The human immune system is the most amazing defense system on earth. The immune system has four recognized basic functions: (1) it recognizes the difference between foreign invasive substances such as bacteria, viruses, parasites, and chemical toxins and its own cells; (2) it responds to each invading pathogen and toxic substance in a unique way; (3) once the immune system encounters a specific invader, it can generate a memory of these foreign substances in order to quickly respond to future invasions; (4) once the immune invaders are subdued, the immune system shifts into a relatively dormant state to await new threats.

This multidirectional immune system is composed of more than 1012 (1 trillion) cells with a total mass of about 1 kg (2.2 lbs). The immune system has two types of responses to foreign and abnormal substances.

The first response involves the secretion into the bloodstream of protein-based immunoglobulins, often called antibodies. The activation of immunoglobulins is called the humoral immune response. This response is directed against foreign microorganisms such as bacteria or viruses.

The 2nd type of response is the cellular immune response or cell-mediated immunity. As the name suggests, this response relies on the interaction between various types of immune cells called lymphocytes. This cell-mediated immune response targets body cells that have become cancerous or infected by pathogens such as viruses, bacteria, fungi, and parasites.

The body's immune cells can react to substances called antigens such as enzymes or proteins. These substances are foreign to our healthy cellular tissues and stimulate the immune system to mount an immune response. It is estimated that the body can react to over 100 million antigens. However, many infectious substances mutate easily, so our immune system is often confronted with a variety of changing antigens, which is why we are prone to recurring viral infections such as colds and flu. Some parasites, such as those that cause malaria, also mutate rapidly to avoid detection by our immune system, which is why people with malaria often experience periodic attacks. Bacteria such as Staphylococcus and Mycobacterium spp. also mutate in these forms to become resistant to antibiotics, and health experts believe that this is a worldwide health problem that requires a one-to-one immune response for each of the viruses, germs, and parasites that continue to change their forms. Surprisingly, however, for the most part, our immune systems are able to quickly adjust to and subdue these life-threatening pathogens.