Humoral immune system: It acts against bacteria and viruses in the body liquids (eg. blood) by means of proteins, called immunoglobulins (also known as antibodies), which are produced by B cells.
Cellular immune system: It destroys virus-infected cells (among other duties) with T cells (also called "T lymphocytes"; "T" means they develop in the thymus). There are two major types of T cells:
Cytotoxic T cells (TC cells): These cells recognize infected cells by using T cell receptors to probe cell surfaces. If they recognize an infected cell, they release granzymes to trigger that cell to become apoptotic ("commit suicide"), thus killing that cell and any viruses that it is in the process of creating.
Helper T cells (TH cells): These cells activate macrophages (cells that ingest dangerous material), and also produce cytokines (interleukins) that induce the proliferation of B and T cells.
In addition, there are regulatory T cells (Treg cells) which are important in regulating cell-mediated immunity.

Intersections between systems
Splitting the innate and adaptive immunity has served to simplify discussions of immunology. However, the systems are quite intertwined in a number of important respects.

One of the most important examples are the mechanisms of 'antigen presentation'. After they leave the thymus, T cells require activation to proliferate and differentiate into cytotoxic ("killer") T cells (CTLs). Activation is provided by antigen-presenting cells (APCs), a major category of which are the dendritic cells. These cells are part of the innate immune system.

Activation occurs when a dendritic cell simultaneously binds itself to a T "helper" cell's antigen receptor and to its CD28 receptor, which provides the "second signal" needed for DC activation. This signal is a means by which the dendritic cell conveys that the antigen is indeed dangerous, and that the next encountered T "killer" cells need to be activated. This mechanism is based on antigen-danger evaluation by the T cells that belong to the adaptive immune system. But the dendritic cells are often directly activated by engaging their toll-like receptors, getting their "second signal" directly from the antigen. In this way, they actually recognize in "first person" the danger, and direct the T killer attack. In this respect, the innate immune system therefore plays a critical role in the activation of the adaptive immune system.

Adjuvants, or chemicals that stimulate an immune response, provide artificially this "second signal" in procedures when an antigen, that would not normally raise an immune response, is artificially introduced into a host. With the adjuvant, the response is much more robust. Historically, a commonly-used formula is Freund's Complete Adjuvant, an emulsion of oil and mycobacterium. It was later discovered that toll-like receptors, expressed on innate immune cells, are critical in the activation of adaptive immunity.

Disorders of the human immune system

The most important function of the human immune system occurs at the cellular level of the blood and tissues. The lymphatic and blood circulation systems are highways for specialized white blood cells to travel around the body. White blood cells include B cells, T cells, natural killer cells, and macrophages. Each has a different responsibility, but all function together with the primary objective of recognizing, attacking and destroying bacteria, viruses, cancer cells, and all substances seen as foreign. Without this coordinated effort, a person would not be able to survive more than a few days, before succumbing to overwhelming infection.

Infections set off an alarm that alerts the immune system to bring out its defensive weapons. Natural killer cells and macrophages rush to the scene to gobble up and digest infected cells. If the first line of defense fails to control the threat, antibodies, produced by the B cells, upon the order of T helper cells, are custom-designed to hone in on the invader.

Many disorders of the human immune system fall into two broad categories that are characterized by:

Attenuated immune response: There are 'congenital' (inborn) and 'acquired' forms of immunodeficiency, characterized by an attenuated response. Chronic granulomatous disease, in which phagocytes have trouble destroying pathogens, is an example of the former, while AIDS ("Acquired Immune Deficiency Syndrome"), an infectious disease caused by the HIV virus that destroys CD4+ T cells, is an example of the latter. Immunosuppressive medication intentionally induces an immunodeficiency in order to prevent rejection of transplanted organs.
Overzealous immune response: On the other end of the scale, an overactive immune system figures in a number of other disorders, particularly autoimmune disorders such as lupus erythematosus, type I diabetes (sometimes called "juvenile onset diabetes"), multiple sclerosis, psoriasis, and rheumatoid arthritis. In these, the immune system fails to properly distinguish between self and non-self, and attacks a part of the patient's own body. Other examples of overzealous immune responses in disease include hypersensitivities, such as allergies and asthma.