Explain the functions of each class of immunoglobulins. Describe the subsets of T-lymphocytes.

Immunoglobulins: Functions of Each Class

Immunoglobulins are glycoprotein molecules produced by plasma cells (differentiated B lymphocytes) that bind specifically to antigens. There are five major classes of immunoglobulins, each with distinct characteristics and functions:

• IgG: This is the most abundant immunoglobulin in serum (75-80%). It provides long-term immunity, crosses the placenta to provide passive immunity to the fetus, neutralizes toxins, opsonizes pathogens (enhancing phagocytosis), and activates the complement system. There are four subclasses: IgG1, IgG2, IgG3, and IgG4, each with slightly different functions.
• IgM: The first antibody produced during an immune response. It is a pentamer (five antibody molecules joined together) and is very effective at activating the complement system. It is primarily found in the bloodstream and is important in combating bacterial infections.
• IgA: Predominantly found in mucosal secretions (saliva, tears, breast milk, respiratory and gastrointestinal tracts). It provides localized immunity by neutralizing pathogens and preventing their attachment to epithelial surfaces. It exists as a dimer in secretions.
• IgE: Involved in allergic reactions and parasitic infections. It binds to mast cells and basophils, triggering the release of histamine and other inflammatory mediators upon antigen exposure.
• IgD: Found on the surface of mature B cells, where it acts as an antigen receptor. Its precise function is not fully understood, but it is thought to play a role in B cell activation.

Subsets of T-Lymphocytes

T-lymphocytes mature in the thymus and play a critical role in cell-mediated immunity. They recognize antigens presented by antigen-presenting cells (APCs) in the context of Major Histocompatibility Complex (MHC) molecules. Different subsets of T-lymphocytes perform specialized functions:

• Helper T Cells (Th): These cells coordinate the immune response by releasing cytokines that activate other immune cells. Several subsets exist:
  • Th1 cells: Promote cell-mediated immunity, activating macrophages and cytotoxic T cells. Important for intracellular pathogens.
  • Th2 cells: Promote humoral immunity, activating B cells to produce antibodies. Important for extracellular parasites and allergens.
  • Th17 cells: Important for immunity against extracellular bacteria and fungi, and contribute to autoimmune diseases.
  • Tfh (Follicular Helper T) cells: Located in the germinal centers of lymph nodes, they provide help to B cells for antibody production and affinity maturation.
• Cytotoxic T Cells (Tc or CD8+ T cells): Directly kill infected cells, tumor cells, and foreign cells by releasing cytotoxic granules containing perforin and granzymes.
• Regulatory T Cells (Treg): Suppress the immune response to prevent autoimmunity and maintain immune homeostasis. They express the transcription factor FoxP3.
• Memory T Cells: Long-lived cells that provide immunological memory. Upon re-exposure to the same antigen, they rapidly proliferate and differentiate into effector T cells, providing a faster and more effective immune response.

The interplay between these T-lymphocyte subsets is crucial for a balanced and effective immune response. Dysregulation of these subsets can lead to immunodeficiency, autoimmunity, or chronic inflammation.