Immunology 101: fundamental immunology for the practicing hematologist.

From an evolutionary perspective, the immune system developed primarily to protect the host from pathogens. In the continuous balance between killing pathogens and protecting host tissues, selective pressures have shaped the discriminatory functions of the immune system. In addition to protection against microbial pathogens, the immune system also plays a critical role in antitumor immunity. Immune dysfunction, either under- or overactivity, is found in a wide range of hematologic disorders. Here we review the fundamental features of the immune system and the key concepts critical to understanding the impact of immune dysfunction on hematologic disorders.