T cells and B cells are central to the human immune system. B cells and T cells operate in the adaptive immune response--the immune system's third and final line of defense. T cells work with B cells in their distinct roles in the immune system.
The immune system can be divided into three parts: surface barriers like skin and mucous membranes, internal defenses like fever, inflammation and phagocytes--cells that engulf and destroy foreign invaders, and adaptive defenses. The adaptive defenses involve the B and T lymphocytes and can be further divided into two subcategories: humoral immunity and cell-mediated immunity. B cells function in humoral immunity. Cell-mediated immunity involves T cells. ??
Humoral immunity relies upon the interaction between antigen and antibody. Antigens are antibody-generating substances--particles recognized as foreign that elicit the binding of antibodies. Antibodies are protein molecules that attach to the surface of antigens. Antibodies are also called immunoglobins. ??
Cell-mediated immunity involves T lymphocytes to directly destroy, or lyse, foreign cells. T cells also release chemicals that further enhance the immune response.
B cells and T cells are both lymphocytes, or white blood cells produced in bone marrow and maturing in the organs of the body's lymphatic system. During maturation, the lymphocytes learn to differentiate between foreign cells and self. This is called self-tolerance. The lymphocytes also learn to recognize a specific antigen and bind to it. This is called immunocompetence.
B Cell Function
B cells mature in the bone marrow. Once activated, B cells have binding sites that are specific to a pathogen. When the antigen is present, it binds to the receptor on the B cell. This triggers the B cell to grow and clone itself. The clones become either plasma cells or memory cells. The plasma cells generate massive amounts of antibodies and release them into the body. The antibody binds to the antigen signaling the cells of the internal defenses to come and kill the pathogen. The plasma cells die within a few days.
Memory cells do not secrete antibodies. Instead, memory cells retain the antibody so that it can be used anytime the antigen is re-encountered. Memory cells are important because they help the body mount a faster and stronger attack the next time the antigen invades. This is called immunological memory. During this secondary immune response, antibodies exist in the body for a much longer period of time--up to months--and antibodies bind more effectively to the antigen.
T Cell Function
B cells are fairly ineffective against pathogens that hide in the body's cells like viruses.T cells are required to search out these foreigners and identify them for destruction. T cells depend upon other cells to present antigenic fragments to them, whereas B cells can search out and recognize whole antigens without much help.
T cells mature in the thymus. Two major T cells result: helper T cells and cytotoxic T cells. These are distinguished based on the glycoprotein exhibited on their outer membranes. Helper T cells exhibit the CD4 glycoprotein and cytotoxic T cells exhibit CD8 glycoprotein. Not all CD4 cells are helper T cells and not all CD8 cells are cytotoxic T cells.
Helper T cells release cytokines--chemical messengers that signal growth, differentiation and the action of other immune cells like macrophages. Helper T cells also help B cells to grow and develop antibodies more quickly.
Cytotoxic T cells patrol the body looking for and can destroy pathogenic cells directly, including cancerous cells. Cytotoxic T cells attach to the compromised cell and then release chemical factors that either help lyse the cell or induce programmed cell death, apoptosis.
Helper T cells help activate or they direct all other immune cells. Therefore, they are essential the the function of the adaptive immune response. HIV invades helper T cells. The number of helper T cells decreases as the infection continues. The rest of the immune system suffers, becoming weaker and weaker, as the amount of helper T cells declines. The body is exposed to opportunistic infections when the immune system is weakened and it is often these infections that become fatal for the AIDS patient.
- Human Anatomy & Physiology, 7th ed., Marieb, 2007
- AIDS.org Factsheet on CD4 Cells
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