Immunology Chapter 6: T Cell-Mediated Immunity

T Cell-Mediated Immunity

• T lymphocytes play a crucial role in defending against infections by various kinds of microbes, particularly those that live and reproduce inside host cells. T cells recognize and eliminate infected cells, thereby removing the source of infection, thereby protecting the host from further harm.
• T cells also provide defense against infections by extracellular microbes, such as bacteria and fungi, which are readily destroyed by phagocytosis, especially by neutrophils.
• In some cases, T cells collaborate with B cells to produce antibodies, which provide a long-term defense against specific pathogens.

Cell-Mediated Immunity

• Cell-mediated immunity provides defense against infections by microbes that live and reproduce inside host cells, including viruses, some bacteria, fungi, and protozoa. This type of immunity is crucial for eliminating infected cells and removing the reservoir of infection.
• The process of cell-mediated immunity involves the activation and proliferation of T cells in response to specific antigens presented by antigen-presenting cells (APCs). Activated T cells then recognize and eliminate infected cells, thereby removing the source of infection.
• Cell-mediated immunity also plays a crucial role in defense against cancer cells, where T cells specifically recognize and eliminate tumor cells.

Types of Intracellular Microbes

• Viruses are among the most common types of intracellular microbes that infect host cells, replicating in the nucleus and cytosol of infected cells.
• Rickettsiae are obligate intracellular parasites that reside in non-phagocytic cells, requiring the presence of host cells to survive.
• Protozoa are another type of intracellular parasite that can infect host cells, with some species exhibiting obligate intracellular behavior.

Extracellular Microbes

• Extracellular microbes, such as bacteria and fungi, are often readily destroyed by phagocytosis, a process mediated by neutrophils and other phagocytic cells.
• Helminthic parasites, on the other hand, are typically eliminated by special types of leukocytes, such as eosinophils.

Line of the Immune Response

• Antigen-specific lymphocytes, including T cells and B cells, expand during an immune response, recognizing and responding to specific antigens.
• As the immune response subsides, many T cells die by apoptosis, allowing for the elimination of activated immune cells and the return of the immune system to a state of homeostasis.
• A pool of surviving memory lymphocytes remains, allowing for rapid responses to subsequent encounters with the antigen.

T Cell Functions

• T lymphocytes mediate defense against microbes that live within host cells, some extracellular microbes, and cancer cells, recognizing and eliminating infected and tumor cells.
• T cells also collaborate with B cells to produce antibodies, providing long-term immunity against specific pathogens.

T Cell Activation and Signaling

• T cells recognize antigen through the TCR (CD3 and ζ chains) and coreceptors CD4 and CD8, which recognize class II and class I MHC molecules, respectively.

• The binding of T cells to APCs is enhanced by adhesion molecules, such as integrins, whose affinity for their ligands is increased by antigen recognition.

• T cell activation and signaling are tightly regulated through a complex interplay of molecular interactions, ensuring proper immune responses and preventing autoimmunity.

• Activated T cells differentiate into memory cells, which survive even after the antigen is eliminated.

• Signaling pathways involve protein tyrosine kinases, which phosphorylate proteins that become docking sites for additional kinases and other signaling molecules.