Exogenous antigens are the antigens that enter into the host from the outside, through inhalation, ingestion, or injection. Exogenous antigens include particles considered foreign within the organism.

For example, allergens (such as pollen), proteins from transplanted tissues and organs, and parts of microorganisms (such as coat, capsule, cell wall, flagella, fimbria, or toxin of bacteria, viruses, etc.) can serve as antigens. These exogenous antigens are taken up by endocytosis or phagocytosis into the antigen presenting cells (APCs), and degraded into fragments.

The fragments are then presented on the surface of APCs by class II histocompatibility molecules to T helper cells (CD4 + or TH). The T helper cells then get activated and release cytokines. The cytokines then activate cytotoxic T lymphocytes (CTL), antibodies secreting B lymphocytes, macrophages and other cells.

Generally microorganisms, pollen, drugs and pollutants are considered as exogenous antigens. The disease causing pathogens induce immune response after entering into the host body.

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Host may develop resistance after the first attack of certain pathogens, for example the virus responsible for small pox and chicken pox induce resistance in the host due to the formation of memory cells developed after the first exposure. But certain pathogens such as virus responsible for common cold, bacteria causing Pharyngitis etc. fail to induce resistance due to various reasons. Frequent alterations in pathogen epitopes due to mutations is one of the reason.