Coreceptors

Coreceptors are cell-surface molecules that work in conjunction with primary receptors to enhance ligand binding, signaling, or cellular responses. They are not the main receptors for their ligands, but they influence the strength, duration, or specificity of signaling. Coreceptors can shape sensitivity to stimuli, help recruit intracellular signaling machinery, or stabilize receptor–ligand interactions.

In the immune system, coreceptors play a central role in tuning responses. T cells use the CD4 or CD8 coreceptors to engage major histocompatibility complex (MHC) molecules presenting antigen. By recruiting kinases such as Lck, these coreceptors help initiate and amplify T cell receptor signaling. B cells rely on a co-receptor complex formed by CD19, CD21, and CD81 to boost B cell receptor signaling, increasing activation and antibody production. Additional co-stimulatory receptors, such as CD28, provide further signals essential for full T cell activation, illustrating how coreceptors integrate with primary receptors to regulate immunity.

Coreceptors also influence pathogen entry in virology. Human immunodeficiency virus (HIV) requires both CD4 and a coreceptor—either CCR5 or CXCR4—to enter susceptible cells. Genetic variants in CCR5, for example, can alter susceptibility to infection, highlighting the clinical relevance of coreceptor biology.

Beyond immunity and virology, coreceptors participate in diverse signaling contexts, including development and tissue homeostasis, where they modulate receptor function for various ligands. Understanding coreceptors helps explain how cells interpret complex environmental cues and offers targets for therapeutic intervention in autoimmunity, infection, and cancer.