Chitin is a structural polysaccharide that contains nitrogen. It is the primary component of exoskeletons of arthropods such as crustaceans and insects. Humans mount an immune response to chitin and it has been implicated in the development of Alzheimer’s Disease.
Possible Role of Chitin-Like Proteins In The Etiology Of Alzheimer’s Disease, Journal of Alzheimer’s Disease, 2018
Chitin levels are elevated in Alzheimer’s disease (AD) patients, not only in the central nervous system but also in the cerebrospinal fluid and plasma.
Chitin can be toxic for neurons, and its accumulation may lead to the development of AD.
Humans mount an immune response to chitin:
Humans and other mammals have chitinase and chitinase-like proteins that can degrade chitin; they also possess several immune receptors that can recognize chitin and its degradation products, initiating an immune response.
Chitin is sensed mostly in the lungs or gastrointestinal tract where it can activate the innate immune system through eosinophils or macrophages, as well as an adaptive immune response through T helper cells. Keratinocytes in skin can also react to chitin or chitin fragments.
 Chitin And Its Effects On Inflammatory And Immune Responses, Clinical Reviews in Allergy and Immunology, April 2018