SARS-CoV-2 is the etiological agent of the coronavirus disease-19 (COVID-19) and is responsible for the pandemic that started in 2020. The virus enters the host cell through the interaction of its spike glycoprotein with the angiotensin converting enzyme-2 (ACE2) on the host cell’s surface. Antibodies present an important role during the infection and pathogenesis due to many reasons, including the neutralization of viruses by binding to different spike epitopes. Therefore, measuring the neutralizing antibody titers in the whole population is important for COVID-19’s epidemiology. Different methods are described in the literature, and some have been used to validate the main vaccines used worldwide. In this review, we discuss the main methods used to quantify neutralizing antibody titers, their advantages and limitations, as well as new approaches to determineACE2/spike blockage by antibodies.

(1) Background: Modification of the structural elements of allergens is widely used in the field of allergies. The goal of the present research was to express, purify, and characterize the shortened recombinant group 5 allergen of Dermatophagoides pteronyssinus (rDer p 5). (2) Methods: rDer p 5 storage stability and aggregation capacity were explored through in silico analysis, dynamic light scattering (DLS), and SDS-PAGE. Serum IgE reactivity and cytokine amount were investigated in sera or cell culture supernatants through ELISA, MULTIPLEX^®, and Western blot analysis using sera from sensitized humans from Brazil, Colombia, and Ecuador. (3) Results: Dimeric rDer p 5 was detected through native PAGE, and this result was confirmed by data from DLS. The protein was thermically stable, as it did not degrade at 4 °C for 21 days. The shortened rDer p 5 was classified as a major IgE allergen in Brazil and Colombia, but minor in Ecuador. IL-13, IL-10, IL-1β, and IL-6 were significantly elevated in the sera of rDer p 5-reactive patients. The same cytokines plus IL-5 were more secreted by human cells upon rDer p 5 stimulation. (4) Conclusions: N-terminal peptide deletion led to a higher rDer p 5 folding stability, which, even though dimeric, was an IgE-reactive protein. Therefore, rDer p 5 could be used for molecular diagnostic applications or as backbone for hypoallergen design.