48.1 Innate Immunity: First Response

• Cells that are always ready to respond confer innate immunity.

• Your ears are protected by waxy secretions and your eyes are protected by tears that contain the enzyme lysozyme, which catalyzes the hydrolysis of the molecules that make up bacterial cell walls.

• The cells responsible for this response are a class of blood cells known as white blood cells, or leukocytes (“whitecells”), to distinguish them from the red blood cells that transport oxygen in vertebrates.

• An antigen is any foreign molecule that can elicit an immune system response.

• Toll-like receptors (TLRs）are a subset of a larger group of proteins called pattern-recognition receptors, which serve as sentinels that detect the presence of molecules associated with pathogens and relay an alert signal to cell.

  • TLRs have also been observed in fungi and plants, suggesting that they arose in a common ancestor of all eukaryotes.

• When human TLR4 is activated by LPS, a signal cascade leads to the production and secretion of cytokines (“cell-movers”)．

  • Cytokines are a class of diverse molecules that signal other immune system cells in various ways, such as increasing white blood cell production, attracting other immune cells to the site of infection, or stimulating other immune cells into action.

• White blood cells known as dendritic (“tree-like”) cells capture antigens and debris from the site of infection by endocytosis, in particular by macropinocytosis (“largecell-drinking”)-uptake of liquids and small particles into a vesicle-phagocytosis, and receptor-mediated endocytosis.

48.2 Adaptive Immunity: Recognition

• Each rabbit produced proteins in its blood, called antibodies, that specifically bound to the particular antigen that was injected.

• The white blood cells that carry out the major features of the adaptive immune response are called lymphocytes.

• To explore the function of the thymus-an organ located in the upper part of the chest of vertebrates-these scientists removed the organ from newborn mice.

• The bursa-dependent lymphocytes, or B cells, produce antibodies.

• The thymus-dependent lymphocytes, or T cells, are involved in graft rejection along with other immune functions, including recognizing and killing host cells that are infected with a virus.

• The immune system cells found in these mucus-secreting tissues are called mucosa-associated lymphoid tissue (MALT).

• Chemical analysis revealed at these surface proteins, now called B-cell receptors (BCRs), had the same overall structure as the antibodies that the B cells produced and secreted into the blood.

• A BCR consists of two distinct polypeptides.

  • The smaller polypeptide is called the light chain.

  • The larger polypeptide is roughly twice the size of 1e light chain and is called the heavy chain.

• Immunoglobulins ( antibodies, BCRs, and TCRs) do not bind to entire antigens.

  • Instead, they bind to a selected region of the antigen called an epitope.

48.3 Adaptive Immunity: Activation

• In the 1950s, Frank Bernet and colleagues developed the clonal selection theory to explain how only the most useful lymphocytes are activated during infection.

• The surface proteins responsible for presenting these epitopes are called major histocompatibility (MHC) proteins.

  • MHC proteins have a groove that binds to small epitope-containing antigen fragments that are typically 8 to 20 amino acids in length.

• MHC proteins come in two types, called class I and class II MHC proteins.

  • The antigens that are processed and loaded onto class I MHC proteins are derived from the cell’s interior, while those loaded onto class II MHC proteins are obtained from outside the cell.

  • Class I MHC loading takes place inside the endoplasmic reticulum (ER); class II MHC proteins are loaded inside endosomes.

48.4 Adaptive Immunity: Response and Memory

• The humoral (immune) response is promoted by TH2 cells and involves the production of antibodies and other proteins secreted into the blood and lymph by activated B cells.

• The cell-mediated (immune) response is promoted by TH1 cells and involves the activation of phagocytic cells and cytotoxic T cells (activated CD8+ cells), among others.

  • This response primarily takes place via cell-cell contact.

• Once in the cytoplasm, the T cell proteins activate a signaling cascade that causes the target cell to self-destruct via apoptosis.

• Besides producing 1e cel at implement the humoral and cell mediated responses, activated B cells and T cells produce specialized daughter cells called memory cells.

• Memory cells do not pa icipate in the initial adaptive immune response- the primary immune response.

• If the same antigen enters the body a second time, memory cells are able to recognize certain epitopes of the antigen and will trigger a second adaptive immune response, or secondary immune response.

• The launching of a secondary immune response by means of memory cells is known as immunological memory.

48.5 What Happens When the Immune System Doesn’t Work Correctly?

• Molecules that trigger allergic reactions are called allergens.

• The production of lgE antibodies in an allergic reaction triggers a series of events known as the hypersensitive reaction.

• An immune response directed against molecules or cells that normally exist in the host is known as autoimmunity.

  • Autoimmune reactions often result in disease due to the destruction of the body’s own cells and structures by lymphocytes of the adaptive immune response.