**3. Phytochemicals**

Phytochemicals or phytonutrients are produced by plants. These chemicals are the basic principle for the nutritive as well as pharmacological action of plants. These chemicals also provide a defense mechanism to the plants against plant pathogens.

The main pathogens belong to the class of bacteria, fungi, protozoa, and viruses [9]. The common vegetables and fruits, such as broccoli, berries, carrot, tomato, garlic, seeds, and onion, contain these chemicals in large quantities which increase the immune capacity of the body.

There are two types of metabolites in all living organisms including plants. They are primary metabolites, such as sugar, amino acids, nucleotides, and lipids, which are essential for their functioning. Plants also produce certain molecules which are not used directly in their life process such chemicals are called secondary metabolites synthesized by biochemical pathways, such as alkaloids, flavonoids, glycosides, phenolics, and terpenoids discovered with various properties, such as induction of flowering, protection from pathogens, protection from the external environment, attractant or repellant for pollination, antimicrobial, antioxidant, anti-inflammatory, and antitumor activity. These chemicals and their product are used in herbal medicine and modern medicines [10–12].
