leukocyte mediated immunity

Definition

Target type: biologicalprocess

Any process involved in the carrying out of an immune response by a leukocyte. [GO_REF:0000022, GOC:add, ISBN:0781735149]

Leukocyte-mediated immunity is a crucial component of the adaptive immune system, involving the coordinated action of various white blood cells (leukocytes) to recognize, target, and eliminate pathogens and other harmful substances. This process encompasses a complex interplay of cellular interactions, signaling pathways, and effector mechanisms.

**1. Antigen Recognition and Presentation:**

* Immune cells, primarily dendritic cells (DCs) and macrophages, act as antigen-presenting cells (APCs). They engulf and process foreign antigens (e.g., viral proteins, bacterial components) into smaller peptides.
* These processed peptides are then displayed on the surface of APCs bound to major histocompatibility complex (MHC) molecules, MHC I for intracellular antigens and MHC II for extracellular antigens.

**2. T Cell Activation:**

* Naive T lymphocytes (T cells) circulate in the blood and lymph.
* When a T cell encounters an APC presenting its specific antigen on MHC, it becomes activated.
* CD4+ helper T cells recognize antigen presented on MHC II, while CD8+ cytotoxic T cells recognize antigen on MHC I.
* Activation involves signaling pathways that stimulate T cell proliferation and differentiation into effector T cells.

**3. Helper T Cell Functions:**

* Activated CD4+ T cells differentiate into various subtypes, each with distinct roles:
* Th1 cells promote cell-mediated immunity by secreting cytokines (e.g., IFN-γ) that activate macrophages and cytotoxic T cells.
* Th2 cells mediate humoral immunity by producing cytokines (e.g., IL-4, IL-5) that stimulate B cell differentiation and antibody production.
* Th17 cells promote inflammation and recruitment of neutrophils.

**4. Cytotoxic T Cell Function:**

* Activated CD8+ T cells differentiate into cytotoxic T lymphocytes (CTLs).
* CTLs directly kill infected cells by releasing cytotoxic granules containing perforin and granzyme.
* Perforin creates pores in the target cell membrane, allowing granzyme to enter and induce apoptosis (programmed cell death).

**5. B Cell Activation and Antibody Production:**

* B lymphocytes (B cells) also express antigen receptors on their surface.
* Upon antigen recognition, B cells become activated and differentiate into plasma cells.
* Plasma cells secrete antibodies, which are specialized proteins that can neutralize pathogens, opsonize them for phagocytosis, or activate complement pathways.

**6. Memory T and B Cells:**

* A subset of activated T and B cells differentiate into memory cells.
* Memory cells persist in the body and can rapidly respond to subsequent encounters with the same antigen, providing long-lasting immunity.

**7. Regulation and Tolerance:**

* The immune system employs regulatory mechanisms to prevent excessive or harmful immune responses.
* Regulatory T cells (Tregs) suppress immune activation and maintain tolerance to self-antigens.

In summary, leukocyte-mediated immunity involves a complex and intricate process of antigen recognition, T cell activation, differentiation of effector T cells, antibody production by B cells, and the generation of memory cells. This finely tuned system ensures effective protection against a wide range of pathogens while avoiding autoimmune reactions.
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