B cells recognize antigens in their native form. Once a B cell encounters its cognate (or specific) antigen (and receives additional signals from a helper T cell (predominately Th2 type)), it further differentiates into an effector cell, known as a plasma cell. Plasma cells are short-lived cells (2–3 days) that secrete antibodies. These antibodies bind to antigens, making them easier targets for phagocytes, and trigger the complement cascade. About 10% of plasma cells survive to become long-lived antigen-specific memory B cells. Already primed to produce specific antibodies, these cells can be called upon to respond quickly if the same pathogen re-infects the host, while the host experiences few, if any, symptoms. Alternative systems In jawless vertebrates Primitive jawless vertebrates, such as the lamprey and hagfish, have an adaptive immune system that shows 3 different cell lineages, each sharing a common origin with B cells, αβ T cells, and innate-like γΔ T cells. Instead of the classical antibodies and T cell receptors, these animals possess a large array of molecules called variable lymphocyte receptors (VLRs for short) that, like the antigen receptors of jawed vertebrates, are produced from only a small number (one or two) of genes. These molecules are believed to bind pathogenic antigens in a similar way to antibodies, and with the same degree of specificity. In insects For a long time it was thought that insects and other invertebrates possess only innate immune system. However, in recent years some of the basic hallmarks of adaptive immunity have been discovered in insects. Those traits are immune memory and specificity. Although the hallmarks are present the mechanisms are different from those in vertebrates. Immune memory in insects was discovered through the phenomenon of priming. When insects are exposed to non-lethal dose or heat killed bacteria they are able to develop a memory of that infection that allows them to withstand otherwise lethal dose of the same bacteria they were exposed to before. Unlike in vertebrates, insects do not possess cells specific for adaptive immunity. Instead those mechanisms are mediated by hemocytes. Hemocytes function similarly to phagocytes and after priming they are able to more effectively recognize and engulf the pathogen. It was also shown that it is possible to transfer the memory into offspring. For example, in honeybees if the queen is infected with bacteria then the newly born workers have enhanced abilities in fighting with the same bacteria. Other