Lecture 2: Adaptive Immune System

The Immune System and Adaptive Immunity

• Defined by 3 characteristics:

1. Systemic effects

2. Specificity

3. Memory

• Types of adaptive immunity:

1. Cellular immunity

2. Humoral immunity

• Active immunity: Body makes its own antibodies or T cells

• Passive immunity: Body acquires these from another person or animal

Components of Adaptive Immunity

• Antigens: Any molecule that triggers an immune response (epitopes are part of the molecule that triggers the immune response.)

• Antibodies: AKA immunoglobulins (Igs), proteins that play a variety of roles in defense

• Three classes of lymphocytes: Natural killer (NK) cells, T cells, and B cells

• Macrophages

• Dendritic cells

• T Lymphocytes (T Cells):

1. Produced: Red bone marrow

2. Mature: Thymus

3. Cortex: Can recognize antigens (immunocompetent)

4. Medulla: Distinguish self vs non-self. If it can’t distinguish self it will lead to autoimmune disorders

5. Disperse throughout the body; colonize Lymphatic tissue and organs all over the body

6. Cannot recognize antigens by itself, requires assistance

• B Lymphocytes (B Cells):

1. Formation and maturation in red bone marrow

2. Adults produce 50 million/day only 10% enter circulation

3. Immunocompetent B cells disperse and colonize same locations as T cells

4. Main function: Produce antibodies

Antigen Presenting Cells

• Include: Dendritic cells, macrophages, reticular cells and B cells

• Major Histocompatibility Complex: Proteins on the APC surface - have an elongated groove for holding the foreign antigen

• MHC’s are unique to you - they are “identification tags” that label every cell of your body

Cellular (Cell-Mediated) Immunity

• Cytotoxic T (T_C) Cells: Carry out the attack on foreign cells

• Helper T (T_H) Cells: Promotes action of T_C cells playing key roles in humoral and innate immunity (AKA T4, CD4, or CD4⁺)

• Regulatory (T_R or T-regs) Cells: Limit immune response by inhibiting activities of other T cells; down regulate the response to limit tissue damage (AKA T4, CD4, or CD4⁺)

• Memory T (T_M) Cells: Responsible for memory in cellular immunity

• Adaptive immunity occurs in three stages:

1. Recognition

2. React (Attack)

3. Remember

Cellular (Cell-Mediated) Immunity: Recognition

• T cells respond to 2 classes of MHC proteins:

1. MHC I: Proteins on every cell

2. MHC II: Proteins occurring only on foreign antigens

• Activation occurs when a T_C or T_H cells binds to an MHC protein that the T cell recognizes (must also bind to a second protein)

• Successful costimulation activates clonal selection

Cellular (Cell-Mediated) Immunity: Attack

• When T_C cells recognize an MHC complex, they directly attack and kill other cells

• Specifically, helper T cells secret interleukins that cause 3 main effects:

1. Attract neutrophils/NKCs

2. Attract macrophages, stimulate phagocytosis & keep in area

3. Stimulate T and B cell mitosis and maturation

• Perforin and Granzymes

• Interferons - Activate macrophages

• Tumor Necrosis Factor (TNF) - Activate macrophages

• When T_H cells recognize an MHC complex, it secretes interleukins

Cell (Cell-Mediated) Immunity: Memory

• Primary response is followed by immune memory

• T cell recall response

Humoral Immunity: Recognition

• When antigens bind to several receptors and takes them in by receptor-mediated endocytosis

• Antigen is processed, linked to MHC-II proteins and displayed

• Binding of T_H leads to secretion of interleukins and B cell activation

Humoral Immunity: Attack

• Once released by plasma cells, antibodies use 4 mechanisms to render antigens harmless

• Attack on antigens steps:

1. Neutralization: Mask critical region of an antigen antibodies

2. Complement Fixation: Antibodies bind complement proteins to an enemy cell

3. Agglutination: Clumping of enemy cells by antibodies

4. Precipitation: Antigen molecules are clumped by adhesion to antibodies

• Note: Antibodies don’t directly destroy antigens, they render them harmless

Humoral Immunity: Memory

• Primary Response: When a person is exposed the first time

• Antibody titer (measurement of concentration of antibodies) increases until day 10

• Leaves an immune memory

• Memory B cells mount a quick secondary (anamnestic) response upon re-exposure