**7. Dendritic cells and its potential benefits to combat different diseases**

DCs are considered key cells as the first line of defense against viruses and to induce adaptive defense. In the innate immune response, they can exert virus phagocytosis and produce cytokines to activate NK cells to eliminate virus infected cells. In adaptive immune response, DCs induce differentiation of Th1-cells that in turn induce activation of antigen specific cytotoxic cells, macrophages, and antibody production to participate in viral clearance.

For the elimination of bacteria, a specific immune response is required, for intracellular bacteria a Th1 response is required as well as cytotoxic T lymphocytes, the latter to produce IFN-γ and can kill the cells that have been infected, in this response the Il −12 is important and its production by DCs requires stimuli derived from pathogens as well as from CD4+ T-cells; on the other hand, for extracellular bacteria a Th17 response is required, in this response DCs play an important role in producing pro-inflammatory cytokines so that a Th17 response can be given, thus these cells coordinate the recruitment of neutrophils that phagocytize extracellular bacteria and thus eliminate the bacterial infection.

DCs participate in the immune response against different opportunistic fungi, the latter are capable of producing different diseases including vulvovaginal candidiasis, oral candidiasis or disseminated candidiasis (*Candida albicans*), invasive pulmonary asperilosis (*Aspergillus fumigatus*), pneumonia (*Pneumocystis carinii*), cryptococcosis (*Cryptococcosis neoformans*). DCs recognize specific structures of fungi such as carbohydrates, proteins, and nucleic acids. This recognition through the PPR activates signaling pathways that lead the DCs to a state of maturation and secretion of cytokines which play an important role in host defense against fungal infections, generating a response either of the Th1 type or Th17.

During parasitic infections, DCs play an important role, since, through them, the body can mount a specific immune response, mainly mediated by T lymphocytes. The DCs recognize the antigens of the parasites, and in the first instance, they induce a Th1-type immune response, characterized mainly by the production of pro-inflammatory cytokines and mediators. Nevertheless, parasites have the ability to polarize, through the activation of DCs, towards a Th2-type immune response, characterized mainly by the production of anti-inflammatory cytokines, eosinophilia and mastocytosis. However, due to the great diversity of parasites that exist, as well as their phenotypic variability, which involves different stages of antigenicity, conditioned by the life cycle of the parasite itself, these microorganisms have the ability to develop strategies that allow them to evade the immune system and facilitate their survival and spread in the host. Despite the different immune responses that the host assembles in contact with the different diseases caused by these microorganisms, DCs are very important, since they represent the junction point between the innate and adaptive immune responses, allowing the host to differentiate the type of microorganism by which it has been invaded and thus be able to mount a specific immune response.

### **8. Conclusions**

Dendritic cells are a key cell type in the recognition of intracellular and extracellular pathogens through the different receptors that they express. The maturation of the DCs is an important event since through this mechanism these cells acquire the ability to express MHC as well as costimulatory molecules, thus conditioning the presentation of the antigen, producing cytokines and mounting immune in order to kill the invading pathogen. The response can be mediated by the PRRs as they will recognize different structures of the invading microorganism and execute a defensive response with the purpose of eliminating the invading microorganism through the production of antimicrobial cytokines and intermediaries, as well as activating transcription factors to produce cytokines that have an important role in the polarization of the T helper cell during priming by DCs.

### **Acknowledgements**

Thanks to the authors who collaborated in the writing of this chapter: Dr. en C. José Luis Muñoz-Carrillo; Dr. en C. Oscar Gutiérrez-Coronado; Dr. en C. Juan Francisco Contreras-Cordero; Dra. en C. Paola Trinidad Villalobos-Gutiérrez; Dr. Luis Guillermo Ramos-Gracia, and Dra. Jazmín Monserrat Vargas-Barboza; as well as the Universities involved: Cuauhtémoc University Aguascalientes, University of Guadalajara, and Autonomous University of Nuevo Leon for financial support for chapter publication.
