occurs mainly by non-covalent forces. Successful binding of the antigen receptor to the epitope, if accompanied by additional signals, results in: Stimulation of the cell to leave G0 and enter the cell cycle. Repeated mitosis leads to the development of a clone of cells bearing the same antigen receptor; that is, a clone of cells of the identical specificity. BCRs and TCRs differ in: their structure the genes that encode them the type of epitope to which they bind B cells BCRs bind intact antigens (like diphtheria toxoid, the protein introduced in the diphtheria-tetanus-pertussis vaccine). These may be soluble molecules present in the extracellular fluid; or intact molecules that the B cell plucks from the surface of antigen-presenting cells like macrophages and dendritic cells. The bound antigen molecules are engulfed into the B cell by receptor-mediated endocytosis. The antigen is digested into peptide fragments by various proteasomes and are then displayed at the cell surface attached along with a class II histocompatibility molecule. Helper T cells specific for this structure (i.e., with complementary TCRs) bind this B cell and secrete lymphokines that: Stimulate the B cell to enter the cell cycle The B cell undergoes repeated mitotic cell division, resulting in a clone of cells with identical BCRs; The B cells switch from synthesizing their BCRs as integral membrane proteins to a soluble version; The clonal cells differentiate into plasma cells that secrete these soluble BCRs, which we now call antibodies T cells There are two types of T cells that differ in their TCR: alpha/beta (αβ) T cells: Their TCR is a heterodimer of an alpha chain with a beta chain. Each chain has a variable (V) region and a constant (C) region. The V regions each contain 3 hypervariable regions that make up the antigen-binding site. gamma/delta (γδ) T cells: Their TCR is also a heterodimer of a gamma chain paired with a delta chain. They show characteristics of both innate immune response and acquired immune response; hence, regarded as the bridging between the two immune systems. The discussion that follows now concerns alpha/beta T cells. The TCR (of αβ T-cells) binds a bimolecular complex displayed at the surface of some other cells called an antigen-presenting cell (APC). This complex consists of: a fragment of an antigen lying within the groove of a histocompatibility molecule. The complex has been compared to a "hot dog in a bun". See