Introduction to Antigens and Antibodies
Forensic scientists use various methods to accurately identify the species of suspected biological samples in criminal investigations. The principle behind species identification is based on the fact that different animal species, including humans, have unique protein markers, also known as antigens, on their cells that distinguish them from one another. By identifying the species-specific antigens and the corresponding antibodies that interact with them, forensic investigators can determine the species of origin of a biological sample.
WHAT ARE ANTIGENS AND ANTIBODIES?
A) Antigens
Antigens are molecules, often proteins, found on the surface of cells or particles that can elicit an immune response. Antigens can be classified into different types:
a) Exogenous antigens: These antigens come from outside the body. They come from external sources, such as pathogens (bacteria, viruses, and fungi) and foreign particles (pollen, dust).
b)Endogenous antigens: These antigens come from within the body, often as a result of cellular damage, abnormal cellular processes, and infected or malfunctioning cells.
c) Autoantigens: These are self-antigens that may trigger an immune response in certain conditions, leading to autoimmune disorders.
B) Antibodies
The immune system responds to antigens by producing specific proteins called antibodies, also known as immunoglobulins (Ig). Antibodies are designed to recognize and bind to specific antigens, marking them for destruction by other components of the immune system. The ability of antigens to elicit an immune response is crucial for the body's defence against infections and the recognition of non-self and potentially harmful substances.
Structure of Antibodies
Figure : Structure of Antibody
Antibodies have a distinctive Y-shaped structure, resembling two identical “arms” extending from a central “stem” or “hinge” region.
The two arms are formed by the pairing of one heavy chain with one light chain, connected by disulfide bonds. Each heavy and light chain consists of a constant region (also called C region or Fc region) and a variable region (also called V region or Fab region).
The variable regions of both heavy (VH) and light (VL) chains are highly variable in amino acid sequence among different antibodies and contain specific binding sites. Each antibody has two identical binding sites, allowing them to recognize and bind to a wide array of antigens with high specificity.
The constant regions make up the stem and hinge of the Y-shaped antibody. They determine the class of an antibody (IgM, IgG, IgA, IgD, IgE) and play a role in effector functions, such as neutralization of pathogens, opsonization (marking pathogens for phagocytosis), activation of the complement system, and modulation of inflammatory responses.
WHAT HAPPENS WHEN ANTIBODIES DETECT ANTIGENS?
When antigens and antibodies combine, a series of events occur that are part of the immune response. The binding of antibodies to antigens is a crucial step in the ability of our immune system to recognize and neutralize foreign substances. Here's a general overview of what happens when antigens and antibodies interact:
1. Recognition and Binding:
Antibodies have specific binding sites that can recognize and attach to complementary regions (also called epitopes) on antigens. The interaction between an antibody and its antigen is highly specific, similar to a lock-and-key mechanism.
2. Formation of Antigen-Antibody Complex:
The binding of antibodies to antigens results in the formation of an antigen-antibody complex. This process can give rise to a cascade of reactions that trigger the immune response, depending on the nature of the antigens and antibodies involved.
3. Neutralization, Precipitation and Agglutination:
If the antigen is a virus or toxin, the binding of antibodies can neutralize its biological activity. This prevents the pathogen from entering host cells or carrying out harmful functions. For larger pathogens like bacteria, the binding of antibodies can lead to the clumping or agglutination of these pathogens, making it easier for immune cells to recognize and engulf them. Additionally, smaller soluble antigens and antibodies can form an insoluble complex that precipitates out of the solution.
The amount of precipitate formed when increasing amounts of an antigen are added to a constant amount of antibody is represented by a precipitation curve. Different regions of the plot are discussed below:
Figure: Precipitation curve depicting Pro-zone, Post-zone and Zone of equivalence in an antigen-antibody reaction
a) Pro-zone (Zone of Inhibition): During the initial stages of the reaction, there is an excess of antibodies compared to the amount of antigen present. The excess antibodies may saturate the antigen, preventing the formation of a visible precipitate.
b) Zone of Equivalence: The zone of equivalence is the ideal concentration range where the antigen and antibody are present in optimal proportions, leading to the formation of a visible and measurable precipitate.
c) Post-zone: The postzone occurs when there is an excess of antigens compared to the amount of antibodies present. Excess antibodies can saturate the available antigens, leading to the formation of large immune complexes and non-specific binding. The precipitation line may also appear fuzzy or weak.
4. Activation of the Complement System:
Binding of antibodies to antigens can activate the complement system, a group of proteins that work together to enhance the immune response. The activated proteins of the complement system have several effector functions, including opsonization (marking pathogens for phagocytosis), chemotaxis (attracting immune cells to the site of infection), and cell lysis (formation of membrane attack complexes leading to the destruction of pathogens).
5. Memory Response:
Successful binding of antibodies to antigens triggers the activation and differentiation of B cells into memory B cells. These memory B cells “remember” the specific antigen, allowing for a faster and more robust immune response upon subsequent exposures to the same pathogen.