BIO 301
Human Physiology

The word Blood
& Body Defenses II


Body Defenses


Major targets of body defense system:Drawings of a typical virus and typical bacterium


Bacteria and viruses  

Anti-bacterial defenses


Illustation of the various components of the innate and adaptive immune systems
The innate immune response functions as the first line of defence against infection. It consists of soluble factors, such as complement proteins, and diverse cellular components including granulocytes (basophils, eosinophils and neutrophils), mast cells, macrophages, dendritic cells and natural killer cells. The adaptive immune response is slower to develop, but manifests as increased antigenic specificity and memory. It consists of antibodies, B cells, and CD4+ and CD8+ T lymphocytes. Natural killer T cells and gamma-delta T cells are cytotoxic lymphocytes that straddle the interface of innate and adaptive immunity (Dranoff 2004). Dendritic cells are leucocytes that get their name from their surface projections that resemble the dendrites of neurons. They are found in most tissues of the body and are particularly abundant in those that are interfaces between the external and internal environments, e.g., skin and the lining of the gastrointestinal tract. Dencritic cells present antigen/self-antigen complexes that activate T-cells. Gamma-delta cells are primarily found in the intestine, the lining of the vagina, and the skin. They encounter antigens at those locations and, therefore, serve as a first line of defense.

Nonspecific Immune Responses

Inflammation  (Check this animation and this one )

Bacterial invasion or tissue damage

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Release of histamine by mast cells (plus chemotaxins by damaged cells)

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Arterial vasodilation & Increased capillary permeability

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Increased blood flow to tissue & accumulation of fluid

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Increased numbers of phagocytes & more clotting factors into surrounding tissues

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Defense against foreign invader plus 'walling off' of inflamed area

Illustration of the events that occur during inflammation

Immunology in the skin

Immunology in the gut mucosa

Interferon (check these animations - Antiviral activity of interferon & Interferons):

Virus enters a cell

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Cell releases interferon

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Interferon binds with receptors on uninvaded cells

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Uninvaded cells produce enzymes capable of breaking down viral mRNA

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Virus enters previously-uninvaded cell (now with interferon)

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Virus-blocking enzymes are activated

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Virus unable to multiply in newly invaded cells

Animated gif showing how viruses cause cells to produce and release interferon

Interferon and the immune response

Natural killer cells

Illustration of how natural killer cells kill target cells

Natural killer cells    

The Complement System (check this animation - Activation of complement)

Illustration of the complement system cascade
The complement cascade after activation by pathogens. Tthe complement cascade is usually activated by antibody complexes (classical pathway). This activation leads opsonization and phagocytosis, as well as lysis as a result of the formation of the membrane attack complex (MAC). These combined actions of complement lead to the elimination of pathogenic cells (Figure modified from Ricklin and Lambris 2007).

Complement system

Complement system (more detailed!)

Drawing showing how complement proteins act as opsonins
Under certain circumstances of infection, bacteria or viruses may become coated with opsonins (C3b, a complement protein, or IgG, an antibody). Such microbes are said to be opsonized (opsonin comes from a Greek word meaning "sauce" or "seasoning"; they make the bacterium or virus more palatable and more easily ingested by a phagocyte.) Opsonins dramatically increase the rate of adherence and ingestion of a pathogen  (Source:


Specific Immune Responses

Drawing showing that stem cells in the bone marrow can become either t-cells or b-cells
Lymphocytes originate as stem cells in the bone marrow.
Some migrate to the Thymus & develop into T-cells; others remain in the bone marrow & develop into B-cells.
Both B- & T-cells then migrate to lymphoid tissue.

Drawing showing location of lymph tissues in humans
Organs and tissues of the immune system

B lymphocytes (or B cells) are most effective against bacteria & their toxins plus a few viruses, while T lymphocytes (or T cells) recognize & destroy body cells gone awry, including virus-infected cells & cancer cells.

Illustration of how stem cells via clonal selection and deletion generate B-cells and T-cells
  Clonal selection (and expansion of the vertebrate immune response (Figure from Bergstrom and Antia 2006)

The adaptive immune response of vertebrates works by clonal selection. Independent of exposure to an antigen or pathogen, the immune system generates a repertoire of immune cell lineages or clones (labeled 1–8 in the above Figure), each encoding a receptor with a predetermined shape and specificity. The human immune system creates in excess of 10,000,000 different clones. As a first approximation, those that react with self-antigens (numbers 3, 5, and 8 in Figure I) are deleted shortly after they mature. When the individual is infected with a pathogen, those clones that are specific for the pathogen (number 2 in the above Figure) will proliferate, producing a pathogen-specific immune cell population that is large enough to control that pathogen. This process is known as clonal expansion. After the pathogen is cleared, some of the pathogen-specific immune cells survive and confer immune memory (Bergstrom and Antia 2006).

How do B & T cells recognize unwanted cells & other material?Drawing of a protein formed by the virus that causes lyme disease

B-cells: Antibody-mediated immunity


Illustration of how B-cells are activated to become plasma cells that produce antibodies
Source: National Cancer Institute
Activation of B Cells to Make Antibody

The B cell uses its receptor to bind a matching antigen, which it proceeds to engulf and process. Then it combines a fragment of antigen with its special marker, the class II protein. This combination of antigen and marker is recognized and bound by a T cell carrying a matching receptor. The binding activates the T cell, which then releases lymphokines—interleukins—that transform the B cell into an antibody- secreting plasma cell.

Photomicrograph of a plasma cell
Plasma Cell

Humoral immunity  

Humeral response

Antibody production




Illustration of how an allergen cause produce allergic symptoms
Source: NIAID

Understanding allergies

Drawing of an antibody

Drawing of an antibody
Source: Check (2007).

Drawings of IgA and IgM
Source: National Cancer Institute
IgA and IgM

IgA -- a doublet -- concentrates in body fluids such as tears, saliva, and the secretions of the respiratory and gastrointestinal tracts. It is, thus, in a position to guard the entrances to the body. 

IgM usually combines in star-shaped clusters. It tends to remain in the bloodstream, where it is very effective in killing bacteria.

Animated gif showing how antibodies can neutralize toxins

Drawing showing the different biological effects of antibodies
The different biological effects of antibodies (Casadevall et al. 2004).

Plasma cells vs. Memory cells

Drawing showing how B-cells and T-cells form effector cells and memory cells
(a) Development of memory B cells and plasma cells.
Development of memory T cells. After activation, cells differentiate into effector T cells and effector memory T cells (Gray 2002).

Plasma cells:

Memory cells:

Primary response vs. Secondary response:

Graph showing antibody production after first exposure and after second exposure to an antigen

Active immunity vs. Passive immunity

Drawing showiing examples of long-term immumity and short-term immunity
As long ago as the 5th century B.C., Greek physicians noted that people who had recovered from the plague would never get it again - they had acquired immunity. This is because, whenever T cells and B cells are activated, some of the cells become "memory" cells. Then, the next time that an individual encounters that same antigen, the immune system is primed to destroy it quickly. The degree and duration of immunity depend on the kind of antigen, its amount, and how it enters the body. An immune response is also dictated by heredity; some individuals respond strongly to a given antigen, others weakly, and some not at all.

Infants are born with relatively weak immune responses. They have, however, a natural "passive" immunity; they are protected during the first months of life by means of antibodies they receive from their mothers. The antibody IgG, which travels across the placenta, makes them immune to the same microbes to which their mothers are immune. Children who are nursed also receive IgA from breast milk; it protects the digestive tract. Passive immunity can also be conveyed by antibody-containing serum obtained from individuals who are immune to a specific infectious agent. Immune serum globulin or "gamma globulin" is sometimes given to protect travelers to countries where hepatitis is widespread. Passive immunity typically lasts only a few weeks.

"Active" immunity (mounting an immune response) can be triggered by both infection and vaccination. Vaccines contain microorganisms that have been altered so they will produce an immune response but will not be able to induce full-blown disease. Some vaccines are made from microbes that have been killed. Others use microbes that have been changed slightly so they can no longer produce infection. They may, for instance, be unable to multiply. Some vaccines are made from a live virus that has been weakened, or attenuated, by growing it for many cycles in animals or cell cultures.

T Lymphocytes: Cell-mediated Immunity

Cytotoxic T cells: (check this animation)

Illustration of how cytotoxic T-cells are activated
Source: National Cancer Institute

Drawing of an antigen-presenting cell
Figure slightly modified from Iwasaki and Medzhitov 2010

Cytotoxic T cell attacking cancer cell

Helper T cells (check this animation)

Illustration of how helper T-cells are activated
Source: National Cancer Institute


Suppressor T cells (also called Regulatory T cells):


Drawing showing functions of regulatory T-cellsRegulatory T (TReg) cells are essential for maintaining tolerance, preventing
autoimmune diseases, and limiting chronic inflammatory diseases (Vignali et al. 2008).


Autoimmunity may arise in several ways:

Graph showing incidence of major autoimmune diseases in males and females
Major autoimmune diseases (Source:

Blood typing

Red Blood Cells:

ABO system:

Blood type Antigen present
A & B
neither A nor B

Drawing showing how blood types are determined genetically


Blood type
A & B
both anti-A & anti-B

If you mix anti-A antibodies with blood cells that have the A antigen OR mix anti-B antibodies with blood cells that have the B antigen, the results will be AGGLUTINATION (or clumping of red blood cells). This reaction can be used to type blood. You simply take two drops of 'unknown' blood and place a drop of anti-A antibody solution on one blood drop & a drop of anti-B antibody solution on the other blood drop. Then, look closely to see if any clumping occurs. If clumping occurs in the drop of blood where you added the anti-A antibodies, then you know that the A antigen is present (and, of course, if there is no clumping, then the A antigen is not present). If clumping occurs in the drop of blood where you added the anti-B antibodies, then you know that the B antigen is present (and, of course, if there is no clumping, then the B antigen is not present). Using this information, you can determine the blood type:

Drop of blood in which 
anti-A antibody was added
Drop of blood in which
anti-B antibody was added
Blood type
No clumping
No clumping
No clumping
No clumping

Type O blood is the most common blood type, followed by type A, type B, and, the least common blood type, AB.

Pie chart showing frequency of different blood types among people in the United States
O+ 37%, O- 6%, A+ 34%, A- 6%,
     B+ 10%, B- 2%, AB+ 4%, AB- 1%

In the above chart, the blood types are listed with either a + or -. The + or - refers to the presence or absence of the Rh factor.

Type O:

Type AB:

Rh system:

Erythroblastosis fetalis (also called Rh disease):

Rh factor

Literature cited:

Bergstrom, C. T. and R. Antia. 2006. How do adaptive immune systems control pathogens while avoiding autoimmunity? Trends in Ecology and Evolution 21:22-28.

Iwasaki, A., and R. Medzhitov. 2010. Regulation of adaptive immunity by the innate immune system. Science 327:291-295.

Related links:


Acute Inflammation

Lymphatic System & Immunity

General Immunology

Introduction to Immunology

Cell Mediated and Humoral Immunity

Understanding the Immune System

Humoral Immunity

Immuno Biology Animations

Blood Types Tutorial

Animated gif of a dog wagging its tailBack to 301 syllabus

Lecture Notes 1 - Cell Structure & Metabolism

Lecture Notes 2 - Neurons & the Nervous System I

Lecture Notes 2b - Neurons & the Nervous System II

Lecture Notes 3 - Muscle

Lecture Notes 4 - Blood and Body Defenses I

Lecture Notes 5 - Cardiovascular System

Lecture Notes 6 - Respiratory System

Literature cited:

Casadevall, A., E. Dadachova, and L.-a. Pirofski. 2004. Passive antibody therapy for infectious diseases. Nature Reviews Microbiology 2: 695-703.

Check, E. 2007. Immunology: pimp my antibody. Nature 446: 964-966.

Dranoff, G. 2004. Cytokines in cancer pathogenesis and cancer therapy. Nature Reviews Cancer 4: 11-22.

Gray, D. 2002. A role for antigen in the maintenance of immunological memory. Nature Reviews Immunology 2: 60-65.

Ricklin, D. and J. D. Lambris. 2007. Complement-targeted therapeutics. Nature Biotechnology 25: 1265-1275.

Vignali, D. A. A., L. W. Collison, and C. J. Workman. 2008. How regulatory T cells work. Nature Reviews Immunology 8: 523-532.