In the human function, which system functions is primarily to defend the body against diseases

The answer to this is the Immune System.

The Immune system is made up of a network of cells and tissues and organs that work together to protect the body. The white blood cells (also known as Leukocytes) is part of the immune system, they help destroy disease causing organisms or substances.
That is why when you fall ill and like a few days later you are all better, that is because of the immune system.
Sometimes when somebody has a weak immune system they fall ill more easily and could die because the bacteria that makes you sick isn't being destroyed by the immune system. Sometimes the person has to go to the hospital for treatment and some medications that can help support their immune system.

Hope this helped :)
Have a great day 

The immune system.

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The basic building blocks of dna and rna are nucleotides