This immune system has a variety of functions that can be classified under innate immunity and adaptive immunity. A variety of soluble and membrane factors allow many immune cell types to communicate with each other to recruit and activate the relevant actors and finely coordinate the body’s defenses. This chapter will describe the ten key characteristics of immunotoxicants and explore how they can be disrupted by chemical exposure leading to either immunosuppression (increased susceptibility to infections and cancers) or immunostimulation (potentially resulting in autoimmune diseases or allergies). Many substances, both synthetic and natural, have been identified as immunotoxicants, disrupting various aspects of the immune response, such as cell proliferation, differentiation, communication and signaling. This highlights the complexity of immunotoxicity and the need for further research in this field, to carry out a risk assessment and protect the general population. Two focuses are made: first, on the immunotoxicity of the PFAS (per- and polyfluoroalkyl substances) and second, on the interplay between the AhR signaling and the immune response.