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Immunity System


“Immunity System is one of the 11 systems in human body which protect animal from pathogens and diseases and independent of nervous and endocrine system.”

These are the organisms that can invade our body and can make us sick.

PATHOGEN: A disease causing organism which also causes infection.

Pathogenic Bacteria: disease causing bacteria.

Pathogenic Fungi: Fungi only infect the skin and only get life threatening when they infect lungs.

Example: Athlete’s foot, ringworm, yeast infection

Pathogenic viruses

Single Celled Protists/Parasites: Amoebic dysentery, malaria, African sleeping sickness, giardia (by drinking untreated water)

“The IMMUNE SYSTEM is a host defense system comprising many biological structures and processes within an organism that protects against disease.”

Two Divisions of Mammal Immune System


This system merely prevents pathogen from entering the body or destroys it before even identifying it. The defense is not concern with the kind of pathogen. It’s shoots first and asks questions later.

Several Layers of Defense in Innate Immune System:

A. Barrier defense

B. Inflammatory defenses

C. Cellular defense

D. Natural killer cells

E. Antimicrobial defenses

A. Barrier Defenses:

The epithelial surfaces form a physical barrier that is impermeable to most infectious agents, acting as the first line of defense against invading organisms.  Shedding of skin epithelium also helps remove bacteria and other infectious agents that have adhered to the epithelial surfaces.

  • Epidermis: an impenetrable barrier to pathogens
  • Oil and sweat: have a low pH that reduces pathogen growth
  • Resident flora: your own colonies of bacteria that lives on your skin out. Compete with harmful bacteria for space
Mucus Membrane:
  • Un-keratinized skin of mucus membrane has different layers of defense
  • Mucus trap pathogens like paper fly etc.
  • They are present in mouth, respiratory and digestive tract.
  • In respiratory tract, cilia help in removing dirty mucus out of body
  • Saliva and tears have this destructive enzyme which is another chemical protection that can break down proteins.


  • The complement system also functions in the inflammatory response. The swellings and pain that alert you to splitter in skin are result able due to local inflammatory response.
  • This is brought about by signaling molecule releasing by injury or infection.
  • One important signaling molecule is histamine which is stored in the granule of mast cells that are found in connective tissues.
  • Histamine release at sights of damage, trigger by near blood vessels. Macrophages become activated and perform diapedesis and attack and destroy pathogens.
  • But dead microbes and cell debris accumulate at the sight of inflammation.
  • Macrophages may also release pyrogens which are chemicals inducing and causing fever.



“Phagocytic white blood cell that has surface receptors that detect specific pathogenic compounds called antigens.”

  • TLR (toll like receptors) bind to specific surfaces.
  • TLR3 binds to double stranded RNA found in some viruses.
  • TLR4 binds to lipids found on surface of pathogenic bacteria.
  • TLR5 binds to bacterial flagellin protein

After detecting pathogens the phagocytic cells may engulf them, trapping them in vacuum and combine with lysosome they destroy pathogens.

4 Types of Phagocytic Cells:

1 .Neutrophils:

“They can flow along the blood stream and can engulf and destroy microbes and tissues. They are the most numerous white blood cells in the body.”

Diapedesis: Movement of leukocytes out of the circulatory system and towards the site of tissue damage or infection due to specific chemicals.

2.  Macrophages:

“They are the part of lymphatic system and present throughout the body but are completely embedded in spline and lymph nodes where they destroy pathogens.”

3. Eosinophil:

They stay embedded in tissues and discharge destructive enzyme.

4. Dendric Cells:

They have low phagocytic activity but are effective against multi-stages pathogens like parasite worms.


“They circulate in entire body and can detect normal proteins with viruses or cancerous cells. They don’t phagocytize the defected cells but rather secret cytotoxic chemicals that can trigger defected cells to destroy themselves and save other cells from infection.”


“ The lymphatic system is part of the circulatory system and a vital part of the immune system, comprising a network of lymphatic vessels and nodes  that carry a clear fluid called lymph directionally towards the heart.”

Some macrophages which are called lymph nodes which engulf pathogens which comes from instentional fluid to lymph. Dendric cells migrate to lymph nodes after interacting with the pathogen , there the dendroid cells are alert of infection by communicating to other immunity process, starting to adaptive immunity where cells learn the type of pathogen.

E. ANTIMICROBIAL PROTEINS: (Inhibit production of pathogens)

  • Peptides and protein function in innate defense by attacking microbes directly or impeding their reproduction.
  • Interferon protein provide innate defense against viruses and help activate macrophage by signals. They are secreted by virus infected cells.
  • Complement system about 30 different protein make up infection fighting components. these proteins circulates in an inactive form and activated by surfaces of many microbes and pathogens.


Adaptive immune system is specific and found in vertebrates including humans that make your body immune to infectious diseases like flu, chicken pox etc. The downside is that you normally get sick first and then your immune system develops immunity so you can’t be infected second time by the some virus etc. The acquired response is called “adaptive” because it prepares the body’s immune system for future challenges. Adaptive response realizes on B cells and T cells which are type of white blood cells collectively called lymphocytes. Like all blood cells, lymphocytes are made in the red marrow bone.

  • T cells: some lymphocytes migrate to thymus gland. These lymphocytes mature in T cells.
  • B cells: some lymphocytes remain in bone marrow and develop into B cells.
  • Natural killer cells: still some other lymphocytes remain in blood and become NK Cells.

Antigen: substances that trigger an immune response for example antibody production by B cells.

Antigen receptors: In adaptive immunity antigen are bind through specific receptor proteins, called antigen receptors. T and B cells have receptor proteins that can recognize and bind to antigens. These are specific to bind just one part of the molecule of pathogen. All the antigen receptors of T and B are identical. Infection of virus, bacteria or other pathogen triggers or activates the B and T cell due to specific antigen receptor.


  • B cell binds specific antigen in the antigen receptor and internalizes it.
  • B cell engulf and transfers antigen to a Class II MHC receptor and returns antigen now bound to MHC back to the surface of the cell. B cell is now sensitized.
  • A helper T cell joins the action and specific for the antigen recognizes and binds to the antigen in the Class II MHC and becomes activated.
  • The activated helper T cell secretes cytokines on the B cell.
  • Cytokines activate the B cell and other lymphocytes.
  • The activated B cell undergoes clonal selection producing Memory B and Plasma cells.
  • Plasma cells secrete antibodies that are specific for the original antigen and produce millions of copies in one hour.

Antibodies / Immunoglobulin: A special protein produced by plasma cells (B cells) that will recognize and bind to its specific epitope of an antigen via its antigen binding sites.

  • IgG antibodies
  • IgM antibodies
  • IgA antibodies
  • IgD antibodies
  • IgE antibodies

The major class of immunoglobulin in the blood is IgG, which is a four chain monomer produced in large quantities during secondary antibody responses. Besides activating complement, the tail region of an IgG molecule binds to specific receptors on macrophages and neutrophils.

Major Histocompatibility Complex:  a present piece of antigen on infected cell surface so that a white blood cell can identify it for destruction. Every cell is cover with MHC.


“It is an immune response that that does not involve antibodies but rather involve the activation of phagocytes, killer T cells and cytokines in response to antigen.”

  1. Helper cells activate cells related to cell mediated immunity. It helps in overall response by immune system.
  2. TH (Helper T cells) is activated by antigen in Class II MHC -respond by secreting cytokines to influence other immune cells.
  3. It causes the killing of infected cells by cytotoxic T cells.


  1. Cytotoxic T cell
  2. Natural killer cell

Both contain target oriented granules of chemical and both kill on contact with target by delivering burst of chemicals.

  • T cells also have antigen-specific surface molecules called T cell receptors (TCRs)
  • If a cell is infected by a virus (or other intracellular microbe) or some of the peptides presented on its class I MHC will be viral peptides. The right T cell receptor will bind to the viral peptide antigen + MHC.

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