HOST DEFENSE AGAINST
INFECTION Since human beings exist in a veritable sea of microbes, defense against microbial attack is important for survival of the species. The skin, mucous membranes (interior of the nose, mouth, vagina, rectum), and intestines are colonized by large numbers of bacteria. These
organisms are called the normal flora. They are protective and form a barrier against invasion by foreign
microbes due, in part, to the metabolic products of these organisms, which may be harmful to potential invaders. In addition, utilization of essential nutrients by the normal flora may "starve" pathogenic invaders. If the normal flora are destroyed by agents such as antibiotics, infection may become more likely. The skin and skin secretions are an effective barrier to invasion by most microorganisms. Tears and saliva have intrinsic antibacterial activity. The respiratory tract is kept clean by coughing and sneezing and by the continuous upward movement of particle-trapping mucus, which is transported by the continuous beating of tiny fibrils, or cilia, of the respiratory lining cells. Once microorganisms invade body tissues, mechanisms come into play that strive to neutralize and
destroy them. These defenses may be divided into two major groups: humoral and cellular (for example,
phagocytic cells). The humoral (or fluid) mechanisms include the antibody and complement systems.
Antibodies Antibodies are proteins produced by specialized white blood cells called lymphocytes. Antibodies have a strong chemical affinity for specific types of foreign biologic matter. Antigens are components of foreign material (for example, microbes) that interact with antibodies. Thus, specific anti-influenza virus antibodies or specific antistaphylococcal antibodies may be present. The interaction of antibodies and components of microbes may serve to destroy, inactivate, or prevent the multiplication of microorganisms. In addition, when an antibody attaches to the surfaces of microorganisms, it makes these microbes more ingestible by phagocytic cells, which engulf and destroy invading microbes.
Complement
In addition to antibodies, another group of proteins circulates in the blood and body fluids, constituting the complement system. These proteins have the ability to destroy certain pathogens by interacting with them. This interaction may be promoted by antibodies or may take place in the absence of antibodies. Thus, blood serum from healthy individuals, lacking antibodies, will destroy many species of bacteria, and this additional means of protection may partially explain why most bacterial species are unable to cause disease in humans.
lymphocytes There are two major types of lymphocytes, B and T. B lymphocytes have the ability to produce antibodies directed against specific foreign antigens to which they are exposed. This is the basis for prevention of disease by immunization. For example, patients may be infected with an attenuated, or weakened, strain of polio virus. The lymphocytes will then make antibodies against the polio virus, which will also be active against wild type, fully virulent organisms. Thus, when patients later come in contact with these organisms, they are immune. T lymphocytes respond to foreign antigens by releasing control substances that help or suppress the function of B lymphocytes and phagocytic macrophages. Certain T lymphoctyes have the ability to kill infected cells.
Inflammation
Inflammation is the phenomenon by which the body responds to an irritant or to an infection. Small blood vessels dilate and leak fluid, producing swelling, redness, and warmth. Phagocytic white blood cells enter the area, adhere to the lining of the blood vessels, and migrate into the tissue to attack microbial invaders. Fever, or elevation of the normal body temperature (37¡ C/98.6¡ F), is a common response to many infections. Fever appears to aid the host in fighting infection by enhancing the immune response. Feverresults when a product of phagocytic cells known as endogenous pyrogens acts on the hypothalamus of the brain, which controls body temperature, to raise the body's "set point."
Phagocytic Cells
Three major types of phagocytic cells are involved in protecting human beings against infection. The polymorphonuclear neutrophil is a motile phagocytic cell that can engulf and destroy pathogens. Ingestion is more efficient when the pathogens have been coated with antibodies or complement or both. Neutrophils, the cells that constitute pus, migrate rapidly to the site of trauma or infection in an attempt to destroy microbes. Macrophages are larger, slower-moving phagocytic cells that have the special capability of killing organisms that can survive ingestion by polymorphonuclear neutrophils. Thus, such organisms as Mycobacterium tuberculosis (the agent that causes tuberculosis) are not destroyed by polymorphonuclear neutrophils, but they are destroyed by macrophages, whose activity is increased by substances produced by T lymphocytes. Macrophages are found in the liver, spleen, lungs, and bone marrow. The eosinophil is a cell that has special potential for killing multicellular microbes such as helminths. Patients with infections such as trichinosis caused by a helminth will frequently have high eosinophil count in the blood. Phagocytic cells kill organisms by engulfing them and enclosing them in a pouch called a phagosome. This pouch then becomes bathed with active metabolites of oxygen, including hydrogen peroxide and superoxide, and various enzymes found in the phagocytic cell. Most microbes are rapidly killed by these cells.
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