He was right, and a global theory of the immune system took flight over the next decade. Humans lack bursas, but, like all vertebrates, do have the two cell lines that chickens have, now known as T cells (for thymus) and B cells (for bursa, not bone marrow as many assume). In the fight against invaders, the B-cells are the foot soldiers. They make antibodies which attack incoming viruses and remember them for future battles. Meanwhile, T cells serve as generals. They direct B cell activity and carry out mercy kills on infected cells. Like Batman and Alfred, the two cell types rely on each other, and any explanation based on just one was always doomed to fail. The new theory gave immunologists a whole new way to think about immune diseases like Wiscott-Aldrich syndrome, where a mutated protein leads to ineffective T cells, breaking up the lymphocyte team.