Cell-mediated immunity is a crucial component of the adaptive immune response that involves various cells, particularly cytotoxic T-cells, to eliminate infected or abnormal cells. Here’s a general sequence of events that leads to this form of immunity:
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Antigen Presentation: The process begins when an antigen-presenting cell (APC), such as a dendritic cell, macrophage, or B cell, encounters a pathogen. The APC engulfs the pathogen, processes it, and presents its antigens on the surface of its cell membrane using a molecule called Major Histocompatibility Complex (MHC) Class II. This presentation is essential for the activation of T-cells.
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Activation of Helper T-cells: The APC then interacts with naïve CD4+ T-cells (helper T-cells) through the recognition of the presented antigen by the T-cell receptor (TCR) on the CD4+ T-cell. This interaction, along with additional signals (such as co-stimulatory molecules), activates the helper T-cells, promoting their proliferation and differentiation.
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Activation of Cytotoxic T-cells: The activated helper T-cells release cytokines that assist in activating naïve CD8+ T-cells (cytotoxic T-cells). These cytotoxic T-cells are now primed to recognize and destroy infected or malignant cells carrying the specific antigen presented by the MHC class I molecules of the infected cells.
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Clonal Expansion: The activated cytotoxic T-cells proliferate and differentiate into effector cytotoxic T-cells. Some of these cells also become memory T-cells, which persist long-term in the body and are primed to respond more rapidly and effectively upon future encounters with the same antigen.
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Target Cell Recognition and Cell Lysis: The effector cytotoxic T-cells circulate throughout the body and patrol for cells expressing the specific antigen. Upon encountering an infected or cancerous cell presenting the antigen on its surface, the cytotoxic T-cell binds to it through its TCR and the antigen-MHC complex. This binding, accompanied by the release of cytotoxic granules containing perforin and granzymes, results in cell lysis, where the target cell is destroyed.
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Memory Formation: After the immune response has cleared the infection, many of the cytotoxic T-cells undergo apoptosis (programmed cell death), but a significant number remain as memory cells. These memory cytotoxic T-cells are essential for providing long-lasting immunity, enabling the immune system to respond quicker and more effectively if they encounter the same antigen in the future.
In summary, cell-mediated immunity involves the recognition and destruction of infected or abnormal cells through a series of events initiated by antigen presentation, activation of T-cells, and the eventual lysis of targeted cells, complemented by the establishment of memory for future protection.