First-class proteins are proteins found from animals, and second-class proteins are proteins found from plants. First-class proteins come from meat, eggs, fish and dairy products while non-animal related foods, like vegetables and fruits, provide second-class proteins.
Both types of proteins provide valuable nutrients as well as essential amino acids. Many of the amino acids that first- and second-class proteins provide aren't naturally made by the human body, which makes them very important to obtain through a healthy, balanced diet.
First-class proteins contain a necessary amount of these important amino acids, but second-class proteins only contain some of them. Because of this, vegetarians must be very careful about eating a balanced, nutritious diet as they could begin to suffer from protein deficiency. This is a very serious problem and it can lead to health issues. A sign of protein deficiency is when someone's abdomen begins to swell.
Growing children especially need protein as do pregnant mothers. Protein is essential to help children grow and be healthy, and since pregnant women are rearing another living human, they need lots of protein to keep themselves and their baby healthy and strong. Even mothers that have already had a child need to make sure they have plenty of protein in their diet because if they are breastfeeding, their body is processing lots of nutrients for the baby's sustenance and they need to replace those nutrients constantly in order to stay healthy themselves.