Adaptive Immunity

Adaptive immunity is a complex immune response that specifically targets pathogens.
Humoral response is a key component of this system, characterized by antibody-mediated actions.

Definition: The humoral immune response is distinct for being mediated by antibodies against extracellular antigens.
Pathogen Interaction: The response is particularly involved with pathogens that remain outside of human cells (extracellular).

B cells: These are the primary cells that mediate the humoral response. They differentiate into plasma cells that produce antibodies.
Helper T cells: These cells assist in activating B cells and are crucial in the humoral response. They help by releasing cytokines.

Pathogens such as bacteria or spirochetes enter the body and trigger a response.
Trillions of B cells exist within the body, each capable of recognizing a specific antigen.

B Cell Receptor Binding: Each B cell has a unique receptor that binds to its specific antigen on the pathogen.
Internalization: The B cell internalizes the antigen after binding.
Processing: The antigen is degraded into fragments within the B cell.
MHC Class II Presentation: These fragments are presented on the B cell's surface via MHC class II molecules, indicating to helper T cells what is present.

Recognition of Pathogenic Antigens: A helper T cell identifies an antigen presented by a B cell that is pathogenic.
Cytokine Release: The helper T cell secretes cytokines to activate the B cell.
Clonal Expansion: Activated B cells undergo proliferation, leading to the creation of numerous clones.
Differentiation into Plasma and Memory B Cells: Some B cells become plasma cells that produce antibodies, while others become memory B cells that remain for future responses.

If a B cell presents a harmless antigen, the helper T cell will recognize it as non-threatening.
Cytokine Response: Instead of activating, the helper T cell will secrete cytokines that inactivate the B cell, known as anergy, preventing unnecessary immune reactions to benign substances.

Origin: B cells are produced and mature in the bone marrow.
Naive B Cells: These immature B cells reside in lymph nodes awaiting activation.
Clonal Expansion and Differentiation: Upon activation, they undergo clonal expansion and differentiation into plasma or memory B cells.

Definition: Antibodies are proteins generated by plasma cells, commonly referred to as immunoglobulins (Ig).
Y-Shaped Structure: Consists of two heavy chains (green) and two light chains (gray) connected by disulfide bonds.
Regions of Antibodies:
- Constant Region: Determines the class of immunoglobulin; type is critical for antibody function.
- Variable Region: Unique for each antibody where antigen binding occurs, specifically at the arms of the Y.
Binding Sites: Two antigen binding sites per antibody, allowing for effective immune response against specific pathogens.
- One is called the stem (Fc region). It serves as a red flag or a signal. This lets phagocytes know hey there is a problem here and I need your help.
- The second is known as the fab region (antigen binding sites). The epitope is a segment of antigen where antibodies bind.
  - Epitopes (small protein subunit): The specific sites on antigens where antibodies bind, usually comprising a few amino acids (4-16).

IgM
- Structure: Pentamer (five antibodies linked); large size limits its transfer across biological barriers. (10 antigen binding sites)
  - Too large to pass through most barriers.
  - First class of antibody made against a specific antigen. So if a plasma cell has been introduced to an antigen for the first time. IGM is the first class to be made against that antigen.
- Function: First antibody to be produced against a specific antigen; plays a key role in initial immune response.
IgG
- Structure: Smaller than IgM, abundant in circulation; can cross the placenta to provide fetal immunity.
  - Abundant in the body and the second class to be made.
  - It’s smaller therefore it can be transported from the placenta to the fetuses blood stream.
- Function: Produced during a secondary immune response; essential for long-term immunity.
IgA
- Structure: Dimeric form (two Y shapes linked); abundant in mucosal areas.
  - The MOST abundant class of all the immunogobulins.
  - IgA is so important for mucosal immunity. IgA is found on the lining of mucus membranes.
    - Remember mucus membranes is where most pathogens enter from.
  - IgA is very important for preventing pathogens in the mucous membrane area.
  - IgA is also secreted in breast milk, tears, saliva, and any bodily fluids like that.

IgD
- Function: Less than 1% of antibodies; thought to be involved in development of immune responses.
IgE
- Function: Involved in allergic reactions and defense against helminths diseases like a parasitic worm; barely detectable in blood.

Neutralization: Antibodies block pathogen binding sites of the virus or the toxin.
- Which results in viral spikes not being able to attach to the host receptor because antibodies are blocking them. Or a toxin can’t bind to the host because antibodies are blocking them.
  - Happens when an antibody binds to a virus. A bunch of antibodies have bound all over the virus or an antibody binds all over to an exotoxin that is secreted from a bacteria.
Opsonization (easier for phagocytosis): Antibodies coat pathogens, which make it easier for phagocytes to recognize and engulf.
- The stem on the antibody will signal or flag the phagocytes to come in and engulf.
Complement Activation: If an antibody binds to a pathogen it can activate the complement activation system which is part of your innate immunity.
Immobilization and Prevention of Adherence: Antibodies could bind to the flagella of a bacteria and this prevents the flagella from working properly so the bacteria are immobilized they can’t move.
Cross-linking: Is where one antibody bind to two bacteria two epitopes from two different antigens at the same time.
- This creates a giant antibody pathogen clump. This is going to make it easier for phagocytes to come in and engulf all of this clump at once instead of one pathogen at a time.

Antibody-dependent Cellular Cytotoxicity (ADCC): Antibodies coat all the way around an infected human host cell with a virus which signals for a natural killer cells to come in and signal the host cell to undergo apoptosis in order to prevent the spread of the virus.
- Natural killer cells will secrete cytokines so the human host cell knows to kill itself by apoptosis.