Chapter 198

A detailed overview of the immune system is provided in Chapter 186. This chapter is focused upon those aspects of inflammation, tolerance, and genetics that are particularly relevant to autoimmune disorders.

The immune system exists to defend against infection and it has developed a beautiful yet complex system to distinguish the “self” from foreign moieties that are considered a threat. The immune system may be broadly divided into the innate immune system and the adaptive immune system. The innate immune system is distinguished by hard-wired programs to distinguish patterns characteristic of pathogens, also known as pathogen-associated molecular patterns (PAMPs). The adaptive immune system consists of T-cell and B-cell functions. T cells and B cells adapt to the environment and undergo a training process to learn to distinguish foreign antigens from self-proteins. As the training process will be distinct for each individual, there is opportunity for errors. Accordingly, most autoimmune disorders are thought to arise from errors in the process of establishing self-tolerance. In contrast, defects in the regulation of innate responses lead to autoinflammatory disorders. The distinction is based on the finding of autoreactive B cells and T cells in autoimmune disorders, as opposed to autoinflammatory diseases, which are associated with considerable inflammatory changes but an absence of autoreactive cells.

Neutrophilic infiltrates accompany many autoimmune diseases of childhood. Many of the vasculitides and nearly all types of arthritis are accompanied by a neutrophilic infiltrate. These cells may be extremely destructive, although they are ideally suited for eliminating bacteria. Housed within granules are a multitude of proteolytic enzymes, antimicrobial peptides, and proteins, which are released into the phagosome holding the bacteria. Of relevance to immune and inflammatory diseases is the potential for neutrophils to damage healthy tissue in a bystander phenomenon. Recurrent infections lead to end organ damage as a consequence of neutrophil release of granule material into the tissue. In autoimmune or autoinflammatory disorders, the neutrophils are recruited inappropriately but similarly cause substantial damage to tissues over time.

Tissue macrophages represent one of the main sentinel cells in the body; others are mast cells and dendritic cells. Certain pathogens are resistant to neutrophil killing, such as mycobacteria and fungi. In some autoimmune and autoimflammatory disease, characterized by high levels of TNF-α, macrophages can become activated in a poorly understood process, then aggregate and form granulomas.6,7 All granulomas are comprised of activated macrophages, called epithelioid cells, but not all activated macrophages are found in granulomas. These disorders are effectively treated with TNF-α inhibitors.

Natural killer (NK) cells were originally defined by their ability to kill tumor cells in vitro. NK cells kill their targets by forming a synapse and releasing cytotoxic granules into the synaptic cleft.10 The granules contain granzyme B, which activates apoptosis in the target cell, and perforin, which is thought to act as a pore to facilitate entry of granzyme B. The role of the NK cell is ...

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