Innate immunity is the defense you are born with. It responds fast and in the same way to any invader, buying time for the slower adaptive immune system.
Barriers
Intact skin, mucus, airway cilia, stomach acid and tears block or wash away most germs. A break in a barrier, such as a burn, a wound or an IV line, opens a door for infection.
Phagocytes and killer cells
Neutrophils arrive first and engulf bacteria; macrophages follow, clean up and signal other immune cells. Natural killer cells destroy virus-infected cells and some cancer cells.
Inflammation
Damaged tissue and immune cells release chemicals such as histamine that widen local blood vessels and make them leaky. This causes the classic redness, heat, swelling and pain, and brings more cells and proteins to the site.
Fever and complement
Signaling proteins called cytokines reset the brain's thermostat and cause fever, which slows the growth of many germs. Complement proteins in the blood coat microbes for phagocytes and can punch holes in their membranes.
At the bedside
Fever, a raised white blood cell count and a raised CRP are early clues that innate immunity is fighting an infection. When the response spreads through the body with low blood pressure or organ dysfunction, think of sepsis.
Related guides
Written by Making Synapses. Topic reference: OpenStax, Anatomy and Physiology 2e (no text adapted).