Immune System Basics
The human immune system is made up of lymph nodes and a point of pathogen entry, or vaccination site. The human immune system uses two general mechanisms to protect the body against environmental pathogens: innate responses and adaptive responses. The innate immune system recognizes specific molecules that are present only in connection with pathogens, such as double-stranded RNA or CpG. These pathogen-associated molecules trigger an immediate, non-specific inflammatory response, such as Type 1 interferon production in response to viral infection.
The human immune system is distributed across many different areas within the body. These areas include the skin and mucosal tissues, lymph nodes, spleen, bone marrow, blood, and lymphatic fluids. Immune cells often have different functions or differentiation pathways in the local environments that exist in these distinct areas (known as “compartments”). The three-dimensional nature of the immune system has increasingly been recognized as critical to its function. A class of immune cells in one compartment, such as skin, recognize a newly introduced antigen (e.g., through vaccination) and travel to a different compartment, such as the lymph node, where they pass biological signals to specialized immune cells that are co-located in that compartment. Thus, the compartments serve to not only separate different types of immune cells, but also to provide the appropriate conditions for immune regulation and response by the adaptive immune system.
The human immune system is made up of many tissues (of which two are lymph nodes) and a point of pathogen entry, or vaccination site. Click the image for a larger, more detailed version.
In contrast, the adaptive immune response recognizes “non-self” molecules and mounts a specific attack against those biomolecules over time. Adaptive immunity is mediated by specialized immune cells called B and T lymphocytes (or simply B and T cells). The adaptive immune system produces a custom-tailored response to a pathogen. T cells have a wide array of functionalities. For example, populations of T cells: (1) serve as regulators of the immune response; (2) provide help to B cells in the formation of antibodies; and (3) act as cytotoxic cells attacking and killing pathogen infected cells as well as tumor cells. B cells develop a specific immunoglobulin (antibody) response to parts of molecules that are present on or in the pathogen (called “antigens”). The adaptive immune response also has specific memory for antigenic structures, so repeated exposure to the same antigen rapidly increases the level of induced protection against that particular pathogen.
| Innate | Adaptive | |
|---|---|---|
| Response Time | hours | days |
| Specificity | limited, fixed | highly diverse, improves during the course of immune response |
| Response to Repeat Infection | identical to primary response | much more rapid than primary response |
An important class of immune cells that play roles in the innate and adaptive immune response are dendritic cells (DCs). These are professional antigen presenting cells that communicate with the B and T cells and galvanize a response against the antigen. Succinctly, antigenic molecules introduced into the body are acquired by dendritic cells (DCs), the sentinels of immunity. The DCs then migrate to a lymph node, where they present the antigen to T lymphocytes, activating their immune function. Activated helper T lymphocytes co-stimulate B lymphocytes to induce antibody production, while activated cytotoxic T lymphocytes lyse antigen-bearing cells.
Perhaps the most important compartment for the adaptive immune system is the lymph node. An important functional element of a human lymph node is the follicle, which develops a germinal center when stimulated by an antigen. A germinal center is made up of follicular dendritic cells (FDCs), B cells, and helper CD4+ T cells. The follicular dendritic cells secrete chemokines which attract B cells and T cells to form the germinal center. Various studies of germinal centers in vivo indicate that many important events occur there, including immunoglobulin (Ig) class switching, rapid B cell proliferation (GC dark zone), production of B memory cells, accumulation of select populations of antigen-specific T cells and B cells, hypermutation, selection of somatically mutated B cells with high affinity receptors, apoptosis of low affinity B cells, affinity maturation, induction of secondary antibody responses, and regulation of serum immunoglobulin G (IgG) with high affinity antibodies.
Therapy
Helping the immune system to combat pathogens by vaccination has been one of the great triumphs of Western medicine. Most vaccines aim to prime the immune system to eliminate dangerous viruses, bacteria, or parasites quickly, before the pathogens can gain a stronghold in the body. Vaccines achieve this effect by "fooling" the immune system into behaving as if the body was already beset by a microorganism that was multiplying unabated and damaging tissues extensively.
For More Information
For more information on the immune system, visit the Immune System page at Wikipedia.