and innate immune responses.
  1. T<sub>h</sub>1 cells, which produce interferon gamma and lymphotoxin alpha.
  2. T<sub>h</sub>2 cells, which produce interleukin 4 (IL-4), interleukin 5 (IL-5), and interleukin 1 (IL-13).

A third category called T helper 17 cells (T<sub>h</sub>17) were also discovered which are named after their secretion of interleukin 17 (IL-17).

CD8<sup>+</sup> cytotoxic T-cells may also be categorized as:

Common innate lymphoid progenitors may then be differentiated into a natural killer progenitor (NKp) or a common helper like innate lymphoid progenitor (CHILp). NKp cells may then be induced to differentiate into natural killer cells by IL-15. CHILp cells may be induced to differentiate into ILC1 cells by IL-15, into ILC2 cells by IL-7 or ILC3 cells by IL-7 as well.