 Adaptive immunity is a second line defence that includes T cellular and B humoral cell mediated responses. This is specific, targets only pathogens and not self, and has memory to sustain a long-lasting immunity against reinfection. Although the innate immune system lacks the fine specificity of adaptive immunity it can distinguish self from non self. Innate immune recognition is mediated by a system of germline-encoded receptors named pattern recognition receptors PRRs that recognize conserved molecular patterns pathogen-associated molecular patterns, PAMPs that are associated with microbial pathogens. These receptors are coupled to signal transduction pathways that control expression of a variety of inducible immune-response genes. 