# Helper T Cells - Background
https://www.ncbi.nlm.nih.gov/books/NBK26827/

## Introduction
- Used in almost all adaptive responses
- Activate B cells to secrete Ab and macrophages to destroy microbes
- Activate cytotoxic T cells
- AIDS lack of helper T cells
- Must become effector cells first
- Activated on antigen-presenting cells:
  - Innate response to infection dictates effector differentiation
  - This dictates type of overall immune response

## Antigen-Presenting Cells (APC) and Costimulatory Proteins
- APC provides 2 signals:
  - 1. Foreign peptide bound to protein on cell surface
  - 2. Costimulatory proteins (incl B7) induced by pathogens
- Effector T cell promote B7 expression yielding positive feedback loop to amplify T cell response

Signal 1 alone causes apoptosis or deactivation (fail-safe to become tolerant to self-antigens). Combined signals differentiate T cell into effector cell by mechanism:
- T cell secretes IL-2 causing synthesis of IL-2 receptors
- IL-2 binding to IL-2 receptors causes gene switching to induce T-cell differentiating
- Safety design so T cells differentiate only when many responding in same location

## Binding to APC
- Once T cell bound to APC