Antibodies are crucial proteins in our immune system, acting as molecular soldiers against invaders. They have a unique Y-shaped structure that allows them to bind to specific antigens and trigger immune responses. Understanding antibodies is key to grasping how our bodies fight off diseases.

Antibody production involves B cells, which create diverse antibodies to combat various threats. This process includes affinity maturation and isotype switching, enhancing our immune defenses. However, cross-reactivity can sometimes lead to autoimmune disorders, highlighting the complexity of our immune system.

Antibody Structure and Function

Structure and function of antibodies

• Antibodies, also known as immunoglobulins (Ig), are Y-shaped proteins produced by B cells
  • Composed of four polypeptide chains: two heavy chains and two light chains connected by disulfide bonds
  • Heavy chains determine the class of antibody (IgG, IgM, IgA, IgE, or IgD)
• Antibodies have two main regions: the variable region (Fab) and the constant region (Fc)
  • Variable region (Fab) located at the tips of the Y-shaped structure contains unique antigen-binding sites that allow antibodies to recognize and bind to specific antigens
    • The specific part of the antibody that binds to the antigen is called the paratope
  • Constant region (Fc) located at the stem of the Y-shaped structure interacts with other components of the immune system, such as complement proteins and immune cells (macrophages, natural killer cells)
• Antibodies function in the immune response by neutralizing toxins and pathogens, opsonizing pathogens for phagocytosis, activating complement proteins to enhance the immune response, and facilitating antibody-dependent cell-mediated cytotoxicity (ADCC)

Antibody Production

Process of antibody production

• B cells, originating from hematopoietic stem cells in the bone marrow, are responsible for producing antibodies
  • Each B cell expresses a unique antigen receptor (BCR) on its surface that binds to a specific antigen
• Antibody production is triggered when a B cell encounters its specific antigen, which binds to the BCR and activates the B cell
• Activated B cells proliferate and differentiate into:
  1. Plasma cells: specialized antibody-secreting cells derived from activated B cells that produce and secrete large quantities of antibodies specific to the activating antigen
     • Plasma cells have an enlarged endoplasmic reticulum to facilitate antibody production
  2. Memory B cells: long-lived cells that provide rapid antibody response upon subsequent exposure to the same antigen by quickly differentiating into plasma cells, leading to a faster and stronger immune response

Antibody Diversity and Maturation

• Affinity maturation: process by which B cells produce antibodies with increased affinity for a specific antigen over time
• Isotype switching: mechanism that changes the constant region of an antibody while maintaining antigen specificity, allowing for different effector functions

Types of Antibody Preparations

• Polyclonal antibodies: mixture of antibodies produced by different B cell clones that recognize various epitopes on the same antigen
• Monoclonal antibodies: identical antibodies produced by a single clone of B cells (hybridoma) that recognize a specific epitope on an antigen

Cross-Reactivity and Autoimmunity

Cross-reactivity in immune responses

• Cross-reactivity occurs when an antibody recognizes and binds to an antigen that is similar, but not identical, to the original antigen due to shared epitopes (antigenic determinants)
  • Positive implications: provides protection against related pathogens that share similar epitopes and enhances the efficiency of the immune response by reducing the number of unique antibodies needed
  • Negative implications: may lead to autoimmune disorders if antibodies cross-react with self-antigens
• Autoimmune disorders can result from cross-reactivity between foreign antigens and self-antigens through molecular mimicry, when a foreign antigen shares structural similarities with a self-antigen
  • Examples of autoimmune disorders related to cross-reactivity:
    • Rheumatic fever: antibodies against Streptococcus bacteria cross-react with heart tissue
    • Multiple sclerosis: antibodies against viral antigens may cross-react with myelin in the central nervous system