Principles of single-cell annotation. (A) Shows the mRNA extracted from scRNA-seq. As the transcriptional product of cells, mRNA reflects the heterogeneity of gene expression and provides important information for cell type annotation and gene function research. (B) Illustrates the basic workflow of single-cell type annotation. Cells are first extracted from tissue, and single-cell sequencing is performed to obtain a gene expression matrix. High-variance genes are then selected for feature selection. Next, annotation models are used to predict cell types, and the annotation results are finally visualized using dimensionality reduction algorithms such as T-SNE. (C) Demonstrates the applications of single-cell type annotation in various fields, including targeted therapy strategies in the immune microenvironment, cell developmental trajectory reconstruction in developmental biology, immune cell activation state tracking in immunology, and drug perturbation response prediction in precision medicine.