Innate immunity consists of four barriers: 1. Physical barriers - e.g. Mucus coating of the epithelium lining the respiratory, gastrointestinal and urogenital tracts. 2. Physiological barrier - e.g. Acid (HCl) in stomach 3. Cellular barriers - e.g. Polymorpho-nuclear leukocytes (PMNL-Neutrophils) 4. Cytokine barrier - e.g. Interferons (secreted by virus infected cells to protect non infected cells from further viral infection)