A healthy immune system keeps the body protected from invading pathogens. It's components, whether they live for a day or for years, are essential in protecting us. Bodily fluids such as tears, respiratory secretions and stomach acid, all play their roles. A well nourished body is actually very well-equipped to handle and remove foreign substances that are unhealthful to the native environment.
The Innate arm of the immune system is our first line of defence. We are born with it and it is made to be ready to take on a range of assaults by having robust control of the body's entry and exit points.
The Adaptive arm however is able to produce uniquely targeted responses to specific threats as they present themselves. It therefore can change over a lifetime as the body builds up memory of the infections experienced since birth.
In a healthy body, the innate defences are enough to keep the invader out before the adaptive defences develop their arsenal. The two systems are closely inter-connected to keep us alive.
For the innate arm we have the skin, the lungs with their secretions, the gut with its stomach acid and various secretions, tears and earwax. The secretions trap the invaders and expel them before they can do any harm.
The adaptive arm produces antibodies. White Blood Cells called lymphocytes have 2 types: B cells and T cells. They are produced in the bone marrow and mature in the bone marrow and Thymus respectively. After maturation, T cells migrate to the spleen and lymph noses while the B cells migrate to lymphoid tissue.
B cells carry antibodies that bind to proteins, carbohydrates or nucleic acids of invading pathogens.
T cells destroy other cells that harbor any foreign material, modulate the activity of other immune cells, can turn off the immune response and in the case of pathogen re-exposure, have the ability to respond rapidly.