**7. Immunologic reactions**

Immunological reactions are inflammatory conditions that clinicians and patients may encounter before, during, or months or years after treatment [62]. Approximately 30–50% of patients are involved. There are two types of reactions that are linked to different immunological mechanisms that are not fully understood: Type 1 and type 2 [63]. These immunological reactions may mimic the drug reaction, the clinician should pay attention to that they are not drug reactions and that treatment should not be interrupted. In both types of reactions, general weakness, fatigue and fever can be observed. Other clinical findings differ according to the developing reaction.

*Type 1 reaction:* Typically occurs in TT and BT. It is due to an increase in cell-mediated immunity and a delayed-type hypersensitivity reaction to *M. leprae* antigens [55, 57, 63, 64]. Characteristic clinical findings are increased inflammation in existing lesions, formation of new lesions, pain and sensitivity in nerves (neuritis), progressive neurological failure.

*Type 2 reaction:* Typically, BL and LL patients develop when treatment begins. Pregnancy and pyogenic infections may induce. The type 2 reaction due to the formation of immune complex with hyper humoral immunity represents cutaneous and systemic small vessel vasculitis [55]. Characteristic clinical findings; painful nodular lesions (erythema nodosum leprosum) that occur suddenly, severe swelling and pain in the joints, iridocyclitis, orchitis, sensitive lymphadenopathy, glomerulonephritis, and hepatosplenomegaly [57, 63, 64].

*Lucio phenomenon* is a rare complication characterized by sudden-onset, necrotizing cutaneous small vessel vasculitis in diffuse, untreated LL patients in the Mexican and Caribbean region [65]. Lesions that are painful but have no increase in temperature can be cured by scarring. Ulceration especially may be observed in knees.
