Chapter Capsule

A. The first line of defense is composed of barriers that block the pathogen at the portal of entry; the second line of defense includes generalized protective cells and fluids in tissues; and the third line of defense includes specific immune reactions with microbes that are required for survival.

B. First Line Defenses: Physical barriers are anatomical structures such as skin, mucous membranes, and cilia; chemical barriers include lysozyme in tears and saliva, and gastric acidity; genetic barriers can be a lack of susceptibility to an infectious agent due to the specialization of a microbe for an exact host.

A. Immunity is the development of specific resistance to an infectious agent. Functions of the immune system are: surveillance by specialized white blood cells (WBCs) for markers or receptors (molecules on surface of cells) in tissues; recognition by WBCs that detect and differentiate normal markers (self) from foreign markers called antigens (nonself). WBCs are programmed to destroy and remove any foreign material, while self is usually unaffected.

B. Components of Immunity: The immune system is a diffuse network of cells, fibers, chemicals, fluids, tissues, and organs that permeates the body. The structure at the cellular level includes separate but interconnected compartments: the reticuloendothelial and mononuclear phagocyte system, a continuous network of fibers and phagocytes that surrounds the tissues and organs; the extracellular fluid (ECF), a liquid environment in which all cells are bathed; the lymphatic system, a series of vessels and organs that carry lymph from tissues; and the bloodstream, which circulates blood to all organs. The constant communication among the compartments ensures that a reaction in one will be transmitted to another.

A. Composition of Whole Blood: Plasma is a clear, complex liquid that contains nutrients, ions, gases, hormones, antibodies, albumin, and waste products dissolved in water. Serum is plasma minus the clotting factors. Blood cells are formed by hemopoiesis in particular bone marrow sites. From stem cells, three main lines of cells are differentiated—white blood cells (leukocytes), red blood cells (erythrocytes), and megakaryocytes that give rise to platelets.

B. Functions of Leukocytes: Leukocytes, the primary cells of host defenses and immunity, are either granulocytes or agranulocytes. Granulocytes contain distinct granules in cytoplasm and include neutrophils, which function as phagocytes; eosinophils, which function in worm and fungal infections; basophils, which are involved in allergic responses, along with tissue mast cells. Agranulocytes lack noticeable granules and include monocytes and lymphocytes. Monocytes function as blood phagocytes and give rise to macrophages in tissues. Two main types of lymphocytes are B cells, which produce antibodies as part of humoral immunities, and T cells, which participate in cell-mediated immunities (CMIs). Leukocytes are motile by ameboid motion, can be carried between the endothelial cells of small blood vessels (diapedesis) and enter surrounding tissues, and can respond to tissue injury or infection by migrating toward chemical signals (chemotaxis).

A. Inflammatory Response: This complex system responds to tissue injury (infection, burn, allergy) by mobilizing the immune system against pathogens, repairing damage, and clearing infection. Its common signs and symptoms are redness, heat, swelling, and pain.

B. Stages of Inflammatory Response: Blood vessels narrow and then dilate in response to chemical mediators and cytokines released by injured tissues and white blood cells. Next, the buildup of fluid from edema swells the tissues and keeps infection from spreading, and attracts neutrophils to engulf debris and microbes. WBCs, microbes, debris, and fluid collect to form pus; macrophages clean up the residue of inflammation; lymphocytes carry out immune reactions such as antibody formation; and healing occurs. Long-term inflammation can result in injury and disease (granuloma). Fever, an increase in body temperature above normal, is due to pyrogens, substances that alter the temperature setting in the brain. Fever can slow microbial multiplication and stimulate the immune response.

C. Phagocytosis is a process whereby foreign materials are engulfed and destroyed. Neutrophils engulf small particles, microbes, molecules; macrophages are larger cells that scavenge large packets of cellular debris and extract antigenic information. Macrophages live in a specific tissue or organ (liver, lung, skin) or are free and wandering. After materials are engulfed by the cell into a phagosome vacuole, lysosomes containing powerful chemicals unite with the phagosome and destroy its contents.

A. Interferon (IFN) is a family of proteins produced by leukocytes and fibroblasts in response to infection, cancer, or various immune signals. Alpha and beta interferon are natural infection-fighting substances, and gamma interferon is an immune activator and regulator. All three also function as antiviral and anticancer cytokines. IFN works by inhibiting the synthesis of viral proteins and by increasing the cellular immune defenses.

B. Complement is a complex chemical defense system that destroys certain pathogens and produces chemical mediators. It involves chemicals called complement (C factor) that act in cascade fashion: One component activates the next in line, which activates the next, and so on. Two major pathways are: (1) classical, which involves activation of complement by specific antibody; and (2) alternative, which is a nonspecific reaction to infections. The result of complement activation is a huge structural protein, the membrane attack complex, that can kill cells and inactivate viruses by digesting holes in their membranes.