UNIT 5: Antigen Recognition and Presentation, Autoimmune Diseases, Immunodeficiency, and Immunodiagnostics

Exam Focus: The comparison table for **MHC Class I vs. Class II** is essential (which T cell interacts, which cells express it, source of antigen). Be prepared to describe the basic principles of **ELISA** and **RIA** and list examples of autoimmune diseases.

Table of Contents

1. Antigen Recognition and Processing
   • Class I & Class II MHC Molecules
   • Antigen Processing and Presentation by MHC Molecule
2. Autoimmune Diseases
   • Organ-specific Autoimmune Diseases
   • Systemic Autoimmune Diseases
3. Immunodeficiency: HIV and AIDS
4. Introduction to Immunodiagnostics
   • ELISA
   • RIA

1. Antigen Recognition and Processing

T cells cannot directly recognize soluble antigens. They require antigens to be processed and presented on the surface of other cells by Major Histocompatibility Complex (MHC) molecules.

Class I & Class II MHC Molecules

Feature	MHC Class I (HLA-A, B, C)	MHC Class II (HLA-DP, DQ, DR)
**Expression**	Expressed on **all nucleated cells**.	Expressed primarily on **Antigen Presenting Cells (APCs)** (Dendritic cells, Macrophages, B cells).
**Source of Antigen**	Endogenous (intracellular, e.g., viral proteins, cancer proteins).	Exogenous (extracellular, e.g., bacteria, toxins).
**T cell Interaction**	Interacts with **Cytotoxic T cells (TC)** via CD8 coreceptor.	Interacts with **Helper T cells (TH)** via CD4 coreceptor.