Types The three major types of lymphocyte are T cells, B cells and natural killer (NK) cells.

The most well known disease that affects the immune system itself is AIDS, an immunodeficiency characterized by the suppression of CD4 ( helper ) T cells, dendritic cells and macrophages by the Human Immunodeficiency Virus (HIV).

They include natural killer cells (NK cells) (which function in cell mediated, cytotoxic innate immunity), T cells (for cell mediated, cytotoxic adaptive immunity), and B cells (for humoral, antibody driven adaptive immunity).

These reactions are mediated by T cells, monocytes, and macrophages.

The source of interferon gamma can be CD4 T cells, CD8 T cells, natural killer cells, B cells, natural killer T cells, monocytes, macrophages, or dendritic cells.

The main response of the immune system to tumors is to destroy the abnormal cells using killer T cells, sometimes with the assistance of helper T cells.

HIV infection leads to low levels of CD4 T cells through a number of mechanisms, including apoptosis of uninfected bystander cells, direct viral killing of infected cells, and killing of infected CD4 T cells by CD8 cytotoxic lymphocytes that recognize infected cells.

Under normal circumstances, many T cells and antibodies react with self peptides.

This allows killer T cells to recognize the tumor cell as abnormal.

T helper cells (also known as effector T cells or Th cells), a sub group of lymphocytes, are responsible for the activation of macrophages.

HIV can infect a variety of immune cells such as CD4 T cells, macrophages, and microglial cells.

Virus infected cells that have been killed by lymphocytes are cleared from the body by phagocytes.

B cells mature into B lymphocytes in the bursa equivalent, which in humans is the GALT, which is thought to be located in the Peyer's patches of the intestine, while T cells migrate to and mature in a distinct organ, called the thymus.

It is impossible to distinguish between T cells and B cells in a peripheral blood smear.

HIV infects vital cells in the human immune system such as helper T cells (specifically CD4 T cells), macrophages, and dendritic cells.

Some phagocytes then travel to the body's lymph nodes and display the material to white blood cells called lymphocytes.

Antibodies can be secreted into the extracellular environment or anchored in the membranes of specialized B cells known as plasma cells.

Therefore, these forms of the receptor are more likely to activate T lymphocytes and initiate the autoimmune process.

Thus HIV infection can only be diagnosed by PCR testing for HIV RNA or DNA, or via testing for the p24 antigen.

They are triggered by IL 4 and their effector cytokines are IL 4, IL 5, IL 9, IL 10 and IL 13.

In the connective tissue deep to the ulcer there is increased vascularity and a mixed inflammatory infiltrate composed of lymphocytes, histiocytes and polymorphonuclear leukocytes.

Phagocytes, in particular dendritic cells and macrophages, stimulate lymphocytes to produce antibodies by an important process called antigen presentation.

This enhances immunity because the lymphocytes respond to the antigens presented by the dendritic cells just as they would at the site of the original infection.

Like all blood cells, plasma cells originate in the bone marrow B cells differentiate into plasma cells that produce antibody molecules closely modeled after the receptors of the precursor B cell.

Gamma delta T cells Gamma delta T cells (γδ T cells) possess an alternative T cell receptor (TCR) as opposed to CD4 and CD8 (αβ) T cells and share the characteristics of helper T cells, cytotoxic T cells and NK cells.

They can communicate with other cells by producing chemicals called cytokines, which recruit other phagocytes to the site of infections or stimulate dormant lymphocytes.

Mature macrophages do not travel far from the site of infection, but dendritic cells can reach the body's lymph nodes, where there are millions of lymphocytes.

The Th cells receiving both signals of activation and proliferation will then become Th0 cells (T helper 0) cell that secrete IL 2, IL 4 and interferon gamma (IFN γ).

At the sites of viral infections, lymphocytes often vastly outnumber all the other cells of the immune system this is common in viral meningitis.

When CD4 T cell numbers decline below a critical level, cell mediated immunity is lost, and the body becomes progressively more susceptible to opportunistic infections.

Analysis of the patient's cerebrospinal fluid (CSF), which is collected by a lumbar puncture ( spinal tap ), reveals an increased number of white blood cells (primarily lymphocytes) and a mildly elevated protein level.

Other mechanisms and evolution It is likely that a multicomponent, adaptive immune system arose with the first vertebrates, as invertebrates do not generate lymphocytes or an antibody based humoral response.

T cells expressing this receptor are referred to as α β (or αβ) T cells, though a minority of T cells express an alternate receptor, formed by variable gamma (γ) and delta (δ) chains, referred as γδ T cells.

Dendritic cells serve as a link between the bodily tissues and the innate and adaptive immune systems, as they present antigen to T cells, one of the key cell types of the adaptive immune system.

Atherosclerosis (also known as arteriosclerotic vascular disease or ASVD) is a specific form of arteriosclerosis in which an artery wall thickens as a result of invasion and accumulation of white blood cells (WBCs).

This process ensures that the selected T cells will have an MHC affinity that can serve useful functions in the body (i.e., the cells must be able to interact with MHC and peptide complexes to effect immune responses).

The adaptive immune system is not dependent on phagocytes but lymphocytes, which produce protective proteins called antibodies, which tag invaders for destruction and prevent viruses from infecting cells.

During the acute phase, HIV induced cell lysis and killing of infected cells by cytotoxic T cells accounts for CD4 T cell depletion, although apoptosis may also be a factor.

The type of white blood cell predominantly present (see table) indicates whether meningitis is bacterial (usually neutrophil predominant) or viral (usually lymphocyte predominant), although at the beginning of the disease this is not always a reliable indicator.

HIV 1 entry to macrophages and CD4 T cells is mediated through interaction of the virion envelope glycoproteins (gp120) with the CD4 molecule on the target cells and also with chemokine coreceptors.

One of the functions of specialized cells (located in the thymus and bone marrow) is to present young lymphocytes with self antigens produced throughout the body and to eliminate those cells that recognize self antigens, preventing autoimmunity.

To actively produce the virus, certain cellular transcription factors need to be present, the most important of which is NF κ B (NF kappa B), which is upregulated when T cells become activated.

T cells have a symbiotic relationship with vitamin D. Not only does the T cell extend a vitamin D receptor, in essence asking to bind to the steroid hormone version of vitamin D, calcitriol, but the T cell expresses the gene CYP27B1, which is the gene responsible for converting the pre hormone version of vitamin D, calcidiol into the steroid hormone version, calcitriol.

Although phagocytes and other components of the innate immune system can, to a limited extent, control viruses, once a virus is inside a cell the adaptive immune responses, particularly the lymphocytes, are more important for defense.

Specific chemotherapeutic agents are associated with organ specific toxicities, including cardiovascular disease(e.g., doxorubicin), interstitial lung disease (e.g., bleomycin) and occasionally secondary neoplasm (e.g., MOPP therapy for Hodgkin's disease).