All animals have protection against the pathogens to which they are susceptible via an innate immune system.
Evolution upped the defensive game for vertebrates, which have an adaptive immune system that can learn and remember attacks to better cope the next time around. Innate immune systems also possess some memory, but it is not as sophisticated as in the adaptive variety.
Taking out the opposition is a team sport. Different cell classes and types are tasked with different roles and responsibilities.
For instance, there are 4 functional types of T cells, which play different roles: hunting (killers), assisting (helpers), not overdoing it (regulators), and archiving the episode (historians).
Killer T’s hunt in small packs. They converse as they prowl, expressing themselves by placing specific proteins on their surfaces.
T talk is tactile. T cells rub up against one another, placing new proteins into the interface between them. These protein paintings are organized into beautiful patterns. One looks like a bullseye. The spacing of the patterns is significant, as is the rigidity of the cell surface on which painting is done. T cells indulge in artful con-versation before terminating pathogens with extreme prejudice.