How is adaptive immunity activated

There are two types of adaptive responses: the cell-mediated immune response , which is carried out by T cells, and the humoral immune response , which is controlled by activated B cells and antibodies. Activated T cells and B cells that are specific to molecular structures on the pathogen proliferate and attack the invading pathogen.

Their attack can kill pathogens directly or secrete antibodies that enhance the phagocytosis of pathogens and disrupt the infection. Adaptive immunity also involves a memory to provide the host with long-term protection from reinfection with the same type of pathogen; on re-exposure, this memory will facilitate an efficient and quick response.

Unlike NK cells of the innate immune system, B cells B lymphocytes are a type of white blood cell that gives rise to antibodies, whereas T cells T lymphocytes are a type of white blood cell that plays an important role in the immune response. T cells are a key component in the cell-mediated response—the specific immune response that utilizes T cells to neutralize cells that have been infected with viruses and certain bacteria.

There are three types of T cells: cytotoxic, helper, and suppressor T cells. Cytotoxic T cells destroy virus-infected cells in the cell-mediated immune response, and helper T cells play a part in activating both the antibody and the cell-mediated immune responses.

Suppressor T cells deactivate T cells and B cells when needed, and thus prevent the immune response from becoming too intense. Not all antigens will provoke a response.

The suppression of immune responses to harmless macromolecules is highly regulated and typically prevents processes that could be damaging to the host, known as tolerance. The innate immune system contains cells that detect potentially harmful antigens, and then inform the adaptive immune response about the presence of these antigens. An antigen-presenting cell APC is an immune cell that detects, engulfs, and informs the adaptive immune response about an infection.

When a pathogen is detected, these APCs will phagocytose the pathogen and digest it to form many different fragments of the antigen. Antigen fragments will then be transported to the surface of the APC, where they will serve as an indicator to other immune cells. Dendritic cells are immune cells that process antigen material; they are present in the skin Langerhans cells and the lining of the nose, lungs, stomach, and intestines.

Sometimes a dendritic cell presents on the surface of other cells to induce an immune response, thus functioning as an antigen-presenting cell.

Macrophages also function as APCs. Before activation and differentiation, B cells can also function as APCs. After phagocytosis by APCs, the phagocytic vesicle fuses with an intracellular lysosome forming phagolysosome. Within the phagolysosome, the components are broken down into fragments; the fragments are then loaded onto MHC class I or MHC class II molecules and are transported to the cell surface for antigen presentation, as illustrated in Figure Note that T lymphocytes cannot properly respond to the antigen unless it is processed and embedded in an MHC II molecule.

Helper T- cells are one of the main lymphocytes that respond to antigen-presenting cells. Lymphocytes in human circulating blood are approximately 80 to 90 percent T cells, shown in Figure Recall that the T cells are involved in the cell-mediated immune response, whereas B cells are part of the humoral immune response. T cells encompass a heterogeneous population of cells with extremely diverse functions. Some T cells respond to APCs of the innate immune system, and indirectly induce immune responses by releasing cytokines.

Other T cells stimulate B cells to prepare their own response. Another population of T cells detects APC signals and directly kills the infected cells. T and B lymphocytes are also similar in that each cell only expresses one type of antigen receptor. Any individual may possess a population of T and B cells that together express a near limitless variety of antigen receptors that are capable of recognizing virtually any infecting pathogen.

T and B cells are activated when they recognize small components of antigens, called epitopes , presented by APCs, illustrated in Figure The requirement for information from the APCs of innate immunity to trigger B cell or T cell activation illustrates the essential nature of the innate immune response to the functioning of the entire immune system. These molecules are important because they regulate how a T cell will interact with and respond to an APC.

The two populations of T cells have different mechanisms of immune protection, but both bind MHC molecules via their antigen receptors called T cell receptors TCRs. Because they assist in binding specificity, the CD4 and CD8 molecules are described as coreceptors. Which of the following statements about T cells is false? Consider the innumerable possible antigens that an individual will be exposed to during a lifetime.

The mammalian adaptive immune system is adept in responding appropriately to each antigen. Mammals have an enormous diversity of T cell populations, resulting from the diversity of TCRs. Each TCR consists of two polypeptide chains that span the T cell membrane, as illustrated in Figure Each polypeptide chain is comprised of a constant domain and a variable domain: a domain, in this sense, is a specific region of a protein that may be regulatory or structural.

The intracellular domain is involved in intracellular signaling. A single T cell will express thousands of identical copies of one specific TCR variant on its cell surface. The specificity of the adaptive immune system occurs because it synthesizes millions of different T cell populations, each expressing a TCR that differs in its variable domain. This TCR diversity is achieved by the mutation and recombination of genes that encode these receptors in stem cell precursors of T cells.

The T H lymphocytes function indirectly to identify potential pathogens for other cells of the immune system. These cells are important for extracellular infections, such as those caused by certain bacteria, helminths, and protozoa. T H 1 cells secrete cytokines to enhance the activities of macrophages and other T cells.

T H 1 cells activate the action of cyotoxic T cells, as well as macrophages. Whether a T H 1 or a T H 2 immune response develops depends on the specific types of cytokines secreted by cells of the innate immune system, which in turn depends on the nature of the invading pathogen. The T H 1-mediated response involves macrophages and is associated with inflammation. Recall the frontline defenses of macrophages involved in the innate immune response.

Some intracellular bacteria, such as Mycobacterium tuberculosis , have evolved to multiply in macrophages after they have been engulfed. These pathogens evade attempts by macrophages to destroy and digest the pathogen.

When M. These stimulated T cells secrete specific cytokines that send feedback to the macrophage to stimulate its digestive capabilities and allow it to destroy the colonizing M.

In the same manner, T H 1-activated macrophages also become better suited to ingest and kill tumor cells. In summary; T H 1 responses are directed toward intracellular invaders while T H 2 responses are aimed at those that are extracellular. A plasma cell is an immune cell that secrets antibodies; these cells arise from B cells that were stimulated by antigens. The B cell receptor has two heavy chains and two light chains connected by disulfide linkages.

Each chain has a constant and a variable region; the latter is involved in antigen binding. Two other membrane proteins, Ig alpha and Ig beta, are involved in signaling.

The receptors of any particular B cell, as shown in Figure In this state, B cells function as APCs. When a T H 2 cell detects that a B cell is bound to a relevant antigen, it secretes specific cytokines that induce the B cell to proliferate rapidly, which makes thousands of identical clonal copies of it, and then it synthesizes and secretes antibodies with the same antigen recognition pattern as the BCRs.

The activation of B cells corresponding to one specific BCR variant and the dramatic proliferation of that variant is known as clonal selection. This phenomenon drastically, but briefly, changes the proportions of BCR variants expressed by the immune system, and shifts the balance toward BCRs specific to the infecting pathogen. T and B cells differ in one fundamental way: whereas T cells bind antigens that have been digested and embedded in MHC molecules by APCs, B cells function as APCs that bind intact antigens that have not been processed.

B cells must be able to bind intact antigens because they secrete antibodies that must recognize the pathogen directly, rather than digested remnants of the pathogen. The main purpose of the innate immune response is to immediately prevent the spread and movement of foreign pathogens throughout the body. The second line of defense against non-self pathogens is called adaptive immune response.

Adaptive immunity is also referred to as acquired immunity or specific immunity and is only found in vertebrates. The adaptive immune response is specific to the pathogen presented. The adaptive immune response is meant to attack non-self pathogens but can sometimes make errors and attack itself. When this happens, autoimmune diseases can develop e. The hallmark of the adaptive immune system is clonal expansion of lymphocytes.

Clonal expansion is the rapid increase of T and B lymphocytes from one or a few cells to millions. Each clone that originates from the original T or B lymphocyte has the same antigen receptor as the original and fights the same pathogen. While the innate immune response is immediate, the adaptive immune response is not. However, the effect of the adaptive immune response is long-lasting, highly specific, and is sustained long-term by memory T cells.

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Download PDF. Abstract The mammalian immune system has innate and adaptive components, which cooperate to protect the host against microbial infections.