AP BIOLOGY ANIMAL FORM AND FUNCTION Immune System

Immune System Animals are constantly under attack by pathogens, infectious agents that cause disease. In response, animals fight back in various ways.

Natural Barriers Prevent Infection 1. Skin (or shells) 2. Chemical secretions that trap or kill microbes. Example: Saliva contains lysozyme, an enzyme that digests microbial cell walls. Also found in tears. 3. Mucus membranes line the passageways of body. Mucus traps microbes and other particles—sent to digestive system.

Non-specific Defenses These immune defenses protect against any type of invader that may get past the barriers provided by skin and mucus membranes. These are not specific to a particular pathogen. Includes: Phagocytes, Complement Proteins, and the Inflammatory Response.

Phagocytes If an invader does get into the body, it may meet special cells called phagocytes. “phago” = eating. These cells roam the body in search of bacteria and dead or damaged cells. 3 main types: Granulocytes (Neutrophils), Macrophages, and Dendritic cells.

Phagocytes—Granulocytes (Neutrophils) Granulocytes: (also called Neutrophils) often take the first stand during an infection. They attack any invaders in large numbers, and "eat" until they die. The pus in an infected wound consists chiefly of dead granulocytes. A small part of the granulocyte community (called Eosinophils) is specialized in attacking larger parasites such as worms.

Phagocytes--Macrophages The macrophages ("big eaters") are slower to respond to invaders than the granulocytes, but they are larger, live longer, and have far greater capacities. Macrophages also play a key part in alerting the rest of the immune system of invaders. Macrophages start out as white blood cells called monocytes. Monocytes that leave the blood stream turn into macrophages.

Phagocytes—Dendritic Cells The dendritic cells are "eater" cells and devour intruders, like the granulocytes and the macrophages. And like the macrophages, the dendritic cells help with the activation of the rest of the immune system. They are also capable of filtering body fluids to clear them of foreign organisms and particles

Complement Proteins Phagocytes are aided by special protein molecules called complement proteins. Some complement proteins poke holes in bacterial cell walls. Complement proteins may coat bacterial cells, stimulating phagocytes to ingest them.

Complement Proteins Aid Immune Response These proteins flow freely in the blood and can quickly reach the site of an invasion where they can react directly with antigens molecules that the body recognizes as foreign substances. When activated, the complement proteins can: Trigger inflammation Stimulate phagocytes Coat pathogens Kill pathogens

Inflammatory Response Let’s say you pick up a splinter as you grab a piece of wood. Ouch! Your immune system will respond with an inflammatory response. Within our tissues lie cells known as Mast Cells. These cells contain the signal histamine, a chemical that activates the inflammatory response. The splinter damages these mast cells, causing them to release histamine, which then migrates through the tissues towards the bloodstream.

Inflammatory Response The release of histamine causes increased permeability and blood flow to the injured tissue. The splinter also causes the release of signals to phagocytes which go to the site and “eat” bacteria and cell debris.

Lymphatic System The lymphatic system consists of lymphatic vessels, through which lymph travels, and various structures that trap “foreign” molecules and particles.

Lymphatic Cells—Important for Immune Response The lymphatic system also involves a transportation system - lymph vessels - for transportation and storage of lymphocyte cells within the body. The lymphatic system feeds lymphatic cells into the body and filters out dead cells and invading organisms such as bacteria. Two main kinds of lymphocytes: B cells and T cells

Acquired Immune Response—B and T Cells B cells and T cells, types of white blood cells called lymphocytes, are critical for the acquired immune response. Lymphocytes that originate from stem cells in the bone marrow, then migrate to the thymus, then mature into T cells (“T” for thymus). Lymphocytes that originate and mature in the bone marrow are B cells (“B” for bone marrow).

Antigens An antigen is a molecule that is foreign to the body and causes the immune system to respond. B and T cells recognize antigens using the antigen-specific receptors embedded in their plasma membranes.

Antigen receptors Every B and T cell has a specific antigen recognition site on its surface. Each person has more than 1 million different B cells and 10 million different T cells, each with a particular antigen-binding specificity.

Helper T cells are the major driving force and the main regulators of the immune defense. Their primary task is to activate B cells and killer T cells.

Activating Helper T Cells Helper T cells themselves must be activated. This happens when a macrophage or dendritic cell, which has eaten an invader, travels to the nearest lymph node to present information about the captured pathogen. The phagocyte displays an antigen fragment from the invader on its own surface, a process called antigen presentation.

Activating the Helper T cell When the receptor of a helper T cell recognizes the antigen, the T cell is activated. Once activated, helper T cells start to divide and to produce proteins that activate B and T cells as well as other immune cells.

Antigen Presentation to Activate the Helper T Cells

B cell activation The B lymphocyte cell searches for antigen matching its receptors. If it finds such an antigen it connects to it, and inside the B cell a triggering signal is set off. The B cell now needs proteins (called cytokines) produced by helper T cells to become fully activated. When this happens, the B cell starts to divide to produce clones of itself. During this process, two new cell types are created, plasma cells and B memory cells.

B Cell activation

Plasma Cells Produce Antibodies The plasma cell is specialized in producing a specific protein, called an antibody, that will respond to the same antigen that matched the B cell receptor. Antibodies are released from the plasma cell so that they can seek out intruders and help destroy them.

Production of Antibodies Plasma cells produce antibodies at an amazing rate and can release tens of thousands of antibodies per second.

Antibodies When the Y-shaped antibody finds a matching antigen, it attaches to it. The attached antibodies serve as an appetizing coating for eater cells such as the macrophage. Antibodies also neutralize toxins and incapacitate viruses, preventing them from infecting new cells

How do antibodies recognize the antigen? Antibodies are protein molecules with two functional regions: 1. fragment antigen binding region or Fab---this is what allows an antibody to recognize a specific antigen. 2. The other end, which binds to effector cells (immune cells), is called the Fe region. There are 5 types of Fe regions, one for each of the five types of antibodies: Ig. A, Ig. D, Ig. E, Ig. M, and Ig. G.

Structure of Antibodies 1. Fab binds to antigen 2. Fe binds to immune cell that destroys antigen 3. heavy chain 4. light chain 5. antigen binding site 6. hinge regions

Memory B and T Cells The Memory Cells are the second type of cell produced by the division of B cells. These cells have a prolonged life span and can thereby "remember" specific intruders. T cells can also produce memory cells with an even longer life span than B memory cells.

Memory B and T cells The second time an intruder tries to invade the body, B and T memory cells help the immune system to activate much faster. The invaders are wiped out before the infected human feels any symptoms. The body has achieved immunity against the invader.

This graph shows how Memory Cells help you to better fight infections. At day 0, someone catches a virus. At day 10, her B-cells start making antibodies, and by day 15 she’s made enough antibodies to destroy all the viruses. Now, she doesn’t make any more antibodies, so fewer and fewer are left in her body. Then, at day 40, the same virus gets in her body again. Since she has Memory B-cells prepared to fight, she can quickly make 100 times more antibodies than she did during the first infection.

Acquired Immune Response & Immunological Memory Not only do B cells and T cells recognize and inactivate foreign cells and molecules. Both also contribute to immunological memory, an enhanced response to a pathogen encountered previously. Vaccines are given to produce an immune response to an inactivated microbe. This provides immunological memory.

Killer T cells (Cytotoxic cells) The killer T cell (cytotoxic cell) is specialized in attacking cells of the body infected by viruses and sometimes also by bacteria. It can also attack cancer cells. The killer T cell has receptors that are used to search each cell that it meets. If a cell is infected, it shows a tiny piece of the antigen and it is swiftly killed.

Natural Killer Cells Natural Killer (NK) cells help recognize and eliminate certain diseased cells. All cells in the body (except RBCs) normally have on their surface a protein called a class 1 MHC molecule. Following viral infection or conversion to a cancerous state, cells sometimes stop expressing this protein. The NK cells that patrol the body attach to such stricken cells and release chemicals that lead to cell death, inhibiting further spread of the virus or cancer.

When Things Go Wrong…. Allergies—Occur when the immune system overreacts to the presences of certain antigens. This causes an overwhelming release of histamines and a (sometimes extreme) inflammatory response.

HIV/AIDS Human Immunodeficiency Virus (HIV) attacks the Helper T cells of the immune system. Since the Helper T cells regulate the immune system response, without them the person is susceptible to any pathogen. When Helper T cell numbers reach a low enough level, the person is diagnosed with AIDS (Acquired Immune Deficiency Syndrome). This virus is spread with direct contact in the blood stream with infected body fluids.