Innate resistance against infections is evolutionarily the earliest protective mechanism and exists in practically all multicellular organisms from plant to human. A significant portion of microorganisms is inactivated by mechanisms of innate resistance during the first hours and days after their penetration into the organism or long before the development of a lymphocyte-mediated immune reaction. Only those types of infections or doses of pathogen that are able to evade or overcome mechanisms of innate resistance become "available" to lymphocytes. However lymphocyte response is impossible without innate inflammation.
Primary receptors for pathogens, the complement system, phagocytosis, endogenous antimicrobial peptides, and interferons (cytokines that inhibit viral growth) play very important roles in the innate immune protection of the organism.
RECEPTORS THAT RECOGNIZE "NONSELF"
On their surfaces microorganisms express repetitive structures composed of carbohydrates and lipids, but the host cells are mostly free of these structures. Receptors recognizing the specific "pattern" on pathogen surfaces are called Pattern Recognition Receptors (PRR). They allow the cells of innate resistance to identify microbial cells. Currently, there are two known types of PRR: secreted and membrane-bound.
• Soluble receptors for pathogens are serum proteins synthesized by the liver: Lipopolysaccharide-Binding Protein (LPB), collagen-like component of the complement System, C1q, and acute phase proteins - Man-nose-Binding Lectin (MBL) and C-Reactive-Protein (CRP). They bind microbial products directly and control their absorption by phagocytes. They are known as opsonins (from Greek opsonein, which means "to prepare food"). In addition some of them promote activation of the complement system.