Immune System - Anatomy & Physiology

What is the immune system

• Our primary line of defense

• The enemies: bacteria, fungi, protists, viruses, and parasites

Parts of lymphatic system

• Lymphatic vessels

• Lymph nodes

• Lymphatic organs

Lymphatic vessels

Blind ended, one-way vessels that pick up excess fluid in the tissues and return it to the blood stream

Lymph nodes

• We have 500-600 lymph nodes

• Job: filter lymph

Lymphatic organs

• Spleen: filters blood (instead of lymph) removing bacteria, viruses, etc. and destroys worn out blood

• Thymus: site where T cells mature

2 types of body defenses

• Non-specific defenses (also called Natural/Innate immunity)

• Specific defenses (also called Adaptive/Acquired immunity)

4 parts of Non-specific defenses

1. Physical Barriers

2. Cells

3. Antimicrobial Chemicals

4. Other defenses

Physical barriers (nonspecific defense)

• Keratinized skin + acidic secretions

• Mucous membranes

Cells (nonspecific defense)

• Phagocytes, macrophages, and neutrophils

• Natural killer cells: kill virus-infected and malignant cells (cancerous)

Antimicrobial cells (nonspecific defense)

• Interferons: Secreted by virus-protected cells to protect neighbors

• Lysozyme: enzyme that kills bacteria (in tears, saliva, mucus, human milk)

Other defenses (non-specific)

• Fever

• Inflammatory response

  • Calor, dolor, rubor, tumor (heat, pain, redness, swelling)

  • Tries to keep pathogen in one place

  • Cleans damage/sets stage for repair

• Complement: plasma proteins that bind foreign cells and either punch holes in them (lyse) or make them more “tasty” to phagocytes (opsonization) 

4 characteristics of specific defenses

1. Specificity

2. Diversity

3. Self/Non-Self Recognition

4. Memory

Specificity

• Antigen (Ag): any substance capable of causing an immune response. (Antibody Generating)

• Antibodies (Ab) are produced in response to specific antigens.

Diversity

The immune system can recognize and respond to millions of different invaders

Self/Non-self recognition

• Cells of the immune system are “trained” to recognize and not attack “self” proteins

  • Self proteins are generally non-antigenic

Memory

• immune system can “remember” antigens it has encountered and react more quickly on subsequent exposures (acquired immunity). 

• A large population of memory cells are produced.

Names for cytotoxic t-cells

• T_C cells

• CD₈ cells

• MHC I dependent cells

Names for helper cells

• T_H cells

• CD₄ cells

• MHC II dependent cells

Specific defenses

Specific identification of antigen/pathogen generates a response

1. Humoral (antibody mediated)

2. Cellular (cell-mediated) response

Cellular (cell-mediated) Immune Response

1. Cytotoxic T-cells (aka CD₈, MHC I dependent)

   • Recognize + kill self-altered cells (virally infected cells and cancerous cells)

   • Can only see antigens presented in MHC I proteins

2. Helper T cells (aka CD₄, MHC II dependent)

   • Help activate cytotoxic t-cells

   • Help activate B-cells

   • Help make the macrophages better phagocytes

   • Can only see antigens presented in MHC II proteins

Major Histocompatibility Complex (MHC)

• Proteins expressed on outside of cells

• MHC I and MHC II

MHC I

• Expressed on almost all body cells

• Express self proteins when cell is healthy

• Express viral proteins when cell is infected and altered self-proteins when cell is cancerous

MHC II

• Expressed on Antigen presenting cells (APC): macrophages, B-cells, and dendritic cells

• Present Ag (antigen) to T_H cells (helper cells)

Antibodies

• A class of proteins called immunoglobins (Ig) made by B cells

• Basic antibody structure: 2 heavy protein chains and 2 light chains with constant and variable regions

5 types of antibodies

• IgM: free in plasma, attached to B cells, activate complement, released during primary response

• IgA: in saliva, tears, milk, gastric juices

• IgD: attached to B cells

• IgG: free in plasma, activate complement, cross placenta from mom to fetus, released during secondary response

• IgE: in GI and respiratory tract secretions, involved in allergic response

4 functions of antibodies

1. Opsonization: coat the pathogen and make it more tasty to macrophages

2. Neutralization: bind to the pathogen to block its entry into your cells

3. Agglutination: Ab crosslink pathogens together into clumps that are easier for the phagocytes to eat

4. Complement fixation (activation): protein cascade activates opsonization

2 types of Humoral (antibody mediated) immunity

1. Active immunity (long term)

2. Passive immunity (short term)

Active humoral immunity

• Is direct exposure to an antigen so you acquire immunity to it

  1. Get sick with disease and then get better

  2. Get vaccinated against the disease

• B cells bind antigen, B cells become active plasma B cells that secrete antibodies specific to the antigen

• Some B cells become memory B cells that produce a secondary response if pathogen is encountered in the future

• Has memory (long-term immunity)

Passive humoral immunity

• When you are given the antibodies rather than making them yourself

• Lasts only a few weeks/months (temporary immunity)

• No memory is established because it wasn’t your B cells that made the antibodies → memory B cells are not produced

• 2 ways to gain passive immunity

  1. Transferred from mother to child across placenta (IgG) or breast milk (IgA)

  2. Injecting the antibodies into the person

Primary vs. secondary immune response

1) Primary

• Takes 10 days to reach peak Ab levels

• Response lasts less than 2 weeks

• Lower number of Ab produced

2) Secondary

• Takes 2-3 days to reach peak Ab levels

• Response lasts weeks to months

• Higher number of Ab produced

Vaccination

• Administration of weakened/dead pathogen or component of pathogen designed to trigger the primary immune response so when the pathogen is encountered in “real life,” the rapid secondary response takes care of it before illness results