Briefly describe the development of Cell-Mediated Immunity. List the cell types involved, along with their actions.

Development of Cell-Mediated Immunity

The development of cell-mediated immunity is a complex, multi-step process initiated upon the body's encounter with an intracellular pathogen or an abnormal cell. It primarily involves T lymphocytes and antigen-presenting cells (APCs).

1. Antigen Presentation: The process begins when an Antigen-Presenting Cell (APC), such as a dendritic cell or macrophage, engulfs a pathogen or a cancerous cell. The APC then processes the foreign antigen into smaller peptides. These antigenic peptides are loaded onto Major Histocompatibility Complex (MHC) molecules (Class I or Class II) and displayed on the APC's surface.
2. T-cell Activation: The APC then travels to secondary lymphoid organs (like lymph nodes or spleen) where it encounters naive T lymphocytes.
   • Naive CD4+ T helper cells recognize antigens presented on MHC Class II molecules by APCs. This interaction, along with co-stimulatory signals, activates the T helper cell.
   • Naive CD8+ cytotoxic T cells recognize antigens presented on MHC Class I molecules by APCs or infected cells. Full activation often requires help from activated CD4+ T helper cells and appropriate co-stimulation.
3. Clonal Expansion and Differentiation: Upon activation, both CD4+ T helper cells and CD8+ cytotoxic T cells undergo rapid proliferation, generating numerous effector cells specific to the encountered antigen. These effector cells then differentiate into various subsets, each with specialized functions.
4. Effector Phase: The activated effector T cells migrate to the site of infection or tumor.
   • Cytotoxic T Lymphocytes (CTLs) directly identify and kill infected or cancerous target cells.
   • Helper T cells secrete cytokines that amplify the immune response, activating other immune cells like macrophages and B cells.
5. Memory Formation: After the pathogen is cleared, most effector T cells die, but a subset of them survives as memory T cells. These memory cells persist in the body, providing long-lasting immunity and enabling a faster, more robust response upon subsequent exposure to the same antigen.

Cell Types Involved and Their Actions

Cell-mediated immunity is orchestrated by several key cell types, primarily T lymphocytes, with crucial support from Antigen-Presenting Cells and other innate immune cells.

Cell Type	Primary Action(s)	Role in CMI
Antigen-Presenting Cells (APCs) (e.g., Dendritic Cells, Macrophages)	Phagocytosis of pathogens/abnormal cells. Processing and presentation of antigenic peptides on MHC Class I and Class II molecules. Provide co-stimulatory signals for T-cell activation.	Initiates the adaptive immune response by activating naive T cells. Dendritic cells are considered the most potent APCs.
Helper T Cells (TH cells / CD4+ T cells)	Recognize antigens presented on MHC Class II molecules. Secrete various cytokines (e.g., IL-2, IFN-γ, TNF-α). Activate cytotoxic T cells, macrophages, and B cells.	Central to coordinating and amplifying both cell-mediated and humoral immune responses. Essential for full activation of CD8+ T cells.
Cytotoxic T Lymphocytes (CTLs / CD8+ T cells)	Recognize antigens presented on MHC Class I molecules on infected or cancerous cells. Induce apoptosis (programmed cell death) in target cells by releasing perforins and granzymes.	Directly kill cells infected with intracellular pathogens or cancerous cells, thereby eliminating the source of infection or malignancy.
Memory T Cells (Memory CD4+ and CD8+ T cells)	Long-lived cells that persist after the primary infection. Mount a rapid and robust immune response upon re-exposure to the same antigen.	Provide immunological memory, ensuring faster and more effective protection against recurrent infections.
Regulatory T Cells (Treg cells)	Suppress immune responses to maintain self-tolerance and prevent autoimmunity. Secrete immunosuppressive cytokines (e.g., IL-10, TGF-β).	Crucial for modulating the intensity and duration of the immune response, preventing excessive inflammation and damage to host tissues.
Natural Killer (NK) Cells (Innate Lymphoid Cells)	Recognize and kill target cells that lack MHC Class I molecules (often seen in virus-infected or tumor cells). Release cytotoxic granules (perforins and granzymes). Part of innate immunity but collaborate with adaptive CMI.	Provide an immediate, non-specific defense against infected and cancerous cells, complementing the specific action of CTLs.