There are some interesting theories that our personalities are determined by the blood type.

According to WellnessKeen:

“Blood type, also known as blood group, though not unique, differs from person to person. For a safe transfusion, blood types need to be matched in a specific way. Broadly, there are four major groups of blood, namely A, B, AB, and O. These are determined by the presence or absence of two antigens, A and B, on the surface of red blood cells (RBCs).

Blood types are classified according to what is known as the ABO blood grouping system. These groups are further classified as per the Rh factor (rhesus factor). The Rh factor is the presence or absence of D antigen on the red blood cells.

The negatives A-, B-, AB-, and O- are the blood groups that do not have any antigens on the red blood cells. The positive ones, of course, have antigens present on them.

This ABO system, which was established in the early 20th century, has an international standing, and has a long time affiliation with the International Society of Blood Transfusions.”

Historically, it is believed that people with type 0 blood observed their environment and were aggressive hunters. It is the oldest blood type, tens of thousands of years old, so people with it are considered ‘universal donors.’

Here are some important facts about people with this blood type:

• These people have strong immune systems, strongest stomach acids, the thinnest blood, and live the longest.
• Dr. Peter D’Adamo’s ‘blood type’ diet recommends these people to eat lean meat, and avoid grains and dairy, as well as alcohol and caffeine.
• •People with type 0 blood are predisposed to iodine deficiency, ulcers, and thyroid issues, and the unhealthy stress levels and poor habits make them prone to insulin resistance, obesity, low activity of the thyroid gland, anger, and hyperactivity.
• People with Type O blood are twice as likely to be bitten by mosquitos.
• Individuals with Type O blood are more than one-third less likely to develop pancreatic cancer than other blood types.
• When it comes to their personality traits, they are energetic, focused, productive, and possess leadership skills. Moreover, blood type 0 is linked to organization, commitment, practicality, and logic.
• Individuals with this blood type are ambitious, honest, passionate, optimistic, and confident.
• Their high adrenaline and noradrenaline levels make them prone to the flight or flight instinct, so they should avoid gambling and extreme physical activity, especially when stressed or tired. 
• When under stress or irritated, these people should take part in some physical exercise for at least half an hour. They will benefit from brisk, regular exercise, and it will maintain their emotional balance. Exercise is especially important for them and combined with a proper diet, it will do wonders for their well-being.

Source: theheartysoul.com