In discussions about immune responses to the coronavirus, much of the conversation has focused on antibodies — Y-shaped proteins that can latch onto the surfaces of pathogens and block them from infecting cells. But antibodies represent just one wing of a complex and coordinated squadron of immune soldiers, each with its own unique modes of attack. Viruses that have already invaded cells, for instance, are cloaked from antibodies, but are still vulnerable to killer T cells, which force infected cells to self-destruct. Another set of T cells, nicknamed “helpers,” can coax B cells to mature into antibody-making machines.