Adaptive immunity, where an organism uses a host of specialized blood cells to defend against pathogens is (as far as we've been able to figure out) an evolutionary phenomenon of vertebrate animals. Multicellular animals also have innate immunity that likely evolved from unicellular defenses against infection by viruses and against predation. Some of the adaptive immunity blood cells (macrophages) look and act like single-celled amoebae in how they chase after and engulf their food. If you're not familiar with Kurzgesagt - In A Nutshell, they have a wonderful series of videos on this (and many other) science topics. I assign these videos to my entry-level college biology students to help them understand immunity.

• Bateriophages - viruses that use bacteria as their hosts
• The Complement system
• The Immune System Explained - Bacterial infection
• How the Immune Sytem Actually Works
• You Are Immune Against Every Disease

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