What Is the Acquired (Adaptive) Immune System? How Does It Protect Against Disease?

When a microorganism (such as a bacterium, fungus or virus) enters the body, the innate immune system is used first. When this is insufficient, for example during a viral infection , the acquired immune system, a more specific and effective secondary system, comes into action. One of the body’s two main immune systems, the acquired (adaptive) immune system, has two divisions: humoral (fluid-based) and cellular.

What is humoral immunity?

Humoral immunityis a defense system against extracellular microorganisms using antibodies . Antibodies are proteins produced specifically against a pathogen entering the body. They have many functions. Some attach to bacterial antigens, enabling engulfment by phagocytes, while others bind viral antigens and prevent viruses from infecting cells.

How are antibodies produced in the body?

Antibodies are produced by plasma cells differentiated from B cells. B cells differentiate in bone marrow before entering circulation. Once differentiated, they recognize antigens and produce specific antibodies binding them. They also present antigens to T cells and are therefore called antigen-presenting cells (APCs). The body contains large numbers of B cells with many different combinations. During infection, whichever antibody binds the microorganism’s antigen, the B cell carrying it proliferates. This is called clonal expansion. Some resulting B cells differentiate into plasma cells and produce antibodies, while others become memory cellsand enable a faster response to the next infection.

What is cellular immunity?

Cellular immunity is responsible for eliminating virus-infected cells or destroying tumor cells. It involves T cells , divided into cytotoxic (killer) and helper T cells. Cytotoxic T cells use special receptors on their surfaces to recognize antigens that should not normally be present. They then activate mechanisms causing the antigen-bearing cell’s death. Helper T cells recognize antigens through similar but different surface receptors and secrete chemicals stimulating phagocytes and other lymphocytes.

AIDSis caused by HIV, which recognizes helper T cells through specific surface receptors, infects them and prevents their function. Because helper T cells are primarily responsible for recognizing virus-infected cells, cytotoxic T cells in someone infected with HIV cannot recognize cells infected by viruses entering the body and cannot fight them. AIDS is therefore fatal because of this effect on immunity rather than the virus itself.

Another immune-system-related disease, MS (multiple sclerosis), arises from a problem during T-cell formation. This involves self-tolerance, the immune system’s tolerance mechanism toward our own tissues and cells. Normally, T cells recognize the body’s own cells and do not attack them; in multiple sclerosis, however, immune cells attack nerve cells.

The importance of adaptive immunity in cancer

New-generation immunotherapy drugs, immune checkpoint inhibitors, are antibody drugs developed to prevent T cells’ blindness to cancer cells. Cancer cells try to appear friendly to T cells through various surface molecules, the best-known being CTLA4, PD-1 and PD-L1. Immunotherapy drugs called checkpoint inhibitors block this false hand of friendship.

Besides B and T cells, acquired immunity also involves antigen-presenting cells . These present antigens of infecting pathogens through special surface receptors to stimulate lymphocytes. Examples include dendritic cells . Presenting tumor-specific antigens to lymphocytes through dendritic cells can activate lymphocytes against tumors. In many cancers, immune cells cannot recognize cancer cells as sufficiently foreign, so genetically modified and stimulated dendritic cells offer promise as an immunotherapy. For example, in prostate cancer treatment, FDA -approved Sipuleucel-Tis a dendritic cell cancer vaccine. Dendritic cell cancer vaccines are personalized . Dendritic cells, a type of white blood cell, are separated from blood taken from the patient, and the remaining blood is returned (leukapheresis). In the laboratory, the isolated cells are stimulated with prostate-specific antigen, a prostate cancer protein, and immune cell growth factors (GM-CSF). Once prepared, they are administered as a vaccine. This prostate cancer vaccine is FDA-approved for patients in good general condition whose disease has metastasized and is resistant to hormone suppression therapy. Because it is prepared individually, it is currently available only in the United States.

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What is the acquired immune system? How does it protect against disease?