Introduction
The immune system activates when the innate immune response is not sufficient to control the infection. Adaptive responses are of two types, that include the cell-mediated immune response produced by T cells and the humoral immune response, which is activated by B cells and their antibodies. T cells and B cells (activated) tend to proliferate and attack the pathogen as they are specific to the molecular structure of the pathogen. The attack can kill pathogens directly and secrete antibodies to enhance phagocytosis and reduce the infection. Adaptive immunity provides the host with long-term protection from the same pathogen and provides an efficient and quick response a second time.
What Are Antigen-Presenting Cells?
B lymphocytes are white blood cells forming antibodies, and T lymphocytes are white blood cells that play an important role in immunity. T cells are essential for a cell-mediated response, whereas the specific immune response uses T cells to neutralize cells infected with viruses and bacteria. In addition, there are cytotoxic, helper, and suppressor T cells. Cytotoxic T cells kill virus-infected cells, and helper T cells activate the antibody along with the cell-mediated immune responses. Suppressor T cells further deactivate T cells and B cells.
An antigen is a foreign macromolecule and is exposed to foreign antigens. The suppression of immune responses prevents damage to the host. The innate immune system has cells that harm antigens and activate an adaptive immune response. An antigen-presenting cell detects, engulfs, and informs the adaptive immune response. When a pathogen is detected, the pathogen undergoes phagocytosis and digests it to form antigen fragments. Antigen fragments transport to the surface. Dendritic cells are immune cells in the skin, nose lining, lungs, stomach, and intestines. Dendritic cell presents on the surface of cells to induce an immune response and act as an antigen-presenting cell.
The phagocytic vesicle and lysosome form a phagolysosome. In the phagolysosome, the components break down into fragments; and are transported to the cell surface for antigen presentation. Helper T-cells are the main lymphocytes that respond to antigen-presenting cells.
What Are T and B Lymphocytes?
Lymphocytes in circulating blood are around 80 to 90 percent T cells and 10 to 20 percent B cells. Cell-mediated immune responses include T cells, whereas B cells belong to the humoral immune response.
T and B cells bind specific antigens, followed by activation and self-maturation to bind the antigen of the infecting pathogen. T and B lymphocytes express one antigen receptor. T and B cells are activated by recognizing antigens and epitopes; an antigen is a macromolecule that reacts with immune system components. T cells express CD4 or CD8 on their surface and are classified as CD4+ or CD8+ cells. CD4+ cells are stimulated to become helper T lymphocytes, and cells produce B cells or cytotoxic T cells directly or secrete cytokines to activate target cells. CD8+ cells are stimulated to form cytotoxic T lymphocytes, kill infected cells by apoptosis, and release cytokines.
B Lymphocytes
B cells differentiate in antibody-secreting plasma cells. A plasma cell is an immunity-related cell that secretes antibodies. Like T cells, B cells have B cell receptors and membrane forms of immunoglobulin. The B cell receptor consists of two light chains and two heavy chains linked by a disulfide bond. Every chain has a constant and variable region; the variable component helps antigen binding. Two proteins, immunoglobulin (Ig) alpha, and Ig beta, help in signaling.
T and B cells differ in that the T cells bind digested antigens, and B cells bind intact antigens that have not been processed.
Cytotoxic T Lymphocytes
Cytotoxic T cells clear infections by attacking and destroying infected cells. These cells protect against viral infections because viruses replicate within cells.
Mucosal Surfaces and Immune Tolerance
Mucosal immunity is by mucosa-associated lymphoid tissue, independent of the systemic immune system, and has innate and adaptive components. Mucosa-associated lymphoid tissue includes lymphatic tissue that combines with epithelial tissue at the mucosal lining of the body. Activated T cells migrate through the lymphatics into the circulatory system to mucosal areas of infection.
What Is Immunological Memory?
The adaptive immune system also shows a memory component for an efficient response upon reinvasion by the same pathogen. The adaptive immune response to a new pathogen is termed a primary response, plasma cells secrete the antibodies, and hence the T cells increase, becoming constant. B and T cells convert into effector cells and differentiate into B and T memory cells.
A memory cell is a specific antigen B or T lymphocyte that does not further form effector cells during the initial immune response but can form effector cells due to re-exposure to the same pathogen.
The Rh antigen is present in Rh-positive red blood cells. A Rh-negative female carries a Rh-positive fetus till birth without difficulty. However, if she conceives a second Rh-positive fetus, her body activates an immune attack that causes hemolytic disease in the newborn.
Antibodies are secreted initially from plasma cells as a primary response to infection. Upon re-exposure to a similar pathogen, memory cells differentiate into antibody-secreting plasma cells.
Vaccination is exposure to non-infectious antigens derived from pathogens, which generates a favorable primary immune response. The host does not perceive the immune response to vaccination as an illness but still confers immune memory. The reaction is the same as secondary exposure exposed to the same pathogen vaccinating an individual. Although reinfection generates abundant memory cells and increased resistance to the pathogen, vaccines involve one or more booster vaccinations.
What Are the Primary Centers of the Immune System?
The immune system is defined by circulating cells in the body and the regulation, maturation, and communication of immune agents. The blood circulates immune cells and proteins. Most cells in the blood are erythrocytes, and 1 percent are white blood cells. Lymph is a watery fluid with protective white blood cells but does not contain erythrocytes.
The immune system cells originate from hematopoietic stem cells in the bone marrow. Cytokine stimulates these cells and differentiates them from immunity cells. The maturation of B cells occurs in the bone marrow, and T cells move from the bone marrow to the thymus for maturation.
The spleen stores B and T cells, macrophages, dendritic cells, and NK cells. Antibodies are secreted from the spleen; the spleen filters the foreign substances and antibody-complexed pathogens. The spleen is a lymph node but is extensive and filters blood, not lymph.
Conclusion
The adaptive immune response is slower, more long-lasting, and more specific than the innate one. However, the adaptive response needs information from the innate immune system T cells to differentiate and proliferate, becoming TH cells or CTLs. B cells differentiate into plasma cells and further secrete antibodies, whereas CTLs induce apoptosis. Memory cells persist after the primary exposure to a pathogen. The mucosal immune system is independent of the systemic immune system but protects the body's mucosal surfaces.
