Cell-mediated Immunity

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This revolves around T-cells. T-cells like to get their hands dirty. They get in, grab the invader and punch holes in it. B-cells don't do that, they circulate antibodies out into the environment and circulate in the blood.

Cell-mediated immunity describes a pathway that does not result in antigen production but instead targets endogenous antigens

• Cancerous and virus-infected cells involve the body’s own cells and thus are not recognized as foreign, evading normal detection
• These cells may instead present antigenic fragments as a complex with their own self markers (MHC class I)
• When helper T cells identify these cells, they stimulate a second type of T lymphocyte – cytotoxic T cells (T_C cells)
• Cytotoxic T cells show specificity to particular antigens and will bind to the presented antigen and release perforating enzymes
• These enzymes cause the infected / cancerous cell to by lysed, preventing the further spread of infection
• Virus infected cells can also be destroyed non-specifically by NK cells, which respond to interferon released by the infected cell

Cellular and humoral immunity work hand in hand, they aren't independent.

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Understanding Cell-mediated Immunity

Cell-mediated immunity is a critical component of the adaptive immune system, primarily responsible for defending the body against intracellular pathogens, such as viruses and some bacteria, as well as cancer cells. Unlike humoral immunity, which involves antibodies produced by B cells, cell-mediated immunity does not rely on antibodies. Instead, it involves the activation of various immune cells that directly attack and eliminate infected or abnormal cells.

Key Components of Cell-mediated Immunity

Cell-mediated immunity is orchestrated by several types of immune cells, each playing a unique role in the immune response:

1. T Lymphocytes (T Cells): These are the primary cells involved in cell-mediated immunity. They originate from the bone marrow and mature in the thymus. T cells are further classified into different subsets based on their functions:

* Cytotoxic T Cells (CD8+ T Cells): These cells directly kill infected or cancerous cells by recognizing antigens presented by Major Histocompatibility Complex (MHC) class I molecules on the surface of target cells.

* Helper T Cells (CD4+ T Cells): These cells assist other immune cells by releasing cytokines, which are signaling molecules that enhance the immune response. They recognize antigens presented by MHC class II molecules.

* Regulatory T Cells (Tregs): These cells help maintain immune tolerance and prevent autoimmune responses by suppressing the activity of other immune cells.

2. Antigen-presenting Cells (APCs): These cells, including dendritic cells, macrophages, and B cells, process and present antigens to T cells, initiating the immune response. They play a crucial role in activating T cells by displaying antigens on their surface in conjunction with MHC molecules.

3. Natural Killer (NK) Cells: Although part of the innate immune system, NK cells contribute to cell-mediated immunity by targeting and destroying cells that lack MHC class I molecules, a common feature of virus-infected and cancerous cells.

Mechanism of Action

The process of cell-mediated immunity involves several steps, each crucial for an effective immune response:

1. Antigen Recognition: The immune response begins when APCs capture and process antigens from pathogens or abnormal cells. These antigens are then presented on the surface of APCs in association with MHC molecules.

2. T Cell Activation: T cells recognize and bind to the antigen-MHC complex through their T cell receptors (TCRs). This interaction, along with co-stimulatory signals provided by APCs, activates the T cells.

3. Clonal Expansion: Once activated, T cells undergo rapid proliferation, producing a large number of effector T cells that can target and eliminate infected or abnormal cells.

4. Effector Functions:

* Cytotoxic T Cells: These cells release perforin and granzymes, which induce apoptosis (programmed cell death) in target cells.

* Helper T Cells: They secrete cytokines that enhance the activity of other immune cells, such as macrophages and B cells, and help coordinate the overall immune response.

5. Memory Formation: Some activated T cells differentiate into memory T cells, which persist in the body long-term. These cells provide a faster and more robust response upon re-exposure to the same antigen.

Importance in Health and Disease

Cell-mediated immunity is essential for controlling infections that evade antibody-mediated responses, such as those caused by intracellular pathogens. It also plays a vital role in tumor surveillance, helping to detect and eliminate cancerous cells before they can proliferate.

However, dysregulation of cell-mediated immunity can lead to various health issues:

• Autoimmune Diseases: An overactive cell-mediated response can result in the immune system attacking the body's own tissues, leading to conditions such as rheumatoid arthritis and type 1 diabetes.
• Immunodeficiency Disorders: Conditions like HIV/AIDS impair cell-mediated immunity by targeting CD4+ T cells, leaving individuals vulnerable to opportunistic infections.
• Transplant Rejection: The immune system may recognize transplanted organs as foreign and mount a cell-mediated response against them, leading to rejection.

Therapeutic Applications

Understanding cell-mediated immunity has led to the development of various therapeutic strategies:

• Immunotherapy: Treatments such as checkpoint inhibitors and CAR T-cell therapy harness the power of cell-mediated immunity to target and destroy cancer cells.
• Vaccines: Some vaccines aim to stimulate cell-mediated immunity by presenting antigens that activate T cells, providing protection against diseases like tuberculosis and certain viral infections.
• Immunosuppressive Drugs: These medications are used to dampen cell-mediated responses in conditions like autoimmune diseases and organ transplantation to prevent tissue damage and rejection.