Background on Innate Immunity

The immune system is an interactive network of cellular and molecular systems that are responsible for recognizing and eradicating pathogens and harmful foreign molecules. In general, the immune response to a threat of infection can be divided into three stages as outlined below:

Immune Defences against Infection

Threat detection mechanisms and the immediate responses to infection are highly conserved, being present in even the simplest of animals, including many that do not have an adaptive immune system. Foreign molecules are discriminated from self through pathogen-associated molecular patterns, or "signalling molecules," which are present on microbes, but not on host cells. These signalling molecules are essential to bacterial survival and are therefore highly conserved during environmental adaptation (even during the development of antibiotic resistance).

Immune responses are "triggered" partly by the binding of these signalling molecules to pattern-recognition receptors (including TLRs) on the surface of host cells. Within hours, the non-specific innate immune system is activated.

Local Inflammation plays an important role in this immediate response to infection. Numerous players in the innate immune system, including neutrophils, monocytes, macrophages, complement factors, cytokines, antimicrobial peptides and acute phase proteins, are marshalled rapidly in a complex and highly regulated response to provide immediate defence against infection. These immediate responses kill pathogens and clear infection.

Over the ensuing days and weeks a more lasting immunity develops which involves specific antigen recognition and longer-term mobilisation / differentiation of T & B cells of the immune system. This evolution is the hallmark of the adaptive immune system and generates immune memory of the infection in the form of circulating antibodies and / or antigen-specific T cells. These components of immune memory generate a powerful response to future challenge by the same pathogen.

Crucial differences between the innate and adaptive immune responses are summarized below:

Innate vs. Adaptive Immunity... click for larger view