Which type of antigens are recognized by T-cells?

T-cells specifically recognize processed fragments of protein antigens, also known as peptide antigens. When a pathogen invades the body, its proteins are broken down into smaller peptide fragments within cells. These fragments are then presented on the surface of antigen-presenting cells (APCs) in conjunction with major histocompatibility complex (MHC) molecules. The recognition occurs when the T-cell receptors (TCRs) bind to these peptide-MHC complexes.

This process is crucial for the adaptive immune response, as it allows T-cells to detect and respond to intracellular pathogens, such as viruses or some bacteria, which primarily express their antigens within host cells. By activating T-cells in response to these processed antigens, the immune system is able to mount a targeted response to clear infected cells and assist in orchestrating a broader immune defense.

In contrast, intact pathogen surfaces typically contain a variety of structures, including lipids and polysaccharides, which are recognized by B-cells and not directly processed for T-cell recognition. Soluble molecules in blood do not present their epitopes as processed fragments for T-cells, as these are generally recognized by antibodies from B-cells. Native glycoproteins may be recognized in their intact form