Immune system module 1: anatomy of the lymphatic and immune systems  (Page 2/37)

Anatomy of the lymphatic system

Lymphatic capillaries

In the small intestine, lymphatic capillaries called lacteals are critical for the transport of dietary fats and fat-soluble vitamins to the bloodstream. These fats are abosorbed lacteals and form a milky fluid called chyle    . The chyle then travels through the lymphatic system, eventually entering the liver and then the bloodstream.

The organization of immune function

The immune system is a collection of barriers, cells, and proteins that interact and communicate with each other in extraordinarily complex ways to combat pathogens . A pathogen is anything that can make one sick when it enters the body. The modern model of immune function is organized into three phases based on the timing of their effects. The three phases consist of the following:

• Barrier defenses such as the skin are also referred to as mechanical defenses that act instantly to prevent pathogens from invading the body tissues.
• The rapid, but nonspecific innate immune response    , consists of a variety of specialized cells and chemicals.
• The slower but more specific and effective adaptive immune response    , involves many cell types and chemicals, but is primarily controlled by white blood cells (leukocytes) known as lymphocytes    , which help control immune responses.

The cells of the blood, including all those involved in the immune response are made in the bone marrow. Hematopoietic tissue contains stem cells that produce the blood cells. ( [link] ). Hematopoietic stem cells are present throughout adulthood to replace those lost to age or function. These cells produce the all the classes of white blood cells that are involved in the function of the the immune system. These cells can be divided into three classes based on function:

• Phagocytic cells also called phagocytes, ingest pathogens to destroy them
• Lymphocytes specifically coordinate the activities of adaptive immunity
• Cells containing cytoplasmic granules, which help regulate immune responses against parasites and viruses

Hematopoietic system of the bone marrow

Lymphocytes: b cells, t cells, plasma cells, and natural killer cells

As stated above, lymphocytes are the primary cells of adaptive immune responses ( [link] ). The two basic types of lymphocytes, B cells and T cells, start out as identical cells with a large central nucleus surrounded by a thin layer of cytoplasm. As they develop they start to change and become distinguished from each other by their surface protein markers as well as by the molecules they secrete. While B cells mature in red bone marrow and T cells mature in the thymus, they both initially develop from bone marrow. T cells migrate from bone marrow to the thymus gland where they further mature. B cells and T cells are found in many parts of the body, circulating in the bloodstream and lymph, and residing in the spleen and lymph nodes, which will be described later in this section. The human body contains approximately 10 ¹² lymphocytes.

B cells

B cells    are immune cells that function primarily by producing antibodies. An antibody    is any of the group of proteins that binds specifically to pathogen-(think viruses, bacteria, etc.). An antigen    is a chemical structure on the surface of a pathogen that binds to T or B lymphocyte antigen receptors. Once activated by binding to antigen, B cells change into plasma cells .

T cells

The T cell    , on the other hand, does not secrete antibodies but performs a variety of functions in the adaptive immune response. Different T cell types have the ability to either secrete substances that communicate with other cells of the adaptive immune system or destroy cells infected with pathogens. The roles of T and B lymphocytes in the adaptive immune response will be discussed further in this chapter.

Plasma cells

When a B cell discovers a pathogen, it creates plasma cells. When a signal from the B cell occurs the plasma cell    creates the antibodies that attack the pathogen.

Natural killer cells

A fourth important lymphocyte is the natural killer cell, a participant in the innate immune response. A natural killer cell (NK)    is a circulating blood cell that contains cytotoxic (cell-killing) chemicals. These chemicals destroy the invading cells, literally causing the cell membrane to burst. It shares this mechanism with the cytotoxic T cells of the adaptive immune response. NK cells are among the body’s first lines of defense against viruses and certain types of cancer.

Chapter review

The lymphatic system is a series of vessels, ducts, and trunks that remove interstitial fluid from the tissues and return it the blood. The lymphatics are also used to transport dietary lipids and cells of the immune system. Cells of the immune system all come from the hematopoietic system of the bone marrow. Primary lymphoid organs, the bone marrow and thymus gland, are the locations where lymphocytes of the adaptive immune system proliferate and mature. Secondary lymphoid organs are site in which mature lymphocytes congregate to mount immune responses. Many immune system cells use the lymphatic and circulatory systems for transport throughout the body to search for and then protect against pathogens.