The Body's Defenses
by Carlo Barbasa
The first line of defense is the skin and mucous.
The intact skin is a barrier that cannot normally be penetrated by
bacteria or viruses, but tiny 
abrasions can let them through.
In general, epidermal secretions such as sweat, tears, and oils give the
skin a pH of 3 to 5, making unsuitable conditions for numerous microorganisms.
Saliva and mucous also provide similar protections to tissues such as the
mouth. Mucous also catches and
traps airborne particles that might enter the lungs.
In the windpipe, cilia sweep out microbes trapped by mucous, which is
then expelled.
On the cellular level, the body depends on phagocytosis (the ingestion of invading particles by certain types of white blood cells), the second line of defense. The entire population of white blood cells consists of neutrophils, monocytes, macrophages, and eosinophils.
Neutrophils are the most abundant of the white blood cells, 60%-70%. These are the anti-microbial kamikazes. They drift from the blood and destroy microbes in the infected tissue. Typically, they destroy themselves in the process.
5% of white blood cells are monocytes, which mature into macrophages (big eaters). These are the war-veterans, living the longest of all the cells. They actually consume and digest microbes. But many bacteria have evolved and found ways to evade these big eaters. Some macrophages are permanent residents of organs and connective tissues. They are especially numerous in the lymph nodes and the spleen.
Eosinophils are the least active of the phagocytic cells, consisting of only 1.5% of the population. But this proportion is perfect. If phagocytic cells were military weapons, eosinophils would be the cruise missiles. They take down big parasitic invaders, like worms. They position themselves near the target and discharge destructive enzymes from their cytoplasmic granules.
The immune system is the body’s third line of defense. It develops a specific response to every type of foreign microbe, toxin, or transplanted tissue. The immune system can distinguish between thousands of different antigens (foreign substances) and dispatch an antibody for each one of those potential toxins or microbes. The immune system can also remember antigens that it has previously encountered (acquired immunity) and respond to its treatment more quickly. To prevent the attack of our own body’s cells, the immune system can pick out foreign cells from amongst our own body cells.
Two different types of immunity occur in our bodies. Active immunity is gained when you recover from an infectious disease, such as chickenpox; you’re not very likely to get chickenpox if you got them as a child. We can also gain active immunity through vaccination. Passive immunity is inherited from parents to children, but is only sustainable for a few weeks before the infant must develop its own defenses or be vaccinated.
(figure on right displays the antigen response)
Clonal selection is
the antigen-specific selection and cloning of lymphocytes.
The immune systems ability to defend against microbes depends on the
number of lymphocytes available to recognize antigens.
Each lymphocyte responds to one specific antigen.
Therefore, immunal diversity and specificity depend on there being
millions upon millions of lymphocytes, each one able to respond to a specific
antigen.
(figure on left shows the microbial process of cloning itself to spread over an entire organism)
The immune system’s capacity to distinguish self from nonself is critical in blood transfusion and transplantation. Organs transplanted from a donor to an incompatible recipient will be rejected in a matter of hours, and eventually destroyed. Blood transfusions can be especially dangerous. Type A and type B patients require either type A or B blood, respectively. Type O are universal donors, yet can only receive from O type. Type AB can receive from any donor, but can only donate to type AB.
If the immune system begins
to malfunction, some not so serious and some serious health complications can
occur. Autoimmune diseases
destroy regularly occurring bodily cells, such as insulin, cartilage, and
nucleic acids. Allergies are
simply the body overreacting to the presence of pollen or other allergens.
The worst case is immunodeficiency.
The most common is the acquired immunodeficiency syndrome (AIDS).
Simply put, the virus responsible, the human immunodeficiency
virus (HIV), literally attacks white blood cells and destroys them.
Then the body has no defense of its own to protect itself from even a
common cold virus.
