The Lymphoid System and Immunity Overview

Chapter 22

Functions of the Immune System

Destroy pathogens (organisms that cause disease)

Detect and kill abnormal cells such as cancerous cells

Remove dead cells and debris from the body

PathogensCan be classified according to size and where

they live in the body.

Largest pathogens are the multicellular organisms such as parasitic worms

Others are microorganisms including fungi, protozoa (such as the one that causes malaria), bacteria, and viruses

Pathogens continuedViruses Must

reproduce inside cells

BacteriaProtozoa

Fungi

Some hide inside cells, but most are extracellular

Parasitic worms

Always extracellular

Three main lines of defense

1. Surface Barriers (Innate External Defenses)

Skin and mucous membranes

2. Innate Internal Defenses- identify enemies by recognizing a limited number of markers unique to pathogens. When overwhelmed, they secrete chemical messengers to mobilize adaptive defenses

Cells and chemicals in body fluids

3. Adaptive Defenses- take time to mobilize and train to fight an identified enemy

Lymphocytes- T cells and B cells

Innate and Adaptive Immunity

Innate immunity

Nonspecific defense mechanisms that come into play immediately or within hours of an antigen's appearance in the body. These mechanisms include physical barriers such as skin, chemicals in the blood, and immune system cells that attack foreign cells in the body. The innate immune response is activated by chemical properties of the antigen.

Adaptive Immunity

Antigen-specific immune response. The adaptive immune response is more complex than the innate. The antigen first must be processed and recognized. Once an antigen has been recognized, the adaptive immune system creates an army of immune cells specifically designed to attack that antigen. Adaptive immunity also includes a "memory" that makes future responses against a specific antigen more efficient.

Adaptive defenses differ from innate defenses in four ways:

1. Are specific- directed against an identified enemy

2. Involve B and T lymphocytes

3. Have memory

4. Are systemic- can act anywhere in the body

Adaptive Immunity-B cells and T cells

Recognize a shape on a pathogen called an antigenic determinant and is formed by the 3D structure of a large molecule called an antigen

Antigens are usually proteins, but can also be large carbohydrates or nucleic acids.

The surface of a given pathogen is studded with many different antigens, each having many different determinants.

Antigens Both B and T cells bind to

antigens. In addition, protein molecules called antibodies bind antigens. These antibodies are secreted by the clonal descendants of B cells, called plasma cells.

Humoral Immunity Aspect of specific immunity

which is mediated by B lymphocytes and involves the binding of antibodies to antigens

Directed against extracellular pathogens

Cellular Immunity The branch of adaptive

immunity involving T lymphocytes that directly attack infected, cancerous, or that release chemicals which regulate other functions of the immune system.

Directed against pathogens within the cells.

Circumstances that T cells attack our own cells

A cell has become cancerous.

A cell has been invaded by a virus.

A cell has been transplanted from another individual.

Anatomy ReviewThe immune system is strategically distributed

throughout the body, particularly at those points such as the respiratory and digestive tract mucosae, where pathogens can most easily gain entry to the body.

Two major parts of the immune system:

Specialized immune cells, many of which are leukocytes (white blood cells) and their close relatives.

Lymphoid organs and tissues including the bone marrow, lymph nodes, spleen, and thymus. lymphocytes are produced, reside, and come into

contact with pathogens.

Immune Cells: LeukocytesCells of the immune system:

Originate in the bone marrowMigrate to tissues or circulate through the blood

and lymphatic system and enter tissues when needed.

Are called leukocytes when traveling in the bloodHave traditionally been classified according to

their shape and the colors of their granules, if any, when stained with histological dyes.

Immune Cells besides leukocytes

Phagocytes- engulf and destroy pathogens, dead cells and debris.

Antigen-presenting cells (APCs) process and present antigens to T cells Dendritic cells Macrophages B cells

Effector Cells B cells T cells

Mast cells

Lymphocytes and adaptive immunity

B cells mature in the bone marrow

T cells mature in the thymus

Together the bone marrow and the thymus are the primary lymphoid organs

Lymphatic System A one way system of

vessels called lymphatic vessels.

The fluid in those vessels, called lymph

Lymph nodes