Serology Les. 3 Mod 1

Overview of Antigens and Antibodies

  • This presentation focuses on key concepts in immunology, specifically antigens and antibodies, discussed through PowerPoint slides.

Antigens

  • How to Define Antigens in two ways:

    • Antigens are

    • 1. a genetic marker (or genetic determinant) on tissue that allow the immune system to recognize a cell is part of the body.

    • 2. a foreign substanced introduced into the body that has not been encountered before and (viruses, bacteria, fungus, parasites, and vaccines), elicits the immune system to respond by producing antibodies.

      • These have to enter the body in order cause body to produce an immune system response. Example: Malaria is an antigen that enters the body, causes illness, and then the body is pushed to make antibodies.

  • Key Points on Self and Foreign Antigens:

    • The immune system recognizes and responds to antigens that do not match one’s own, potentially leading to antibody production against foreign antigens.

  • Production of Antibodies

    • The immune system may create an antibody in response to foreign antigens when these antigens are perceived as harmful.

Types of Antigens

  • Common Pathogens as Antigens:

    • Viruses, bacteria, fungi, and parasites are all examples of entities that can be recognized as foreign antigens.

Vaccines and Antigens

  • Vaccine Functionality

    • Vaccines introduce a weakened or inactive form of an antigen to stimulate the immune response without causing disease.

    • When vaccinated with measles or other viruses, the immune system produces a response akin to actual infection, leading to immunity.

    • Immunogens

    • Not all antigens elicit an immune response. Only those that stimulate antibody production are designated as immunogens.

Antibody Behavior and Types

  • Distinction Between Antigens and Immunogens

    • An immunogen will always generate an immune response; an antigen may not if it does not provoke a response.

    • Example: The disparity in antibody production based on environmental exposure and genetic factors.

    • Immunogens are specific; they react with specific antibodies and sensitized T-cells. There is no cross reactivity, likened to a key fitting a lock.

  • Antigenic Determinants

-Epitope- a piece on an antigen that determines what the antigen is. This is the part on an antigen that the body responds to.

For example, during our childhood we know our parents would tell us to do something and we don’t do it. The parents then get mad, but we don’t respond because we wouldn’t notice one way or another. Then the parent wags the finger at us. Then we respond immediately. The finger is the epitope.

Antigen Structure

Antigen’s Chemical Structure

  • Hardy- Not easily destroyed

  • Complex -Has high molecular weight,

  • Are proteins and polysaccharides

  • Can be lipids (fats) but are bad antigens because they are not structurally stable

Antigen’s Physical Nature

  • Foreign -This is the most important aspect of an antigen’s physical nature, otherwise it is not an effective antigen

  • Degradability- they can get rapidly destroyed and come in big numbers in order for them to be effective in causing an immune response

  • Has high molecular weight

  • Must be structurally stable

  • It has to be complex -has lots of proteins that are branched, the more complex, the more likely it is to elicit a strong immune response

  • They can be found on the cell surface and on tissues; they also are viruses, bacteria, fungus, parasites, and products of another cell’s metabolism.

Antibody Structure

  • The Main Function of an Antibody:

-To combine with an antigen. (Antibodies can also combine with a complement)

  • General Structure of Antibodies:

    • Antibodies (or immunoglobulins) are glycoproteins that are found in the plasma. They’re composed of two heavy chains and two light chains, forming a Y-shaped structure.

    • The two heavy chains determine the class of an antibody/immunoglobulin (whether it’s an IgG, IgA, IgM, IgE or an IgD).

    • The two light chains determines whether its a lambda or a kappa immunoglobulin

    • They are often found in urine, not in feces

    • This structure includes:

      • Fab Region (Fragment Antibody):

      • Top part of the Y where antigen binding occurs; varies among different antibodies.

      • Fc Region (Fragment Complement):

      • The lower part of Y responsible for effector functions and can bind to complement proteins only if an antigen-antibody bond exists.

      • The chains that are constant in structure are the ones below the heavy chains in the Fab Region and the two chains on the right side of the Fc Region. These stay the same especially when antigens bond with the antibody at the bonding sites.

      • The chains that are variable are the chains below the binding sites in the Fab Region, which changes with each specific antigen. Like a key in a magic lock.

  • Types of Immunoglobulins:

  • Monomers -These are antibodies that can combine 2 antigens at a time

  • Dimers - These are antibodies that can combine 4 antigens at a time

  • Pentamers -These are antibodies that can combine 10 antigens at a time.

    • IgG, IgA, IgM, IgE, IgD

    • IgG:

      • Most abundant in serum (~75% concentration), can cross the placenta, provides long-term immunity.

      • Important for neutralizing pathogens.

      • This is an antibody that is a monomer

      • These can difuse easily into extravascular space

      • These have 4 subclasses: IgG1, IgG2, IgG3, and IgG4

      • When the IgG complexes are formed, that is when the complements can be activated

    • IgA:

      • Found in secretions (saliva, tears, mucus)

      • This antibody is a Dimer in our secretions

      • This has 2 subclasses: IgA1 and IgA2

      • These can’t cross the placenta

      • This is combined by a J-chain to keep the anibody from flying apart

    • IgM:

      • First antibody produced during an immune response

      • It’s a large pentamer structure allows it to bind ten antigens.

      • These cannot cross the placenta due to it’s size.

      • This is only Intravascular as a result of it’s size

      • It is is hooked together by a J chain

      • This is used to destroy an antigen

    • IgE:

      • Mediates allergic reactions and is elevated in responses to parasites.

      • It is activated by intestinal parasites

      • This antibody is a monomer

      • it binds to mast T-Cells and basophils to release histamine

      • As histamine and IgE increases, so does Eosinophils to inhibit the allergic reaction

    • IgD:

      • It’s associated with B lymphocytes

      • This antibody is a monomer

      • This was often seen with IgM

Antibody Response Phases

Antibody Synthesis happens when the immunes system comes into contact with a foreign antigenic substance and it reacts to it. It starts with a cellular immunity response and then becomes humoral.

  • Phases of Primary Antibody Response (for primarily IgG and IgM):

    1. Lag Phase:

    • In the initial period after exposure with little to no antibody production, the immune system begins to recognize the antigen.

    1. Log Phase:

    • Rapid increase in antibody production as the immune response ramps up. The antibody titers rises exponentially here.

    1. Plateau Phase:

    • Stabilization of antibody levels as the antibody takes a break here

    1. Decline Phase:

    • Antibody levels gradually decrease but do not return to zero; it gets to zero very slowly.

  • IgG lag phase occurs much longer than the IgM antibody, and it doesn’t go to zero, but it picks back up overtime, while IgM very very slowly goes to zero with a slight pickup.

Secondary Immune Response

  • Anamnestic Response:

    • Upon re-exposure to the same antigen, memory B lymphocytes (the clones from the first response) rapidly produce antibodies, leading to a quicker and stronger response due to pre-existing antibodies (IgG).

    • The IgM response is shorter lived as it doesn’t produce as much as the IgG does overtime.

Cross-Reactivity

  • Definition and Implications:

    • Cross-reactivity occurs when antibodies target antigens that are structurally similar, possibly leading to false positives in diagnostic tests (e.g., syphilis tests can show false positives due to heterophile antibodies).

Summary

  • Antigens can be self or foreign substances that ultimately determine immune responses.

  • Antigen-antibody interactions are highly specific; any deviation can lead to immune dysfunction or erroneous diagnoses.

  • Understanding the mechanisms involved in antigen recognition and antibody production is crucial in fields such as immunology and diagnostics.

Review Questions

  • Suggested review materials and questions from textbooks to solidify learning of key concepts were discussed.

  • Chapter review questions were identified to aid in preparation for exams.