Your Body’s Basic Defense System
The world is filled with both beneficial and harmful organisms, which only want to survive and reproduce, even if to the detriment of others, such as a host organism. Protecting yourself against macroorganisms and predators has become fairly easy. However, escaping from microorganisms is virtually impossible. Fortunately, your body has evolved a system that both prevents pathogens from gaining access to your body and destroys the pathogens that do enter before they can cause damage.
Physical and Chemical Barriers
The primary physical barrier your body uses to prevent infection is your skin. Your skin is a contiguous layer of cells and provides no access for pathogens to enter. Also, skin is composed of several cellular layers that pathogens must travel through to gain access to the deeper regions of the body and the circulatory system. If this were not a difficult enough obstacle for pathogens, new skin cells are continually added at the base of the skin and the older cells are progressively moved upward. Therefore, a pathogen’s struggle to penetrate intact skin is much like a salmon’s struggle to swim upstream.
The top layer of cells is shed daily, so pathogens have a limited amount of time before they are removed along with the dead skin cells. Most viruses require a living cellular host they can latch onto in order to survive and reproduce. The outer layers of skin are in fact dead cells that are compacted together into a watertight barrier. Viruses are unable to reproduce in this barrier before being shed from the surface.
While the skin is an extremely efficient pathogenic barrier, there are other means pathogens use to gain access to your internal tissues. Anything you eat or drink may contain pathogenic agents. Although many foods are processed to destroy harmful microorganisms, the act of touching the food with your hands (especially if not washed) may contaminate the food immediately prior to eating. Many of these pathogens, which may be lucky enough to escape the lymphocytes in the tonsils, soon encounter an inhospitable environment in the stomach. The hydrochloric acid (HCl) produced to chemically digest our food denatures and destroys most pathogens that enter the stomach.
Another means of protection relies on trapping pathogens in a thick proteinaceous (protein-filled) substance called mucus. In the nasal and respiratory tract, goblet cells secrete mucus to trap pathogens, then ciliated cells move the mucus upward to the larynx where the material is transferred to the esophagus and the HCl-filled stomach. Similarly, tears are produced to moisten and lubricate the sclera of the eye. Tears also contain mucus and can trap any pathogens on the surface and remove this material from the eye via the nasolacrimal duct, which drains into the nasal cavity.
Phagocytosis and Opsonization
Many white blood cells remove pathogens and debris by phagocytosis. Macrophages are particularly adept at this. These cells remove any foreign material, regardless of what it is. Additionally, when pathogens are decorated with antibodies and/or are covered with certain markers called complement factors (discussed later), the phagocytic activity of these cells is increased. This elevation of phagocytic activity resulting in more rapid removal of pathogens is called opsonization.
Complement Factors
In an immune cascade, not unlike what happens in the clotting cascade, previously inactive complement factors, which are proteins in the blood plasma, become activated during a pathogenic attack.
The classical activation pathway is initiated via antibody opsonization of a pathogen, which triggers complement factor 1 (C1) to activate C4, then C2. These combine to enzymatically activate C3. These factors have two portions: a and b. Thus, C3 is split upon activation into C3a and C3b.
An alternative pathway can lead to the activation of C3 directly when carbohydrates are recognized. Carbohydrates are foreign to the human body, yet rather common in bacterial cells walls.
Once split, each portion of C3 is bioactive, meaning it has an effect on human tissues. C3a along with another fragment, C5a, leads to an increase in inflammation in local tissues. C3b, along with antibodies, further opsonizes pathogens. Additionally, C3b activates factors C5–C9 to form a protein pore, which inserts into the plasma membrane of pathogens and results in their death. The structure that is formed is referred to as the attack complex.
Cytokines
Many chemicals produced and secreted by immune system cells have far-reaching and powerful effects on the body. Collectively these substances are referred to as chemokines, cytokines, or lymphokines. The vast array of these chemical mediators is beyond the scope of this section; however, the general concept of their actions is essential to understanding immune function. Many of these cytokines play critical roles in the activation of the immune system during an immune response, while others are required to slow and even halt such a response.
Inflammation
Several cytokines affect the blood vessels and allow more blood to flow into the tissue (vasodilation) and to make the capillary endothelial cells easier for WBCs to migrate between (increase vascular permeability). As a consequence, more plasma leaks into the interstitial tissues more rapidly than can be removed by lymph vessels. At this point, swelling (edema) occurs. Mast cells (basophils that have left the circulation) secrete powerful inflammatory cytokines such as histamine, which makes the vessels leaky. Additionally, they secrete heparin, a molecule that inhibits the clotting cascade by preventing the activation of thrombin. If the vessels were to leak and heparin was not produced, a blood clot would rapidly form and prevent the passage of fluid and WBCs into the tissues.
Fever
Another effect of some cytokines is to increase the body’s temperature above normal. A group of cytokines is called the endogenous pyrogens because of their ability to cause fever. The increased temperature helps fight an infection. Higher body temperatures destroy some pathogens directly, and reduce the effects of the toxins that bacteria can create in the body. Increased temperature also increases the division, migration, and metabolism of the immune system cells and gives them an attack advantage over many pathogens.
When is a fever dangerous?
Only when the body temperature reaches or exceeds 105°F will your cells be destroyed. However, any fever lasting for more than a few days and in excess of 101–102°F should lead one to seek clinical treatment from a doctor.