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In addition to the antigen receptors, other molecules contribute to cell activation: first, by functioning as coreceptors (eg, CD4, CD8, CD19/CD21); second, by increasing the avidity of the interaction with antigen or the antigen-presenting cell (eg, LFA-1); or, third, by inducing separate signal transduction events that influence the cellular response (eg, CD28, CD40). Coreceptors, such as CD4 or CD8 in T cells or CD19/CD21 in B cells, are of particular interest in the context of antigen receptor signal transduction, since they directly contribute to the formation of the complex between the antigen receptor and the antigen complex, thereby increasing the sensitivity of the interaction. The coreceptors also contribute to the initiation of signals, as we will discuss below. Protein-tyrosine phosphorylation is important in the initiation of cellular responses by antigen receptors on either E3 or T cells. Neither the TCR nor the B cell …