Immune System

Specific Immunity

As a third line of defense, cell-mediated immunity—part of adaptive immunity—involves various types of cells that fight a specific pathogen and recognize antigens, which are molecules that trigger a response from the adaptive immune system, for example, surface proteins of bacteria or viruses.

The body's third line of defense is specific immunity, which is an adaptive immune response that identifies and targets a particular disease causing pathogen using B cell and T cell lymphocytes. This type of immunity also generates immunological memory, which enables the immune system to elicit a heightened immune response following re-exposure to a pathogen. The body utilizes the adaptive immune response by identifying and recognizing many foreign-body proteins, each called an antigen, which is a substance recognized by surface antibodies or other immune cells to prompt an immune response. Any molecule that triggers an antibody-mediated response from the immune system is an antigen. The term antigen comes from the fact that antigens are antibody generating.

Macrophages are phagocytic cells that engulf and digest pathogens. The macrophage forms a vesicle, or phagosome, around the pathogen, and antigenic fragments are left behind in the vesicle when the pathogen is digested. Macrophages display these antigens in special antigen-presenting complexes on their cell surface. Just like a macrophage, a dendritic cell is also an antigen-presenting cell that elicits an adaptive immune response from T cells. Macrophages also secrete substances called cytokines, specifically interleukin-1. Secretion of interleukin-1 attracts a specific type of lymphocyte called a helper T cell. Helper T cells bind to the displayed antigen and secrete interleukin-2, which causes the proliferation of both killer T cells and B cells. These two types of cells are responsible for the two types of specific immunity: antibody-mediated immunity and cell-mediated immunity.

Antibody-mediated immunity generally targets pathogens outside the body's cells, such as bacteria and free viruses. An antibody is a Y-shaped protein, also called an immunoglobulin, or Ig, that recognizes and binds to a specific antigen. Antibody-mediated immunity relies on antibody-producing B cells. Like macrophages, B cells engulf and display antigens to helper T cells, which in turn can activate the B cells. Once activated, B cells divide into different kinds of cells including plasma cells, which rapidly produce large quantities of antibodies specific to the identified antigen. Other B cells, called memory B cells, live for years until the specific antibody they produce is once again required. When a memory B cell is activated by a helper T cell, it rapidly differentiates into plasma cells and resumes production of antibodies for the "remembered" antigen.

After B cells produce antibodies, antibody-mediated immunity works in several different ways. Antibodies coat bacteria and free viruses, and the presence of antibodies on the surface of a pathogen functions as an opsonin, a molecule that promotes phagocytosis by macrophages and other phagocytic immune cells. The antibody coating also prevents pathogens from functioning normally and helps keep them from harming the body, especially because antibodies cross-link with one another. This cross-linkage clumps pathogens into particles that further inhibit their action in the body. Clumping pathogens into particles also makes them easier for phagocytic cells to engulf and digest.

Antibody-Mediated Immunity

Antibody-mediated immunity neutralizes and precipitates pathogens. The presence of antibodies marks pathogens for phagocytosis. Antibodies also activate the complement system, which forms a pore and triggers a cascade of events that eventually kills foreign cells through cytolysis.
In cell-mediated specific immunity, which is different from antibody-mediated immunity, killer T cells target infected, damaged, or dysfunctional cells in the body, such as cancer cells. Cells of organ transplants also are targeted when they display antigens on the surface of their cells. All T cells are coated with receptors that recognize antigens, which is important given that there are millions of different T cells in the body and each one recognizes and binds to just one particular antigen. The appropriate antigen-specific killer T cells are generated in large numbers after a progenitor memory T cell is activated. This happens after a series of events takes place, which first begins with antigen-presenting cells, such as macrophages and dendritic cells, ingesting the pathogenic or infected cell and displaying these digested contents as antigens on their cell surfaces for circulating memory T cells and helper T cells to see. When memory T cells and helper T cells bind to antigens displayed by the antigen-presenting cells, the helper T cell becomes activated, triggering a series of chemical signals inside the T cell that eventually results in the release of cytokines, which causes memory T cells to proliferate, or rapidly undergo cell division. The proliferation of the memory T cell triggers the production of several killer T cells specific to the antigen. When a killer T cell binds to an infected cell, it secretes cytotoxic enzymes that punch holes in the infected cell's membrane, causing cytolysis. Killer T cells also secrete enzymes that trigger apoptosis, or cell death, in infected cells.

Cell-Mediated Immunity

Cell-mediated immunity targets infected or damaged cells. After a macrophage digests an infected cell, it displays antigens on its cell surface. Both helper T cells and memory T cells bind to these displayed antigens. Helper T cells release cytokines, which cause the memory T cell that recognized the antigen to proliferate into an army of killer T cells. These killer T cells recognize and bind to infected cells and release cytotoxic granules that kill the infected cell.