Immune Response to HPIV Infection When immune system is exposed to HPIV, innate immune response is activated which includes CD8+ and CD4+ T-cell responses. This response restricts viral replication of HPIV. When HPIV invades airway, immune system activates mucosal IgA response which protects airway lumen and epithelium by neutralizing the virus within the infected cells. IgA protection is short-lived. For long-term protection, neutralizing antibodies target F and HN glycoproteins. However, IgG build the most durable protection against re-infection. Maternal IgG provides protection to infants for the first few weeks or months of life.