Malachi 4:2a "But unto you that fear my name shall the Sun of righteousness arise with healing in his wings..."
People and mammals have a multi-pronged immune system. When a bacterial infection is detected, cells begin to release nitric oxide, which kills bacteria, and they begin a cascade of other defense strategies. Ultimately, antibodies that directly attack any invader are produced.
In recent years, scientists have discovered that even in certain plants there are immune systems that respond to infectious agents. These researchers, working with mustard plants, subjected them to 15 proteins unique to infectious microbes. In response, the plants began producing nitric oxide. Measurable increases of this gas were noted within two minutes of exposure. And, just as in people and mammals, the nitric oxide began to kill real microbes. It also begins a chain of other strategies that would enable the plant to fight infection. In some cases, the entire plant – and not just the plant cells exposed to the infectious agents – also joined in fighting the "infection." Further research has shown that a number of genes in the mustard plant are involved in defense against infectious agents.
While no plants have yet been found to produce antibodies, it appears that many plants have immune systems similar to our own. At creation God said everything was "very good" so there were no diseases, but He gave us immune systems anyway because He knew we would fall, just as He already had our salvation worked out.