Cellular immunity represents a defense mechanism within the body, operating to identify and eliminate threats directly at the cellular level. This immune response does not rely on antibodies. Instead, it involves specialized cells that target and destroy infected host cells, abnormal cells like cancer cells, and various intracellular pathogens such as viruses, fungi, and certain bacteria. This direct cellular action is fundamental to maintaining health and protecting against internal dangers.
Cells of Cellular Immunity
The primary cells responsible for cellular immunity are various types of T lymphocytes, known as T cells, and natural killer (NK) cells. T cells originate in the bone marrow and mature in the thymus, where they develop unique surface proteins called T-cell receptors (TCRs) that enable them to recognize specific antigens.
Cytotoxic T cells, also known as killer T cells (CD8+ T cells), are direct effectors that destroy infected or cancerous cells by recognizing foreign antigens displayed on their surface. Helper T cells (CD4+ T cells) do not directly kill cells but coordinate the immune response by releasing signaling proteins called cytokines, which activate other immune cells like killer T cells and macrophages.
Memory T cells are long-lived cells that develop after an initial exposure to a pathogen, allowing for a faster and stronger response upon subsequent encounters. Regulatory T cells help to control the immune response, preventing excessive reactions and maintaining self-tolerance. Natural killer (NK) cells are another type of lymphocyte that belongs to the innate immune system and can recognize and destroy infected or abnormal cells without prior activation.
How Cellular Immunity Works
Cellular immunity begins with the recognition of foreign or abnormal proteins, called antigens, presented on the surface of infected or abnormal cells. Antigen-presenting cells (APCs), such as macrophages or dendritic cells, engulf pathogens, break them down into fragments, and display these fragments on specialized molecules called Major Histocompatibility Complex (MHC) proteins.
T cells, particularly helper T cells, then recognize these presented antigens. This recognition, along with co-stimulatory signals, leads to the activation of the helper T cell. Once activated, helper T cells proliferate and differentiate, releasing cytokines that amplify the immune response and activate cytotoxic T cells.
Activated cytotoxic T cells then seek out and directly bind to infected or cancerous cells displaying the specific antigen on their MHC class I molecules. Upon binding, the cytotoxic T cell releases molecules like perforin and granzymes that induce programmed cell death, or apoptosis, in the target cell, thereby eliminating the threat. This process also generates memory T cells, ensuring a quicker response upon re-encounter.
The Diverse Functions of Cellular Immunity
Cellular immunity plays diverse roles in protecting the body. It is effective in combating intracellular pathogens, such as viruses and certain bacteria, which reside and replicate inside host cells, making them inaccessible to antibodies. For example, cytotoxic T cells are important in controlling viral infections by directly eliminating infected cells, thus preventing viral spread.
This immune branch is also important in cancer surveillance, where it identifies and destroys tumor cells that display abnormal antigens on their surfaces. This recognition allows the immune system to remove potentially harmful growths. Cellular immunity also contributes to autoimmune diseases, where immune cells may mistakenly attack the body’s own healthy tissues. Furthermore, it plays a role in transplant rejection, as T cells can recognize transplanted organs as foreign and initiate an immune response against them.
Cellular Immunity Compared to Humoral Immunity
Cellular immunity and humoral immunity are two distinct yet cooperative branches of the immune system. Cellular immunity primarily involves direct cell-to-cell interactions to eliminate threats, relying on T cells and NK cells to target and destroy infected or abnormal cells. Humoral immunity, in contrast, is mediated by B cells and the antibodies they produce.
Antibodies are soluble proteins that circulate in body fluids, like blood and lymph, and primarily target extracellular pathogens, such as bacteria and toxins. While humoral immunity neutralizes free-floating invaders, cellular immunity targets threats hidden within cells, highlighting their complementary actions.