IMMUNOLOGY

Detailed explanation-1: -Helper T cells become activated through a multistep process, which begins with antigen-presenting cells, such as macrophages. These cells ingest an infectious agent or foreign particle, partially degrade it, and export fragments of it-i.e., antigens-to the cell surface.

Detailed explanation-2: -Helper T cells are arguably the most important cells in adaptive immunity, as they are required for almost all adaptive immune responses. They not only help activate B cells to secrete antibodies and macrophages to destroy ingested microbes, but they also help activate cytotoxic T cells to kill infected target cells.

Detailed explanation-3: -Professional antigen presenting cells (APCs), including dendritic cells (DCs) and macrophages, present antigens to T cells to initiate adaptive immunity by sequentially delivering signal 1 (antigen), signal 2 (co-stimulation), and signal 3 (polarizing signals mediated by soluble or membrane-bound factors) (6).

Detailed explanation-4: -T helper cells recognize viral peptides in association with class II MHC protein, usually on the surface of an antigen-presenting cell (APC). These interactions result in T helper cell activation, proliferation and differentiation, providing the affinity of binding is sufficiently high.

Detailed explanation-5: -T cell activation is a key event in the adaptive immune response and vital to the generation of both cellular and humoral immunity. Activation is required not only for effective CD4 T cell responses but also to provide help for B cells and the generation of cytotoxic T cell responses.

Detailed explanation-6: -Professional antigen-presenting cells, including macrophages, B cells and dendritic cells, present foreign antigens to helper T cells, while virus-infected cells (or cancer cells) can present antigens originating inside the cell to cytotoxic T cells.

Detailed explanation-7: -The first set of responses constitutes the innate immune response. Because the recognition molecules used by the innate system are expressed broadly on a large number of cells, this system is poised to act rapidly after an invading pathogen or toxin is encountered and thus constitutes the initial host response.