**16. Coinfections in immunocompromised patients**

The outcome of coinfection is the result of diverse interactions involving the host and the parasite's genetic background and the environment. In these infections, few reports have explained the immune mechanisms implicated, because there is considerable variability in each combination of microorganisms that produces an infection.

The immunological mechanisms that occur in the eye are similar to the rest of the immune system. However, there is more regulation in the silencing response in order to prevent damage from infection and inflammation, and immune mechanisms preserve the functionality of the cornea [63].

In the following, studies that investigated the most common corneal coinfections are reviewed. These reports show the critical role of pathogens and the pathogenesis generated by the host immune response.

Vernal conjunctivitis is an example of how the immune phenotype affects the response to the infection. Patients with vernal keratoconjunctivitis have a family history of atopic diseases such as allergic rhinitis, asthma, and eczema [64]. A theory suggests that patients with a history of atopy are susceptible to intracellular infections because they have a Th2 immune phenotype [65, 66]. Kerr and Stern showed a polymicrobial infection in two patients with vernal kerato‐ conjunctivitis and corneal ulcers [67].

Although regulation of the immune response in the eye is controlled locally, in immunocom‐ promised patients with human immunodeficiency virus (HIV), it is evident that the privilege is broken by the depletion of T CD4+ cells, and infections can occur. In addition, several pathogens can remain latent (herpes virus, bacteria, fungi, parasites). The clinical manifesta‐ tion produced by the herpes virus can be conjunctivitis, blepharitis, intraocular inflammation, retinitis, or keratitis [68]. In particular, herpes simplex virus types 1 and 2 (HSV-1 and HSV-2] infect 50 % to 90 % of the population infected with HIV, causing ocular herpes and genital orofacial herpes in different geographic regions [69]. Herpetic retinitis has a high incidence; however, few cases have been reported. Faber *et. al*., studied eyes from 25 cases with AIDS with an immunohistochemical test. *Cytomegalovirus* was found in 60 % of the cases and was related to retinitis, while in another case series, 36.64 % of 131 patients were diagnosed with *Cytomegalovirus* retinitis [70, 71]. Freigassner *et. al*., documented a case with the Epstein-Barr virus and *Cytomegalovirus* in a patient with AIDS [72]. Other studies showed *Cytomegalovirus* or herpes in isolated cases [73, 74, 75].

Opportunistic microorganisms such as *Toxoplasma* spp., herpes zoster virus (HZV), and *Pneumocystis* spp., participates the least in coinfections.
