T 細胞の種類と機能

T cells differentiate into two primary types based on surface markers. CD4 T cells become helper T cells or regulatory T cells, while CD8 T cells become cytotoxic T cells. Helper T cells include Th1, Th2, and Th17 subsets, each with distinct functions in immune response. Cytotoxic T cells patrol the body to neutralize infected or cancerous cells. Regulatory T cells suppress self-reactive lymphocytes to prevent autoimmunity.

Th1 effector cells activate macrophages by secreting cytokines essential for cell-mediated immunity. They also stimulate dendritic cells to express co-stimulatory molecules and secrete cytokines that activate and differentiate CD8 cells into cytotoxic T cells. This dual role makes Th1 cells critical for coordinating immune responses against intracellular pathogens and infected cells.

Th2 effector cells secrete cytokines like IL-4 that act as co-stimulatory molecules for B cell activation. When Th2 cells bind to B cells displaying specific antigen fragments on class II MHC receptors, they release cytokines that stimulate rapid B cell division and signal antibody production. This interaction is essential for most T cell-dependent antigens to elicit strong immune responses.

IL-17 is a pro-inflammatory cytokine produced by Th17 cells that triggers inflammatory reactions against extracellular microbes. It recruits and activates neutrophils and other immune cells at infection sites, boosting production of pro-inflammatory cytokines and chemokines to clear pathogens. However, unregulated IL-17 production contributes to autoimmune diseases including multiple sclerosis, rheumatoid arthritis, and psoriasis.

Regulatory T cells suppress self-reactive lymphocytes outside lymphoid organs through direct contact or by releasing inhibitory cytokines. This suppression prevents the immune system from attacking the body's own cells and tissues. By dampening inappropriate immune responses, regulatory T cells maintain immune tolerance and prevent the development of autoimmune disorders.

Cytotoxic T cells patrol the body to neutralize host cells that are cancerous, infected by intracellular pathogens like viruses, or foreign cells introduced during transplants. They recognize these abnormal cells through antigen presentation on MHC I molecules. This surveillance function is critical for eliminating threats that have already entered cells, where antibodies cannot reach.

The differentiation of CD4 T cells into helper T cell subsets—Th1, Th2, and Th17—depends on the antigen type, antigen-presenting cell, and regulatory cytokines present during activation. Different antigens and cytokine signals direct T cells toward specific helper phenotypes suited to combat particular threats. This plasticity allows the immune system to mount appropriate responses tailored to different pathogens.