Activation responses vary between the three types of cells, but in general all involve both changes in gene expression and in the initiation of cell division. This binding elicits changes in the activity of the immune cells, termed activation, which is the second step in the adaptive immune response. In order for their immune functions to be elicited, the cells must first encounter antigens by binding specifically to them using specialized membrane proteins. (credit: modification of work by NCI scale-bar data from Matt Russell) * Four Stages of the Adaptive Immune Responseī cells, Helper T cells, and Cytotoxic T cells all respond to antigens in a similar pattern subsequent sections of this chapter will address the specifics of the immune response in each cell type. T and B cells are indistinguishable by light microscopy but can be differentiated experimentally by probing their surface receptors. This scanning electron micrograph shows a T lymphocyte. Helper T cells do not directly kill infected cells, but secrete molecules that are crucial for the function of all other cells in the immune response to a pathogen.įigure 1. Cytotoxic T cells directly kill human cells that are infected or unhealthy. T cells include the Helper T cells and the Cytotoxic, or Killer, T cells. Except in certain immune system diseases called autoimmune diseases, no mature B or T cells are able to recognize and bind to molecules that are found on healthy human cells, but only to molecules found on pathogens or on unhealthy human cells.ī cells are involved in the humoral immune response, which targets pathogens loose in blood and lymph, and B cells carry out this response by secreting antibodies.T cells are involved in the cell-mediated immune response, which targets infected cells in the body. The specificity of these unique surface proteins, or receptors, on the lymphocytes is determined by the genetics of the individual and is present before a foreign molecule is introduced to the body or encountered. In other words, each B or T cell can recognize only a very few different molecules, but together the entire lymphocyte population in a healthy person should be able to recognize molecules from most pathogens. During the maturation process, each B or T cell develops unique surface proteins that are able to recognize a unique set of very specific molecules on antigens (discussed below). The B cells remain in the bone marrow to mature (hence the name “B” for “bone marrow”), while T cells migrate to the thymus, where they mature (hence the name “T” for “thymus”). Whether an immature lymphocyte becomes a B cell or T cell depends on where in the body it matures. The two types of lymphocytes of the adaptive immune response are B and T cells ( Figure 1). Lymphocytes, which are a subclass of white blood cells, are formed with other blood cells in the red bone marrow found in many flat bones, such as the shoulder or pelvic bones. Adaptive immunity also involves a memory to give the host long-term protection from reinfection with the same type of pathogen carrying the same antigens on reexposure, this host memory will facilitate a rapid and powerful response. Their attack can kill pathogens directly or they can secrete antibodies that enhance the phagocytosis of pathogens and disrupt the infection. In adaptive immunity, activated T and B cells whose surface binding sites are specific to the antigen molecules on a pathogen greatly increase in numbers and attack the invading pathogen. The adaptive immune responses depends on the function of two types of lymphocytes, called B cells and T cells. The adaptive immune system works because the immune cells responsible for it are each able to recognize and respond to one specific antigen, or a few very similar ones. There are millions of potential sequences of amino acids, carbohydrates, and other small molecules that can act as antigens. Another example is a short chain of carbohydrates found on the cell wall of Neisseria meningitidis, the bacteria that causes meningitis. One example of an antigen is a specific sequence of 8 amino acids in a protein found only in an influenza virus, the virus responsible for causing “the flu.”. In fact, without information from the innate immune system, the adaptive response could not be mobilized.Īn antigen is a small, specific molecule on a particular pathogen that stimulates a response in the immune system. This part of the immune system works in tandem with the innate immune response to neutralize pathogens. Theme 2: How Does Blood and Organ Donation Work?Ģ.7 Adaptive Immunity Antigens and the Adaptive Immune ResponseĪdaptive immunity occurs after exposure to an antigen either from a pathogen or a vaccination.The adaptive, or acquired, immune response takes days or even weeks to become established-much longer than the innate response however, adaptive immunity is more specific to an invading pathogen.
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