The way  you describe it so far  the sneezing doesnt seem cause for concern and no medication is needed . However if other symptoms like fever , cough , vominting , fast breathing or feeding difficulty develop you must go to a Pediatrician for a physical check up . As far as the immunity query : After birth till about 3 to 5 years of age a newborn baby is exposed to about 5000 viruses . And immunity develops as he is exposed to them and antibodies develop within him . Most of these viral infections are self limiting and spontaneously subside and may only require symptomatic treatment for fever , Nasal  discharge etc . Generally we refrain form giving antibiotics for seemingly viral infections as they cause more harm rather than good

Maintain normal body temperature.Avoid direct use of AC. If sneezing worsens then need consultation with Paediatrician