Adaptive immunity is an immune system that responds precisely to specific antigens. It takes several days to respond to pathogens that innate immunity has failed to eliminate, but it forms immune memory, so it reacts rapidly and strongly when exposed to the same antigen a second time. Adaptive immunity has two main branches: humoral immunity and cell-mediated immunity.
Basic Knowledge
Overview of adaptive immunity: The central cells are lymphocytes. B cells, which mature in the bone marrow, are responsible for humoral immunity, while T cells, which arise in the bone marrow and mature in the thymus, are central to cell-mediated immunity. T cells further differentiate into helper T cells (Th), cytotoxic T cells (Tc/CTL), and regulatory T cells (Treg).
Antigen presentation: Antigen-presenting cells (APCs), such as dendritic cells and macrophages, fragment pathogen proteins and present them on MHC molecules. MHC class II molecules present antigens to helper T cells, whereas MHC class I molecules present antigens to cytotoxic T cells.
Humoral immunity: Antigen presentation → activation of helper T cells → activation and proliferation of B cells → differentiation into plasma cells → large-scale production of antibodies (immunoglobulins). Some B cells remain as memory B cells.
Cell-mediated immunity: Antigen presentation → activation of helper T cells → proliferation of cytotoxic T cells → direct destruction of virus-infected cells and cancer cells. Some cells remain as memory T cells. Cell-mediated immunity is also responsible for transplant rejection and resistance to intracellular pathogens such as tuberculosis bacteria.
State the differences in the types of pathogens handled by humoral and cell-mediated immunity, and explain the targets each is specialized to defend against.
Answer: Humoral immunity is effective against extracellular bacteria, viruses before infection, and toxins by producing antibodies. Cell-mediated immunity is effective against virus-infected cells, intracellular pathogens such as tuberculosis bacteria, cancer cells, and transplanted organs (non-self cells) because cytotoxic T cells directly destroy infected cells.
- B cells = mature in the bone marrow; humoral immunity
- T cells = arise in the bone marrow and mature in the thymus; cell-mediated immunity
- Helper T cells = commanders required to activate both types of immunity
- Plasma cells = produce large amounts of antibodies
- MHC class II → presents to helper T cells
- MHC class I → presents to cytotoxic T cells
- Memory cells (B and T) = make secondary responses faster and stronger
Points to Note
① Helper T cells activate both B cells and cytotoxic T cells. Because HIV destroys helper T cells, both types of immunity become dysfunctional in AIDS. ② MHC class I molecules are expressed by all nucleated cells and are used to identify infected cells. ③ The secondary response (upon reinfection) is faster and stronger than the primary response (the first response).
Practice
- State the organs in which B cells and T cells mature, respectively.
- Explain the role of helper T cells in adaptive immunity from the perspectives of both humoral and cell-mediated immunity.
- Give three targets of defense handled by cell-mediated immunity.