https://sci.mu.edu.iq/%d8%ad%d9%84%d9%82%d8%a9-%d9%86%d9%82%d8%a7%d8%b4%d9%8a%d8%a9-%d8%a8%d8%b9%d9%86%d9%88%d8%a7%d9%86-acquired-immunity/