The peripheral nervous system is composed of autonomic and the somatic parts. The autonomic is composed of sympathetic and parasympathetic parts. The sympathetic part prepares the body for stress, the parasympathetic returns the body to normal. The somatic part is composed of receptor nerves, which receive sensory input from the body, and effector nerves, which trigger muscular activity. The spinal arc can short-circuit the path from sensor nerve to brain to muscle, allowing a faster response to a huge stimulus, such as putting your hand on a hot burner.
White cells leave blood vessels and enter tissues.
Helper T activated both killer T cells and B cells. Killer T cells then destroy cells which are infected, B cells become plasma cells and produce antibodies, which clog up invaders outside of cells. Immune system also produces memory T and memory B cells which can rapidly produce more killer T cells and plasma cells if invader encountered a second time. Clogged up invaders then digested by white blood cells. Helper T cells are also called CD4 lymphocytes Every day during height of infection, HIV infects 2 billion helper Ts and produces 100 million - 1 billion new viruses.