![[Pasted image 20240609065350.png|300]] # Antibodies ## Antibody Function The purpose of all immunoglobulins is to bind to their corresponding antigen. Antibodies specifically will continue on to deliver the bound antigen to various components of the immune system while membrane bound immunoglobulins (BCRs) **Antibodies (Ab)** are the secreted forms of [[Antibodies|B Cells Receptors (BCRs)]] They are large Y-shaped [[Proteins|proteins]] that are capable of **identifying and neutralizing foreign objects.** - Both BCRs and antibodies are also known as **immunoglobulins (Ig)** > [!info]- Key Features of Antibodies > **Antibodies** are: > 1. Extremely specific (high affinity) > 2. Are produced by [[B-Cells]] as a result of the [[Adaptive Immunity|adaptive immune system]] > 3. Circulate our bo[](B%20Cells.md)ajor component of blood and [[The Lymphatic System|lymph]] > 4. Bind to the extracellular spaces (do not enter cells) There are many different functions of antibodies, although the most - Polyclonal (Multiple Clones) - Monoclonal (Single Clone) ### Antibody Binding > See also: > - [[Intermolecular Forces]] The achieve this goal, antibodies will bind to a unique molecule of the target pathogen known as the [[Antigens|antigen]]. Antibodies can bind to: - **Antigen Presenters** (such as [[Major Histocompatibility Complex (MHC)|MHC Class I & II]]) - Directly to the antigen on a **pathogen's surface** --- An antibody’s **valency** is the amount of antigen binding sites it has associated with it. > [!warning] **Affinity vs Avidity** > Affinity: How tightly the antibody binds to the antigen > Some antibodies have high flexibility largely do to their **hinge region** and as a result have a higher binding avidity. Monoclonal vs Polyclonal ### Catalytic Antibodies > See also: > https://pdb101.rcsb.org/motm/21 ![[Pasted image 20240314233449.png|200]] At the end of the day, antibodies are still just proteins and are also *capable of catalyzing biological reactions* the same way any enzyme would. ## Compliment System Interactions > See also: > - [[The Complement System]] Not every antibody is as effective at activating [[the complement system]], with some not being capable of it at all: ![[Relative Complement Fixing Affinity.png|200]] ## Immunoglobulin Structure The overall structure of an immunoglobulin is y-shaped and contains ==four== different polypeptide chains. - Two identical heavy chains (H chains) - Two identical light chains (L chains) CDR: Complementarity-determing region ### Main Structure ![[Pasted image 20240309131548.png|320]] > [!summary] **Paratope vs Epitope** > The **paratope** *(antibody region*) binds to the **epitope** *(antigen region)*. > > An epitope will likely bind to various different paratopes. When an epitope has few or a single associated paratope it is known as **selective** *Pathogen Recognition Receptors (PRRs)* are used by phagocytes to detect *Pathogen Associated Molecular Patterns (PAMPs)* during the innate (primary) immune response Cross-linking can occur when a single antibody binds to multiple different antigens. When a large number of antibodies bind in this way, aggregates known as **immune complexes** can form. ### Antibody Isotypes The Fc region is the backbone > [!tip] These can be remembered using the acronym "G.A.M.E.D." ![[Immunoglobulin Structure.png|400]] There are five different classes (isotypes) of antibodies: > [!info]- **IgG** > - Most abundant isotype in blood (plasma), accounting for 70-75% > - Smaller and better able to pass through tissue towards sites of infection > - Only class that can pass through the placenta > - Likes to o p s o n i z e > - IFN $\gamma$ causes isotype switching to IgG and also promotes the activation of macrophages, making it a wombo combo super combo of a cytokinend X‑Gal that produces blue colonies > - Largely found within blood vessels, protects blood and tissues > - Prevents blood-borne (omg like the game) infections > - Neutralizes toxins/viruses > - Opsonizes bacteria/viruses > [!info]- **IgA** > - Has a dimeric and monomeric form > - Monomeric form is secreted by plasma cells who underwent isotype switching in the [[Lymph Nodes|lymph node]] or spleen > - Dimeric form is made in secondary lymphoid tissue near mucosal surfaces and is found in secreted solutions (mother’s milk) > [!info]- **IgM** > - The first antibody that is formed > - The most prominent during primary immunity (the first encounter) > - Forms a pentamer (5 copies) structure > - This large structure and the general longer peptide chain of IgM makes it less effective at squeezing through tissues to get to infected sites > [!info]- **IgE** > - Lowest Concentration > - Main function is to fight large parasitic organisms (helminths, ) > - Their Fc region binds to the surface of granulocytes, allowing the antibody arm to still bind to antigens on the parasitic organism and promote the release of their granules > [!info]- **IgD** > - Their sole purpose is to promote the development of B-Cells within the bone marrow > - Should stop expression once the B-Cell has matured (but still before activation) and is moving through the lymphatics > #### Isotype Switching > See also: > - [[Transcription]] B-Cells are capable of changing what class of isotype they produce through a process called **isotype switching** ![[Isotype Switching Cytokines.png|400]] IL-21 is considered one of the more important cytokines in this process as it is the most potent inducer of terminal differentiation of B-Cells into plasma cells