Understanding the Immune System

Introduction

• The immune system defends the body against foreign invaders
• It consists of cells, tissues, and organs working together
• Microbes (germs) like bacteria, viruses, parasites, and fungi are examples of foreign invaders
• The immune system's role is to keep them out or destroy them

Self and Nonself

• A healthy immune system distinguishes between the body's own cells (self) and foreign cells (nonself)
• The body’s defenses coexist with cells with distinct “self” marker molecules
• Antigens trigger the immune response and carries marker molecules that identify them as foreign
• Antigens can be microbes, parts of microbes, or tissues/cells from another person (except identical twins)
• In autoimmune diseases, the immune system attacks the body's own cells/tissues
• In allergies, the immune system responds to harmless foreign substances like ragweed pollen, and such antigens are called allergens

The Structure of the Immune System

• Lymphoid organs house lymphocytes (small white blood cells) and are positioned throughout the body
• Bone marrow produces all blood cells, including white blood cells destined to become immune cells
• The thymus is located behind the breastbone and is where T lymphocytes ("T cells") mature
• Lymphocytes travel throughout the body using blood vessels and lymphatic vessels
• Lymphatic vessels run parallel to blood vessels and carry lymph, a clear fluid bathing body tissues
• Lymph nodes are small, bean-shaped structures along lymphatic vessels, containing compartments where immune cells congregate and encounter antigens
• Lymphocytes enter lymph nodes through incoming lymphatic or blood vessels and exit through outgoing lymphatic vessels
• The spleen, in the upper-left abdomen, contains immune cell compartments and serves as a meeting ground for immune defenses and antigens
• Lymphoid tissue is found in the digestive tract, airways, and lungs, including the tonsils, adenoids, and appendix

Immune Cells and Their Products

• The immune system stores lymphocytes and phagocytes
• Some immune cells target all threats, while others are trained on specific enemies
• Immune cells use direct contact or chemical messengers to communicate
• The immune system stores a small number of each cell type to recognize possible enemies
• When an antigen appears, matching cells multiply, and then fade away after the job is done
• All immune cells begin as immature stem cells in the bone marrow and grow into specific cell types such as T-cells, B-cells or phagocytes
• Stem cells can be used to regenerate damaged immune responses in autoimmune and immune deficiency diseases

B Lymphocytes

• B cells and T cells are two main types of lymphocytes
• B cells secrete antibodies into bodily fluids to ambush antigens in the bloodstream
• Antibodies are powerless to penetrate cells
• B cells are programmed to make specific antibodies like blocking a virus that causes the common cold or attacking a bacterium that causes pneumonia
• Plasma cells are large cells producing antibodies after a B cell encounters its triggering antigen
• Each plasma cell descended from a B cell manufactures millions of identical molecules and pours them into the bloodstream

Antibodies and Immunoglobulins

• An antigen matches an antibody like a key matches a lock.
• When an antigen and antibody interlock, the antibody marks the antigen for destruction
• Antibodies belong to the immunoglobulin family, with different types playing different roles
• Immunoglobulin G (IgG) coats microbes speeding uptake by other immune cells
• IgM is highly effective at killing bacteria
• IgA concentrates in body fluids, guarding entrances to the body
• IgE protects against parasitic infections and is responsible for allergy symptoms
• IgD remains attached to B cells and initiates early B-cell response

T Cells

• T cells identify antigens on the surface of cancerous cells
• T cells have two functions: direct and regulate immune responses, and directly attack infected or cancerous cells
• Helper T cells (Th cells) coordinate immune responses by communicating with other cells by stimulating B cells, call in phagocytes and activate other T cells
• Killer T cells (cytotoxic T lymphocytes or CTLs) attack foreign cells carrying particular molecules
• CTLs attack viruses by recognizing small fragments and launching an attack to kill the cell
• T cells recognize an antigen if carried by the body's own MHC (major histocompatibility complex) molecules
• MHC molecules are proteins by which T cells distinguish between self and nonself.

MHC and Natural Killer (NK) cells

• Self MHC molecule presents a foreign antigen to T cells
• MHC molecules are proteins recognized by T cells for distinguishing between self and nonself
• Non-self MHC molecules will lead to the T cell to destroying the cell
• Doctors match donors to recipients to avoid graft rejection
• Natural killer (NK) cells are lethal white cells or lymphocytes
• NK cells are armed with granules filled with chemicals
• NK cells recognize cells lacking self-MHC molecules so they attack foreign cells

Phagocytes and Granulocytes

• Phagocytes are large white cells that engulf and digest microbes and foreign particles
• Monocytes circulate in the blood, developing into macrophages when they migrate into tissues
• Macrophages rid the body of worn-out cells and debris, display bits of foreign antigen, and produce chemical signals known as monokines
• Granulocytes are another type of immune cell, destroying microorganisms with potent chemicals
• Some chemicals, such as histamine contribute to inflammation and allergy
• Neutrophils are a type of granulocyte and phagocyte, using prepackaged chemicals to break down microbes
• Eosinophils and basophils "degranulate," spraying chemicals onto harmful cells or microbes
• The mast cell is like the basophil, but found in the lungs, skin, tongue, and linings of the nose and intestinal tract, where it is responsible for allergy symptoms
• Platelets promote blood clotting and wound repair

Cytokines

• Cytokines are chemical messengers for the immune system to communicate
• These proteins coordinate an appropriate immune response
• Cytokines include interleukins, interferons, and growth factors
• Some cytokines are chemical switches turning certain cell types on and off
• Interleukin 2 (IL-2) triggers the immune system to produce T cells