Chapter 16

Genetics of Immunity  1  - Human  Immune  System

I.  Immunity

Immune system protects organisms from foreign invaders.

Foreign versus self

•To protect the body from harmful organisms, the immune system identifies foreign molecules as “nonself” and destroys those cells.

•Molecules recognized by the immune system are called antigens.  Antigens are usually protein fragments or carbohydrates.

•During development, the immune system removes components that recognize “self”.

•Immune response to one’s own body is called autoimmunity.

Rh factor

•Rh factor or rhesus factor is another blood group affecting cell surface molecules. 

•Three genes affect this phenotype.

     Phenotypes:

     Rh+      produces Rh factor on RBC

     Rh-       no Rh factor on RBC

Rh incompatibility

      Rh incompatibility occurs when an Rh- mother has an Rh+ child.

II.  Human leukocyte antigens (HLA)

•HLA proteins are produced by major histocompatibility complex (MHC) class I and II genes.

•Link sugars to form branched glycoproteins on cell surface of white blood cells.

•HLA glycoproteins can recognize bacterial and viral proteins, marking them for immune system to target, a process called antigen processing.

Antigen Presenting Cells

HLA diversity

   Several genes with multiple alleles determine an HLA type.

Genetic diversity at HLA genes is large.

•Only 1 in 10,000 unrelated people will share an HLA type by chance at the six major HLA genes.

•Matching at least 4 major HLA genes is needed for most transplants to succeed.

HLA genes account for about 50% of the genetic impact on immunity.

III. Components of the immune system

Lymphatic system of duct and nodes

Lymph, fluid filling lymph ducts

1.  Organs involved in production or maturation of immune cells

–Spleen

–Thymus 

–Bone marrow

–Skin

–Small intestine

2.  Immune system cells

Many cells types contribute to immune response

Types of immune cells

Levels of immune protection

Physical barriers

Lymphocytes

Macrophages

Innate immunity

Adaptive immunity

Physical barriers

Unbroken skin

Mucous membranes and secretions

3. Infection fighting chemicals in tears and saliva

Flushing effect of tears, saliva, urination, and diarrhea

Nonspecific innate defense

Phagocytosis

Antimicrobial proteins

•    Complement system 

•    Collectins

•    Cytokines

–          interferon

–          interleukins

–          tumor necrosis factor

4.  Inflammatory response

Fever

Inflammation: an innate defense system

Complement system kills bacteria

Specific, Adaptive Immunity

Requires stimulation.

Response time is in days.

Major features:

Diversity     many different pathogens recognized

Specificity  distinguishes particular molecules

Memory      responds faster with subsequent exposure

Primary immune response is reaction to first exposure.

Secondary immune response is reaction to exposure using “memory” of first response.

Specific, Adaptive Immunity

5.  Two types of response:

Humoral immune response

      B cells, antibodies, memory cells

Cellular immune response:

      T cells, cytokines, memory cells

Activated B cells produce antibodies and memory cells

Function of Antibodies

Types of antibodies or immunoglobins (Ig)

•There are numerous but limited antibody genes in the human genome.

•During early development of B cells, sections of the antibody genes are rearranged along their chromosome.

•Rearrangement of Ig genes creates new versions of the antibody proteins.

•By shuffling 200 genes, over 100 trillion different antibodies can be produced.

•A single stimulated B cell produces the same antibody combinations.