18 Animal Defense Systems Animal defense systems are based on the distinction between self and nonself. There are two general types of defense mechanisms:  Nonspecific defenses, or innate defenses, are inherited mechanisms that protect the body from many different pathogens.  Specific defenses are adaptive mechanisms that protect against specific targets.

18 Animal Defense Systems White blood cells are important in defense. All blood cells originate from stem cells in the bone marrow. White blood cells (leukocytes) are clear and have a nucleus and organelles. White blood cells can leave the circulatory system. The number of white blood cells sometimes rises in response to invading pathogens.

18 Animal Defense Systems There are two main groups of white blood cells: phagocytes and lymphocytes. Phagocytes engulf and digest foreign materials. Lymphocytes are most abundant. There are two types: B and T cells. T cells migrate from the circulation to the thymus, where they mature. B cells circulate and also collect in lymph vessels, and make antibodies.

18 Animal Defense Systems Four groups of proteins play key roles in defending against disease:  Antibodies, secreted by B cells, bind specifically to certain substances.  T cell receptors are cell surface receptors that bind nonself substances on the surface of other cells.  Major histocompatibility complex (MHC) proteins are exposed outside cells of mammals. These proteins help to distinguish self from nonself.  Cytokines are soluble signal proteins released by T cells. They bind and alter the behavior of their target cells.

18 Nonspecific Defenses Vertebrate blood contains about 20 antimicrobial complement proteins. Complement proteins provide three types of defenses:  They attach to microbes, helping phagocytes recognize and destroy them.  They activate the inflammation response and attract phagocytes to the site of infection.  They kill invading cells.

18 Nonspecific Defenses Phagocytes ingest pathogens. There are several types of phagocytes:  Neutrophils attack pathogens in infected tissue.  Monocytes mature into macrophages. They live longer and consume larger numbers of pathogens than do neutrophils. Some roam and others are stationary in lymph nodes and lymphoid tissue.  Eosinophils kill parasites, such as worms, that have been coated with antibodies.  Dendritic cells have highly folded plasma membranes that can capture invading pathogens.

18 Nonspecific Defenses Natural killer cells are a class of nonphagocytic white blood cells They can initiate the lysis of virus-infected cells and some tumor cells.

18 Nonspecific Defenses The inflammation response is used in dealing with infection or tissue damage. Mast cells and white blood cells called basophils release histamine, which triggers inflammation.

18 Specific Defenses: The Immune System Four characteristics of the immune system:  1. Specificity: Antigens are organisms or molecules that are specifically recognized by T cell receptors and antibodies.  The sites on antigens that the immune system recognizes are the antigenic determinants (or epitopes).  Each antigen typically has several different antigenic determinants.  The host creates T cells and/or antibodies that are specific to the antigenic determinants.

18 Specific Defenses: The Immune System  2. Diversity:  It is estimated that the human immune system can distinguish and respond to 10 million different antigenic determinants.  3. Distinguishing self from nonself:  Each normal cell in the body bears a tremendous number of antigenic determinants. It is crucial that the immune system leave these alone.  4. Immunological memory:  Once exposed to a pathogen, the immune system remembers it and mounts future responses much more rapidly.

18 Specific Defenses: The Immune System The immune system has two responses against invaders: The humoral immune response and the cellular immune response. The two responses operate in concert and share mechanisms.

18 Specific Defenses: The Immune System The humoral immune response involves antibodies that recognize antigenic determinants by shape and composition. The cellular immune response is able to detect antigens that reside within cells. It destroys virus-infected or mutated cells. Its main component consists of T cells. T cells have T cell receptors that can recognize and bind specific antigenic determinants.

18 Specific Defenses: The Immune System When the body encounters an antigen for the first time, a primary immune response is activated. When the antigen appears again, a secondary immune response occurs. This response is much more rapid, because of immunological memory.

18 Specific Defenses: The Immune System Artificial immunity is acquired by the introduction of antigenic determinants into the body. Vaccination is inoculation with whole pathogens that have been modified so they cannot cause disease. Immunization is inoculation with antigenic proteins, pathogen fragments, or other molecular antigens. Immunization and vaccination initiate a primary immune response that generates memory cells without making the person ill.

18 B Cells: The Humoral Immune Response Antibody molecules are proteins called immunoglobulins. Five Classes/Types of IG's.  G,M,D,A and E

Figure Structure of Immunoglobulins (Part 1)

Figure IgG Antibodies Promote Phagocytosis

18 T Cells: The Cellular Immune Response T cells, like B cells, possess specific surface receptors. The genes that code for T cell receptors are similar to those for immunoglobulins. T cell receptors also have constant and variable regions. A major difference between antibodies and T cell receptors is that T cell receptors bind only to an antigenic determinant that is displayed on the surface of an antigen-presenting cell.

18 T Cells: The Cellular Immune Response Activated T cells give rise to two types of effector cells. Cytotoxic cells, or T C, recognize virus-infected cells and kill them by causing them to lyse. Helper T cells, or T H cells, assist both the cellular and humoral immune systems. Activated helper T cells proliferate and stimulate both B and T C cells to divide.  HIV

Figure The Interaction between T Cells and Antigen-Presenting Cells (Part 3)

Figure Macrophages Are Antigen-Presenting Cells

Figure (a) Phases of the Humoral and Cellular Immune Responses (Part 1)

Figure (a) Phases of the Humoral and Cellular Immune Responses (Part 2)

Figure (b) Phases of the Humoral and Cellular Immune Responses (Part 1)

Figure (b) Phases of the Humoral and Cellular Immune Responses (Part 2)

18 T Cells: The Cellular Immune Response T cells developing in the thymus are tested to ensure that they will be functional and will not attack normal self antigens. If the developing T cell binds to one of the body’s own normal antigens, it undergoes apoptosis.

18 T Cells: The Cellular Immune Response For organ transplants to be successful, MHC molecules must match; otherwise, these same molecules will act as antigens. The cellular immune system is responsible for rejection. Rejection problems can be controlled somewhat by treating patients with immunosuppressing drugs.

18 Disorders of the Immune System The human immune system can overreact to a dose of antigen and produce an inappropriate immune response. Allergies are the most familiar example. Immediate hypersensitivity. Delayed hypersensitivity.