Our environment is filled with microorganisms such as bacteria, viruses, fungi and parasites that we encounter on a daily basis. Some of these microorganisms can cause disease, yet most of us are reasonably healthy most of the time. Humans and other vertebrates protect themselves with a complex array of defensive mechanisms. The body has three levels of defense against infection. 1. The anatomical and physiological barriers include: o intact skin provides a physical barrier, o mucosal surfaces of the gut, respiratory urogenital and conjunctivae surfaces, ? stomach acid, lysozyme and other antimicrobial body secretions. 2. The innate immune responses (non specific) are the initial internal defenses. These responses are: ? quick and non-specific, o involves phagocytosis and activation of the complement proteins, o limits the spread of infection, o often resolves the infection. o Examples: skin, mucusm lysozymes in eyes, acid in stomach 3. The adaptive immune responses are the last line of defense. These responses are: ? slower to respond but specific to the particular pathogen, o involve antibody-mediated responses and cell-mediated responses. II. Cells of the Immune System
Blood Erythrocytes (RBC) Platelets Leukocytes (WBC) Myeloid cells Lymphocytes Dendritic cells Granulocytes monocyte/ macrophages (in blood) ? B cells NK cells T cells Neutrophils dendritic cell (in tissue) Mast cells CTL basophils Macrophage (in tissue) TH Eosinophils Treg All blood cells, including RBCs, arise from hematopoietic stem cells (HSC, hemato = blood; poiesis = formation). HSCs are: · self-renewing stem cells, . found in the bone marrow, umbilical cord blood and in the blood (a very small number). There are two major cell populations in blood: · erythrocytes (also called red blood cells or RBCs) carry oxygen to the tissues, . leukocytes (also called white blood cells or WBCs) are the cells of the immune system. Found everywhere o Myeloid cells participate in the innate immune responses. Myeloid cells include: · monocytes (that mature into macrophages): phagocytose and kill bacteria, alert the immune system in the presence of an infection. Tissue repair and wound healing. mast cells: constantly search for suspicious objects in the body. When they encounter a foreign object, they release a signal in the form of histamine molecules · granulocytes · Neutrophils: phagocytose and kill bacteria. Most abundant. They patrol the body and kill infectious cells and then die, which leads to puss formation. Largest population is in gum tissue.
· Basophils · Eosinophils · Lymphoid cells. Most types of lymphoid cells (also called lymphocytes) participate in the adaptive immune responses. Lymphocytes include: cytotoxic T cells (CTLs): kill viral infected cells · T helper (Th) cells: provide cytokines and other stimulatory signals to B cells, CTLs and macrophages. CD4 (cluster of differentiation) -- > get depleted as person develops AIDS and adoptive immune response collapses · T regulatory (Treg) cells, B cells: secrete antibodies into body fluids Natural killer cells (NK cells) are a type of lymphoid cell that participates in innate responses. NK cells will not be discussed further in MICB 202. Dendritic cells phagocytose and