Abstract
This chapter describes immune responses to the six major types of pathogens: extracellular bacteria, intracellular bacteria, viruses, parasites, fungi and prions. Innate immunity mediated by neutrophils, NK cells, NKT cells, γδ T cells, complement and microbicidal molecules prevents infection or slows it until adaptive immunity can also respond to the pathogen. Extracellular entities are coated in antibody and cleared by antibody- and complement-mediated mechanisms. Parasitic worms are prevented from anchoring in the host by IgA and IgE antibodies. IgE triggers mast cell, basophil and eosinophil degranulation and the release of toxic mediators. Intracellular bacteria and parasites as well as viruses are eliminated by CTLs, NK cells, NKT cells and γδ T cells secreting cytotoxic cytokines and/or carrying out target cell cytolysis. Macrophage hyperactivation and granuloma formation may be triggered to confine persistent invaders. Th1 and Th17 responses support cell-mediated immunity against internal threats, whereas Th2 responses support humoral immunity against external threats. Each type of pathogen has evolved to evade immune responses by avoiding recognition or inactivating various leukocyte effector mechanisms.