Immune System - An Overview

**trimurtulu** · 02-02-2009, 06:31 PM

Immune System

Introduction

The human immune system is a truly amazing constellation of responses to attacks from outside the body. It has many facets, a number of which can change to optimize the response to these unwanted intrusions. The system is remarkably effective, most of the time. This note will give you a brief outline of some of the processes involved.

An antigen is any substance that elicits an immune response, from a virus to a sliver.

The immune system has a series of dual natures, the most important of which is self/non-self recognition. The others are: general/specific, natural/adaptive = innate/acquired, cell-mediated/humoral, active/passive, primary/secondary. Parts of the immune system are antigen-specific (they recognize and act against particular antigens), systemic (not confined to the initial infection site, but work throughout the body), and have memory (recognize and mount an even stronger attack to the same antigen the next time).

Self/non-self recognition is achieved by having every cell display a marker based on the major histocompatibility complex (MHC). Any cell not displaying this marker is treated as non-self and attacked. The process is so effective that undigested proteins are treated as antigens.

Sometimes the process breaks down and the immune system attacks self-cells. This is the case of autoimmune diseases like multiple sclerosis, systemic lupus erythematosus, and some forms of arthritis and diabetes. There are cases where the immune response to innocuous substances is inappropriate. This is the case of allergies and the simple substance that elicits the response is called an allergen.

The Blood System

The 5 liters of blood of a 70 kg (154 lb) person constitute about 7% of the body's total weight. The blood flows from the heart into arteries, then to capillaries, and returns to the heart through veins.

Blood is composed of 52–62% liquid plasma and 38–48% cells. The plasma is mostly water (91.5%) and acts as a solvent for transporting other materials (7% protein [consisting of albumins (54%), globulins (38%), fibrinogen (7%), and assorted other stuff (1%)] and 1.5% other stuff). Blood is slightly alkaline (pH = 7.40 ± .05) and a tad heavier than water (density = 1.057 ± .009).

All blood cells are manufactured by stem cells, which live mainly in the bone marrow, via a process called hematopoiesis. The stem cells produce hemocytoblasts that differentiate into the precursors for all the different types of blood cells. Hemocytoblasts mature into three types of blood cells: erythrocytes (red blood cells or RBCs), leukocytes (white blood cells or WBCs), and thrombocytes (platelets).

The leukocytes are further subdivided into granulocytes (containing large granules in the cytoplasm) and agranulocytes (without granules). The granulocytes consist of neutrophils (55–70%), eosinophils (1–3%), and basophils (0.5–1.0%). The agranulocytes are lymphocytes (consisting of B cells and T cells) and monocytes. Lymphocytes circulate in the blood and lymph systems, and make their home in the lymphoid organs.

All of the major cells in the blood system are illustrated below.

There are 5000–10,000 WBCs per mm3 and they live 5-9 days. About 2,400,000 RBCs are produced each second and each lives for about 120 days (They migrate to the spleen to die. Once there, that organ scavenges usable proteins from their carcasses). A healthy male has about 5 million RBCs per mm3, whereas females have a bit fewer than 5 million.

The Lymph System

Lymph is an alkaline (pH > 7.0) fluid that is usually clear, transparent, and colorless. It flows in the lymphatic vessels and bathes tissues and organs in its protective covering. There are no RBCs in lymph and it has a lower protein content than blood. Like blood, it is slightly heavier than water (density = 1.019 ± .003).

The lymph flows from the interstitial fluid through lymphatic vessels up to either the thoracic duct or right lymph duct, which terminate in the subclavian veins, where lymph is mixed into the blood. (The right lymph duct drains the right sides of the thorax, neck, and head, whereas the thoracic duct drains the rest of the body.) Lymph carries lipids and lipid-soluble vitamins absorbed from the gastrointestinal (GI) tract. Since there is no active pump in the lymph system, there is no back-pressure produced. The lymphatic vessels, like veins, have one-way valves that prevent backflow. Additionally, along these vessels there are small bean-shaped lymph nodes that serve as filters of the lymphatic fluid. It is in the lymph nodes where antigen is usually presented to the immune system.

The human lymphoid system has the following:

· primary organs: bone marrow (in the hollow center of bones) and the thymus gland (located behind the breastbone above the heart), and

· secondary organs at or near possible portals of entry for pathogens: adenoids, tonsils, spleen (located at the upper left of the abdomen), lymph nodes (along the lymphatic vessels with concentrations in the neck, armpits, abdomen, and groin), Peyer's patches (within the intestines), and the appendix.

Innate Immunity

The innate immunity system is what we are born with and it is nonspecific; all antigens are attacked pretty much equally. It is genetically based and we pass it on to our offspring.

Surface Barriers or Mucosal Immunity

The first and, arguably, most important barrier is the skin. The skin cannot be penetrated by most organisms unless it already has an opening, such as a nick, scratch, or cut.
Mechanically, pathogens are expelled from the lungs by ciliary action as the tiny hairs move in an upward motion; coughing and sneezing abruptly eject both living and nonliving things from the respiratory system; the flushing action of tears, saliva, and urine also force out pathogens, as does the sloughing off of skin.
Sticky mucus in respiratory and gastrointestinal tracts traps many microorganisms.
Acid pH (< 7.0) of skin secretions inhibits bacterial growth. Hair follicles secrete sebum that contains lactic acid and fatty acids both of which inhibit the growth of some pathogenic bacteria and fungi. Areas of the skin not covered with hair, such as the palms and soles of the feet, are most susceptible to fungal infections. Think athlete's foot.
Saliva, tears, nasal secretions, and perspiration contain lysozyme, an enzyme that destroys Gram positive bacterial cell walls causing cell lysis. Vaginal secretions are also slightly acidic (after the onset of menses). Spermine and zinc in semen destroy some pathogens. Lactoperoxidase is a powerful enzyme found in mother's milk.
The stomach is a formidable obstacle insofar as its mucosa secrete hydrochloric acid (0.9 < pH < 3.0, very acidic) and protein-digesting enzymes that kill many pathogens. The stomach can even destroy drugs and other chemicals.