Antigen recognition: T and B cell receptor complexes, antigen processing and presentation.

Antigen recognition by T and B cells, as well as antigen processing and presentation, are fundamental processes in the adaptive immune response that allow the immune system to identify and respond to specific pathogens. Let's delve into these processes:

1. T Cell Receptor (TCR) Complex and Antigen Recognition:

T cells have T cell receptors (TCRs) on their cell surfaces. TCRs are membrane-bound proteins that recognize specific antigens presented by antigen-presenting cells (APCs), such as dendritic cells, macrophages, and B cells.

TCRs have two chains: alpha and beta or gamma and delta, depending on the type of T cell. Each TCR has a unique antigen-binding site.

TCRs can only recognize antigens when they are presented in association with major histocompatibility complex (MHC) molecules on the surface of APCs. There are two main types of MHC molecules: MHC class I and MHC class II.

MHC class I molecules present antigens from within the cell (e.g., viral proteins from infected cells) to cytotoxic T cells (CD8+ T cells).

MHC class II molecules present antigens from outside the cell (e.g., proteins from engulfed bacteria) to helper T cells (CD4+ T cells).

TCR binding to the antigen-MHC complex triggers the activation of T cells, leading to an immune response tailored to the specific antigen.

2. B Cell Receptor (BCR) Complex and Antigen Recognition:

B cells have B cell receptors (BCRs) on their cell surfaces. BCRs are membrane-bound immunoglobulin molecules that function as antibodies.

BCRs, like TCRs, are specific to particular antigens. Each B cell produces BCRs with the same antigen specificity.

B cells recognize antigens in their native form, meaning they can directly bind to antigens without the need for antigen presentation by APCs.

When BCRs bind to antigens, B cells are activated. Activated B cells can then differentiate into plasma cells, which produce large quantities of antibodies with the same antigen specificity.

3. Antigen Processing and Presentation:

Antigen processing and presentation is a crucial step in the immune response, allowing immune cells to display antigens to T cells.

For MHC class I presentation:

Intracellular pathogens, such as viruses, produce antigens inside host cells.

These antigens are broken down into smaller peptide fragments by proteasomes (protein-degrading enzymes).

The peptide fragments are transported into the endoplasmic reticulum (ER) and loaded onto MHC class I molecules.

The MHC class I-antigen complex is transported to the cell surface for recognition by cytotoxic T cells.

For MHC class II presentation:

Extracellular pathogens, like bacteria, produce antigens outside host cells.

Antigen-presenting cells, such as dendritic cells and macrophages, engulf these pathogens.

Antigens from the engulfed pathogens are broken down into peptide fragments within endosomes.

MHC class II molecules are synthesized in the ER and then transported to endosomes, where they bind to the antigenic peptides.

The MHC class II-antigen complex is then presented on the cell surface for recognition by helper T cells.

Antigen recognition by T and B cells, along with the processing and presentation of antigens, is a highly coordinated process that ensures the immune system can specifically target and respond to pathogens while avoiding unnecessary immune responses to harmless substances.