INTRODUCTION TO THE IMMUNE SYSTEM

Innate and adaptive immunity

The course of a typical antibody response: The adaptive response takes 7-14 days to develop and mount a specific, protective immune response

An infection, and the response to it can be divided into stages and involves soluble mediators and cells of the immune system

Circulating lymphocytes encounter antigen in peripheral lymph nodes The adaptive immune response develops in the lymphoid tissues Circulating lymphocytes encounter antigen in peripheral lymph nodes Sentinel cells in the periphery pick up pathogens and transport them to the draining lymph node

The immune response is dynamic, involving cells re-circulating from the tissues via the lymphatics and blood system Innate immunity acts locally in the infected tissue, Adaptive immune responses develop in the lymphoid tissues, Effector cells multiply and migrate back to tissue.

Cellular components of the immune system

Myeloid cells in innate and adaptive immunity

Phagocytic cells Polymorphonuclear neutrophils Blood monocytes

Phagocytosis

The mononuclear phagocyte system

Mobilisation of defensive components of innate immunity

Macrophages and acute inflammation

B cells need T cell “help” to make antibody Lymphocytes mediate adaptive (pathogen specific) immunity: each cell has a unique antigen receptor B cells need T cell “help” to make antibody

Innate and adaptive (acquired) immunity are integrated

SUMMARY The response to an initial infection occurs in three phases: innate, early induced, adaptive.

Cells of the innate immune system:, macrophages, granulocytes, natural killer cells, immature B cells Release of soluble components to opsinise phagocytosis Release of toxic granules to kill micro-organisms, parasites or virally infected cells Release of soluble mediators to attract other leukocytes to site of inflammation Removal of dead or dying cells Transfer of infectious or toxic material to lymph nodes to initiate adaptive immune response

Cells of the adaptive immune system: Dendritic cells, T and B lymphocytes, Lymphocytes are generated with unique antigen receptors on their surface. Dendritic cells (and macrophages) present antigen to lymphocytes and provide activating signals Lymphocytes undergo clonal expansion and migrate back to area of infection as effector cells. T cytotoxic cells selectively kill virally infected cells T helper cells selectively help B cells produce large amounts of high affinity specific antibody. After infection, memory lymphocytes provide protective immunity

Natural killer cells kill virally infected cells Virally induced structures IFN-g released by infected cell “primes”other cells to kill it